Merge commit 'de238b39e335c6814283faa171b35145f124edf2'

7 files changed, 983 insertions, 289 deletions

diff --git a/lisp/progmodes/bug-reference.el b/lisp/progmodes/bug-reference.el
index 759db1f5686..813ecbe3847 100644
--- a/lisp/progmodes/bug-reference.el
+++ b/lisp/progmodes/bug-reference.el
@@ -1,4 +1,4 @@
-;; bug-reference.el --- buttonize bug references
+;; bug-reference.el --- buttonize bug references  -*- lexical-binding: t; -*-
 
 ;; Copyright (C) 2008-2019 Free Software Foundation, Inc.
 
@@ -91,7 +91,7 @@ The second subexpression should match the bug reference (usually a number)."
 (bug-reference-set-overlay-properties)
 
 (defun bug-reference-unfontify (start end)
-  "Remove bug reference overlays from region."
+  "Remove bug reference overlays from the region between START and END."
   (dolist (o (overlays-in start end))
     (when (eq (overlay-get o 'category) 'bug-reference)
       (delete-overlay o))))
@@ -99,7 +99,7 @@ The second subexpression should match the bug reference (usually a number)."
 (defvar bug-reference-prog-mode)
 
 (defun bug-reference-fontify (start end)
-  "Apply bug reference overlays to region."
+  "Apply bug reference overlays to the region between START and END."
   (save-excursion
     (let ((beg-line (progn (goto-char start) (line-beginning-position)))
           (end-line (progn (goto-char end) (line-end-position))))
diff --git a/lisp/progmodes/compile.el b/lisp/progmodes/compile.el
index 5bfb0bf9018..1a0d9bdbb70 100644
--- a/lisp/progmodes/compile.el
+++ b/lisp/progmodes/compile.el
@@ -562,7 +562,7 @@ LINE, END-LINE, COL, and END-COL can also be functions of no argument
 that return the corresponding line or column number.  They can assume REGEXP
 has just been matched, and should correspondingly preserve this match data.
 
-f/usr/shaTYPE is 2 or nil for a real error or 1 for warning or 0 for info.
+TYPE is 2 or nil for a real error or 1 for warning or 0 for info.
 TYPE can also be of the form (WARNING . INFO).  In that case this
 will be equivalent to 1 if the WARNING'th subexpression matched
 or else equivalent to 0 if the INFO'th subexpression matched.
@@ -2056,8 +2056,7 @@ by replacing the first word, e.g., `compilation-scroll-output' from
                             (if (boundp 'byte-compile-bound-variables)
                                 (memq (cdr v) byte-compile-bound-variables)))
                         `(set (make-local-variable ',(car v)) ,(cdr v))))
-                 '(compilation-buffer-name-function
-                   compilation-directory-matcher
+                 '(compilation-directory-matcher
                    compilation-error
                    compilation-error-regexp-alist
                    compilation-error-regexp-alist-alist
diff --git a/lisp/progmodes/grep.el b/lisp/progmodes/grep.el
index c0f47159c95..8c7a58fd8bd 100644
--- a/lisp/progmodes/grep.el
+++ b/lisp/progmodes/grep.el
@@ -959,8 +959,16 @@ substitution string.  Note dynamic scoping of variables.")
 The pattern can include shell wildcards.  As whitespace triggers
 completion when entering a pattern, including it requires
 quoting, e.g. `\\[quoted-insert]<space>'."
-  (let* ((bn (or (buffer-file-name)
-                 (replace-regexp-in-string "<[0-9]+>\\'" "" (buffer-name))))
+  (let* ((grep-read-files-function (get major-mode 'grep-read-files))
+         (file-name-at-point
+           (run-hook-with-args-until-success 'file-name-at-point-functions))
+         (bn (if grep-read-files-function
+                 (funcall grep-read-files-function)
+               (or (if (and (stringp file-name-at-point)
+                            (not (file-directory-p file-name-at-point)))
+                       file-name-at-point)
+                   (buffer-file-name)
+                   (replace-regexp-in-string "<[0-9]+>\\'" "" (buffer-name)))))
          (fn (and bn
                   (stringp bn)
                   (file-name-nondirectory bn)))
diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index 4d91da73340..a0adaa84eeb 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -50,7 +50,6 @@
 (require 'imenu)
 (require 'moz nil t)
 (require 'json)
-(require 'sgml-mode)
 (require 'prog-mode)
 
 (eval-when-compile
@@ -66,7 +65,7 @@
 
 ;;; Constants
 
-(defconst js--name-start-re "[a-zA-Z_$]"
+(defconst js--name-start-re (concat "[[:alpha:]_$]")
   "Regexp matching the start of a JavaScript identifier, without grouping.")
 
 (defconst js--stmt-delim-chars "^;{}?:")
@@ -572,6 +571,119 @@ then the \".\"s will be lined up:
   :safe 'booleanp
   :group 'js)
 
+(defcustom js-jsx-detect-syntax t
+  "When non-nil, automatically detect whether JavaScript uses JSX.
+`js-jsx-syntax' (which see) may be made buffer-local and set to
+t.  The detection strategy can be customized by adding elements
+to `js-jsx-regexps', which see."
+  :version "27.1"
+  :type 'boolean
+  :safe 'booleanp
+  :group 'js)
+
+(defcustom js-jsx-syntax nil
+  "When non-nil, parse JavaScript with consideration for JSX syntax.
+
+This enables proper font-locking and indentation of code using
+Facebook’s “JSX” syntax extension for JavaScript, for use with
+Facebook’s “React” library.  Font-locking is like sgml-mode.
+Indentation is also like sgml-mode, although some indentation
+behavior may differ slightly to align more closely with the
+conventions of the React developer community.
+
+When `js-mode' is already enabled, you should call
+`js-jsx-enable' to set this variable.
+
+It is set to be buffer-local (and t) when in `js-jsx-mode'."
+  :version "27.1"
+  :type 'boolean
+  :safe 'booleanp
+  :group 'js)
+
+(defcustom js-jsx-align->-with-< t
+  "When non-nil, “>” will be indented to the opening “<” in JSX.
+
+When this is enabled, JSX indentation looks like this:
+
+  <element
+    attr=\"\"
+  >
+  </element>
+  <input
+  />
+
+When this is disabled, JSX indentation looks like this:
+
+  <element
+    attr=\"\"
+    >
+  </element>
+  <input
+    />"
+  :version "27.1"
+  :type 'boolean
+  :safe 'booleanp
+  :group 'js)
+
+(defcustom js-jsx-indent-level nil
+  "When non-nil, indent JSX by this value, instead of like JS.
+
+Let `js-indent-level' be 4.  When this variable is also set to
+nil, JSX indentation looks like this (consistent):
+
+  return (
+      <element>
+          <element>
+              Hello World!
+          </element>
+      </element>
+  )
+
+Alternatively, when this variable is also set to 2, JSX
+indentation looks like this (different):
+
+  return (
+      <element>
+        <element>
+          Hello World!
+        </element>
+      </element>
+  )"
+  :version "27.1"
+  :type 'integer
+  :safe (lambda (x) (or (null x) (integerp x)))
+  :group 'js)
+;; This is how indentation behaved out-of-the-box until Emacs 27.  JSX
+;; indentation was controlled with `sgml-basic-offset', which defaults
+;; to 2, whereas `js-indent-level' defaults to 4.  Users who had the
+;; same values configured for both their HTML and JS indentation would
+;; luckily get consistent JSX indentation; most others were probably
+;; unhappy.  I’d be surprised if anyone actually wants different
+;; indentation levels, but just in case, here’s a way back to that.
+
+(defcustom js-jsx-attribute-offset 0
+  "Specifies a delta for JSXAttribute indentation.
+
+Let `js-indent-level' be 2.  When this variable is also set to 0,
+JSXAttribute indentation looks like this:
+
+  <element
+    attribute=\"value\">
+  </element>
+
+Alternatively, when this variable is also set to 2, JSXAttribute
+indentation looks like this:
+
+  <element
+      attribute=\"value\">
+  </element>
+
+This variable is like `sgml-attribute-offset'."
+  :version "27.1"
+  :type 'integer
+  :safe 'integerp
+  :group 'js)
+
 ;;; KeyMap
 
 (defvar js-mode-map
@@ -1485,6 +1597,102 @@ point of view of font-lock.  It applies highlighting directly with
   ;; Matcher always "fails"
   nil)
 
+;; It wouldn’t be sufficient to font-lock JSX with mere regexps, since
+;; a JSXElement may be nested inside a JS expression within the
+;; boundaries of a parent JSXOpeningElement, and such a hierarchy
+;; ought to be fontified like JSX, JS, and JSX respectively:
+;;
+;;   <div attr={void(<div></div>) && void(0)}></div>
+;;
+;;   <div attr={           ← JSX
+;;          void(          ← JS
+;;            <div></div>  ← JSX
+;;          ) && void(0)   ← JS
+;;        }></div>         ← JSX
+;;
+;; `js-syntax-propertize' unambiguously identifies JSX syntax,
+;; including when it’s nested.
+;;
+;; Using a matcher function for each relevant part, retrieve match
+;; data recorded as syntax properties for fontification.
+
+(defconst js-jsx--font-lock-keywords
+  `((js-jsx--match-tag-name 0 font-lock-function-name-face t)
+    (js-jsx--match-attribute-name 0 font-lock-variable-name-face t)
+    (js-jsx--match-text 0 'default t) ; “Undo” keyword fontification.
+    (js-jsx--match-tag-beg)
+    (js-jsx--match-tag-end)
+    (js-jsx--match-expr))
+  "JSX font lock faces and multiline text properties.")
+
+(defun js-jsx--match-tag-name (limit)
+  "Match JSXBoundaryElement names, until LIMIT."
+  (when js-jsx-syntax
+    (let ((pos (next-single-char-property-change (point) 'js-jsx-tag-name nil limit))
+          value)
+      (when (and pos (> pos (point)))
+        (goto-char pos)
+        (or (and (setq value (get-text-property pos 'js-jsx-tag-name))
+                 (progn (set-match-data value) t))
+            (js-jsx--match-tag-name limit))))))
+
+(defun js-jsx--match-attribute-name (limit)
+  "Match JSXAttribute names, until LIMIT."
+  (when js-jsx-syntax
+    (let ((pos (next-single-char-property-change (point) 'js-jsx-attribute-name nil limit))
+          value)
+      (when (and pos (> pos (point)))
+        (goto-char pos)
+        (or (and (setq value (get-text-property pos 'js-jsx-attribute-name))
+                 (progn (set-match-data value) t))
+            (js-jsx--match-attribute-name limit))))))
+
+(defun js-jsx--match-text (limit)
+  "Match JSXText, until LIMIT."
+  (when js-jsx-syntax
+    (let ((pos (next-single-char-property-change (point) 'js-jsx-text nil limit))
+          value)
+      (when (and pos (> pos (point)))
+        (goto-char pos)
+        (or (and (setq value (get-text-property pos 'js-jsx-text))
+                 (progn (set-match-data value)
+                        (put-text-property (car value) (cadr value) 'font-lock-multiline t)
+                        t))
+            (js-jsx--match-text limit))))))
+
+(defun js-jsx--match-tag-beg (limit)
+  "Match JSXBoundaryElements from start, until LIMIT."
+  (when js-jsx-syntax
+    (let ((pos (next-single-char-property-change (point) 'js-jsx-tag-beg nil limit))
+          value)
+      (when (and pos (> pos (point)))
+        (goto-char pos)
+        (or (and (setq value (get-text-property pos 'js-jsx-tag-beg))
+                 (progn (put-text-property pos (cdr value) 'font-lock-multiline t) t))
+            (js-jsx--match-tag-beg limit))))))
+
+(defun js-jsx--match-tag-end (limit)
+  "Match JSXBoundaryElements from end, until LIMIT."
+  (when js-jsx-syntax
+    (let ((pos (next-single-char-property-change (point) 'js-jsx-tag-end nil limit))
+          value)
+      (when (and pos (> pos (point)))
+        (goto-char pos)
+        (or (and (setq value (get-text-property pos 'js-jsx-tag-end))
+                 (progn (put-text-property value pos 'font-lock-multiline t) t))
+            (js-jsx--match-tag-end limit))))))
+
+(defun js-jsx--match-expr (limit)
+  "Match JSXExpressionContainers, until LIMIT."
+  (when js-jsx-syntax
+    (let ((pos (next-single-char-property-change (point) 'js-jsx-expr nil limit))
+          value)
+      (when (and pos (> pos (point)))
+        (goto-char pos)
+        (or (and (setq value (get-text-property pos 'js-jsx-expr))
+                 (progn (put-text-property pos value 'font-lock-multiline t) t))
+            (js-jsx--match-expr limit))))))
+
 (defconst js--font-lock-keywords-3
   `(
     ;; This goes before keywords-2 so it gets used preferentially
@@ -1596,7 +1804,10 @@ point of view of font-lock.  It applies highlighting directly with
                  (forward-symbol -1)
                (end-of-line))
             '(end-of-line)
-            '(0 font-lock-variable-name-face))))
+            '(0 font-lock-variable-name-face)))
+
+    ;; jsx (when enabled)
+    ,@js-jsx--font-lock-keywords)
   "Level three font lock for `js-mode'.")
 
 (defun js--inside-pitem-p (pitem)
@@ -1722,9 +1933,414 @@ This performs fontification according to `js--class-styles'."
                            'syntax-table (string-to-syntax "\"/"))
         (goto-char end)))))
 
+(defconst js--unary-keyword-re
+  (js--regexp-opt-symbol '("await" "delete" "typeof" "void" "yield"))
+  "Regexp matching unary operator keywords.")
+
+(defun js--unary-keyword-p (string)
+  "Check if STRING is a unary operator keyword in JavaScript."
+  (string-match-p js--unary-keyword-re string))
+
+;; Adding `syntax-multiline' text properties to JSX isn’t sufficient
+;; to identify multiline JSX when first typing it.  For instance, if
+;; the user is typing a JSXOpeningElement for the first time…
+;;
+;;   <div
+;;       ^ (point)
+;;
+;; …and the user inserts a line break after the tag name (before the
+;; JSXOpeningElement starting on that line has been unambiguously
+;; identified as such), then the `syntax-propertize' region won’t be
+;; extended backwards to the start of the JSXOpeningElement:
+;;
+;;   <div         ← This line wasn’t JSX when last edited.
+;;     attr="">   ← Despite completing the JSX, the next
+;;             ^    `syntax-propertize' region wouldn’t magically
+;;                  extend back a few lines.
+;;
+;; Therefore, to try and recover from this scenario, parse backward
+;; from “>” to try and find the start of JSXBoundaryElements, and
+;; extend the `syntax-propertize' region there.
+
+(defun js--syntax-propertize-extend-region (start end)
+  "Extend the START-END region for propertization, if necessary.
+For use by `syntax-propertize-extend-region-functions'."
+  (if js-jsx-syntax (js-jsx--syntax-propertize-extend-region start end)))
+
+(defun js-jsx--syntax-propertize-extend-region (start end)
+  "Extend the START-END region for propertization, if necessary.
+If any “>” in the region appears to be the end of a tag starting
+before the start of the region, extend region backwards to the
+start of that tag so parsing may proceed from that point.
+For use by `syntax-propertize-extend-region-functions'."
+  (let (new-start
+        forward-sexp-function ; Use the Lisp version.
+        parse-sexp-lookup-properties) ; Fix backward-sexp error here.
+    (catch 'stop
+      (goto-char start)
+      (while (re-search-forward ">" end t)
+        (catch 'continue
+          ;; Check if this is really a right shift bitwise operator
+          ;; (“>>” or “>>>”).
+          (unless (or (eq (char-before (1- (point))) ?>)
+                      (eq (char-after) ?>))
+            (save-excursion
+              (backward-char)
+              (while (progn (if (= (point) (point-min)) (throw 'continue nil))
+                            (/= (char-before) ?<))
+                (skip-chars-backward " \t\n")
+                (if (= (point) (point-min)) (throw 'continue nil))
+                (cond
+                 ((memq (char-before) '(?\" ?\' ?\` ?\}))
+                  (condition-case nil
+                      (backward-sexp)
+                    (scan-error (throw 'continue nil))))
+                 ((memq (char-before) '(?\/ ?\=)) (backward-char))
+                 ((looking-back js--dotted-name-re (line-beginning-position) t)
+                  (goto-char (match-beginning 0)))
+                 (t (throw 'continue nil))))
+              (when (< (point) start)
+                (setq new-start (1- (point)))
+                (throw 'stop nil)))))))
+    (if new-start (cons new-start end))))
+
+;; When applying syntax properties, since `js-syntax-propertize' uses
+;; `syntax-propertize-rules' to parse JSXBoundaryElements iteratively
+;; and statelessly, whenever we exit such an element, we need to
+;; determine the JSX depth.  If >0, then we know we to apply syntax
+;; properties to JSXText up until the next JSXBoundaryElement occurs.
+;; But if the JSX depth is 0, then—importantly—we know to NOT parse
+;; the following code as JSXText, rather propertize it as regular JS
+;; as long as warranted.
+;;
+;; Also, when indenting code, we need to know if the code we’re trying
+;; to indent is on the 2nd or later line of multiline JSX, in which
+;; case the code is indented according to XML-like JSX conventions.
+;;
+;; For the aforementioned reasons, we find ourselves needing to
+;; determine whether point is enclosed in JSX or not; and, if so,
+;; where the JSX is.  The following functions provide that knowledge.
+
+(defconst js-jsx--tag-start-re
+  (concat "\\(" js--dotted-name-re "\\)\\(?:"
+          ;; Whitespace is only necessary if an attribute implies JSX.
+          "\\(?:\\s-\\|\n\\)*[{/>]"
+          "\\|"
+          "\\(?:\\s-\\|\n\\)+" js--name-start-re
+          "\\)")
+  "Regexp unambiguously matching a JSXOpeningElement.")
+
+(defun js-jsx--matched-tag-type ()
+  "Determine if the last “<” was a JSXBoundaryElement and its type.
+Return `close' for a JSXClosingElement/JSXClosingFragment match,
+return `self-closing' for some self-closing JSXOpeningElements,
+else return `other'."
+  (cond
+   ((= (char-after) ?/) (forward-char) 'close) ; JSXClosingElement/JSXClosingFragment
+   ((= (char-after) ?>) (forward-char) 'other) ; JSXOpeningFragment
+   ((and (looking-at js-jsx--tag-start-re) ; JSXOpeningElement
+         (not (js--unary-keyword-p (match-string 1))))
+    (goto-char (match-end 0))
+    (if (= (char-before) ?/) 'self-closing 'other))))
+
+(defconst js-jsx--self-closing-re "/\\s-*>"
+  "Regexp matching the end of a self-closing JSXOpeningElement.")
+
+(defun js-jsx--matching-close-tag-pos ()
+  "Return position of the closer of the opener before point.
+Assuming a JSXOpeningElement or a JSXOpeningFragment is
+immediately before point, find a matching JSXClosingElement or
+JSXClosingFragment, skipping over any nested JSXElements to find
+the match.  Return nil if a match can’t be found."
+  (let ((tag-stack 1) tag-pos type last-pos pos)
+    (catch 'stop
+      (while (and (re-search-forward "<\\s-*" nil t) (not (eobp)))
+        (when (setq tag-pos (match-beginning 0)
+                    type (js-jsx--matched-tag-type))
+          (when last-pos
+            (setq pos (point))
+            (goto-char last-pos)
+            (while (re-search-forward js-jsx--self-closing-re pos 'move)
+              (setq tag-stack (1- tag-stack))))
+          (if (eq type 'close)
+              (progn
+                (setq tag-stack (1- tag-stack))
+                (when (= tag-stack 0)
+                  (throw 'stop tag-pos)))
+            ;; JSXOpeningElements that we know are self-closing aren’t
+            ;; added to the stack at all (because point is already
+            ;; past that syntax).
+            (unless (eq type 'self-closing)
+              (setq tag-stack (1+ tag-stack))))
+          (setq last-pos (point)))))))
+
+(defun js-jsx--enclosing-tag-pos ()
+  "Return beginning and end of a JSXElement about point.
+Look backward for a JSXElement that both starts before point and
+also ends at/after point.  That may be either a self-closing
+JSXElement or a JSXOpeningElement/JSXClosingElement pair."
+  (let ((start (point)) tag-beg tag-beg-pos tag-end-pos close-tag-pos)
+    (while
+        (and
+         (setq tag-beg (js--backward-text-property 'js-jsx-tag-beg))
+         (progn
+           (setq tag-beg-pos (point)
+                 tag-end-pos (cdr tag-beg))
+           (not
+            (or
+             (and (eq (car tag-beg) 'self-closing)
+                  (< start tag-end-pos))
+             (and (eq (car tag-beg) 'open)
+                  (or (< start tag-end-pos)
+                      (progn
+                        (unless
+                            ;; Try to read a cached close position,
+                            ;; but it might not be available yet.
+                            (setq close-tag-pos
+                                  (get-text-property (point) 'js-jsx-close-tag-pos))
+                          (save-excursion
+                            (goto-char tag-end-pos)
+                            (setq close-tag-pos (js-jsx--matching-close-tag-pos)))
+                          (when close-tag-pos
+                            ;; Cache the close position to make future
+                            ;; searches faster.
+                            (put-text-property
+                             (point) (1+ (point))
+                             'js-jsx-close-tag-pos close-tag-pos)))
+                        ;; The JSXOpeningElement may be unclosed, else
+                        ;; the closure must occur at/after the start
+                        ;; point (otherwise, a miscellaneous previous
+                        ;; JSXOpeningElement has been found, so keep
+                        ;; looking backwards for an enclosing one).
+                        (or (not close-tag-pos) (<= start close-tag-pos)))))))))
+      ;; Don’t return the last tag pos, as it wasn’t enclosing.
+      (setq tag-beg nil close-tag-pos nil))
+    (and tag-beg (list tag-beg-pos tag-end-pos close-tag-pos))))
+
+(defun js-jsx--at-enclosing-tag-child-p ()
+  "Return t if point is at an enclosing tag’s child."
+  (let ((pos (save-excursion (js-jsx--enclosing-tag-pos))))
+    (and pos (>= (point) (nth 1 pos)))))
+
+;; We implement `syntax-propertize-function' logic fully parsing JSX
+;; in order to provide very accurate JSX indentation, even in the most
+;; complex cases (e.g. to indent JSX within a JS expression within a
+;; JSXAttribute…), as over the years users have requested this.  Since
+;; we find so much information during this parse, we later use some of
+;; the useful bits for font-locking, too.
+;;
+;; Some extra effort is devoted to ensuring that no code which could
+;; possibly be valid JS is ever misinterpreted as partial JSX, since
+;; that would be regressive.
+;;
+;; We first parse trying to find the minimum number of components
+;; necessary to unambiguously identify a JSXBoundaryElement, even if
+;; it is a partial one.  If a complete one is parsed, we move on to
+;; parse any JSXText.  When that’s terminated, we unwind back to the
+;; `syntax-propertize-rules' loop so the next JSXBoundaryElement can
+;; be parsed, if any, be it an opening or closing one.
+
+(defun js-jsx--put-syntax-table (start end value)
+  "Set syntax-table text property from START to END as VALUE.
+Redundantly set the value to two properties, syntax-table and
+js-jsx-syntax-table.  Derivative modes that remove syntax-table
+text properties may recover the value from the second property." ; i.e. js2-mode
+  (add-text-properties start end (list 'syntax-table value
+                                       'js-jsx-syntax-table value)))
+
+(defun js-jsx--text-range (beg end)
+  "Identify JSXText within a “>/{/}/<” pair."
+  (when (> (- end beg) 0)
+    (save-excursion
+      (goto-char beg)
+      (while (and (skip-chars-forward " \t\n" end) (< (point) end))
+        ;; Comments and string quotes don’t serve their usual
+        ;; syntactic roles in JSXText; make them plain punctuation to
+        ;; negate those roles.
+        (when (or (= (char-after) ?/) ; comment
+                  (= (syntax-class (syntax-after (point))) 7)) ; string quote
+          (js-jsx--put-syntax-table (point) (1+ (point)) '(1)))
+        (forward-char)))
+    ;; Mark JSXText so it can be font-locked as non-keywords.
+    (put-text-property beg (1+ beg) 'js-jsx-text (list beg end (current-buffer)))
+    ;; Ensure future propertization beginning from within the
+    ;; JSXText determines JSXText context from earlier lines.
+    (put-text-property beg end 'syntax-multiline t)))
+
+;; In order to respect the end boundary `syntax-propertize-function'
+;; sets, care is taken in the following functions to abort parsing
+;; whenever that boundary is reached.
+
+(defun js-jsx--syntax-propertize-tag-text (end)
+  "Determine if JSXText is before END and propertize it.
+Text within an open/close tag pair may be JSXText.  Temporarily
+interrupt JSXText by JSXExpressionContainers, and terminate
+JSXText when another JSXBoundaryElement is encountered.  Despite
+terminations, all JSXText will be identified once all the
+JSXBoundaryElements within an outermost JSXElement’s tree have
+been propertized."
+  (let ((text-beg (point))
+        forward-sexp-function) ; Use Lisp version.
+    (catch 'stop
+      (while (re-search-forward "[{<]" end t)
+        (js-jsx--text-range text-beg (1- (point)))
+        (cond
+         ((= (char-before) ?{)
+          (let (expr-beg expr-end)
+            (condition-case nil
+                (save-excursion
+                  (backward-char)
+                  (setq expr-beg (point))
+                  (forward-sexp)
+                  (setq expr-end (point)))
+              (scan-error nil))
+            ;; Recursively propertize the JSXExpressionContainer’s
+            ;; (possibly-incomplete) expression.
+            (js-syntax-propertize (1+ expr-beg) (if expr-end (min (1- expr-end) end) end))
+            ;; Ensure future propertization beginning from within the
+            ;; (possibly-incomplete) expression can determine JSXText
+            ;; context from earlier lines.
+            (put-text-property expr-beg (1+ expr-beg) 'js-jsx-expr (or expr-end end)) ; font-lock
+            (put-text-property expr-beg (if expr-end (min expr-end end) end) 'syntax-multiline t) ; syntax-propertize
+            ;; Exit the JSXExpressionContainer if that’s possible,
+            ;; else move to the end of the propertized area.
+            (goto-char (if expr-end (min expr-end end) end))))
+         ((= (char-before) ?<)
+          (backward-char) ; Ensure the next tag can be propertized.
+          (throw 'stop nil)))
+        (setq text-beg (point))))))
+
+(defconst js-jsx--attribute-name-re (concat js--name-start-re
+                                            "\\(?:\\s_\\|\\sw\\|-\\)*")
+  "Like `js--name-re', but matches “-” as well.")
+
+(defun js-jsx--syntax-propertize-tag (end)
+  "Determine if a JSXBoundaryElement is before END and propertize it.
+Disambiguate JSX from inequality operators and arrow functions by
+testing for syntax only valid as JSX."
+  (let ((tag-beg (1- (point))) tag-end (type 'open)
+        name-beg name-match-data expr-attribute-beg unambiguous
+        forward-sexp-function) ; Use Lisp version.
+    (catch 'stop
+      (while (and (< (point) end)
+                  (progn (skip-chars-forward " \t\n" end)
+                         (< (point) end)))
+        (cond
+         ((= (char-after) ?>)
+          ;; Make the closing “>” a close parenthesis.
+          (js-jsx--put-syntax-table (point) (1+ (point)) '(5))
+          (forward-char)
+          (setq unambiguous t)
+          (throw 'stop nil))
+         ;; Handle a JSXSpreadChild (“<Foo {...bar}”) or a
+         ;; JSXExpressionContainer as a JSXAttribute value
+         ;; (“<Foo bar={…}”).  Check this early in case continuing a
+         ;; JSXAttribute parse.
+         ((or (and name-beg (= (char-after) ?{))
+              (setq expr-attribute-beg nil))
+          (setq unambiguous t) ; JSXExpressionContainer post tag name ⇒ JSX
+          (when expr-attribute-beg
+            ;; Remember that this JSXExpressionContainer is part of a
+            ;; JSXAttribute, as that can affect its expression’s
+            ;; indentation.
+            (put-text-property
+             (point) (1+ (point)) 'js-jsx-expr-attribute expr-attribute-beg)
+            (setq expr-attribute-beg nil))
+          (let (expr-end)
+            (condition-case nil
+                (save-excursion
+                  (forward-sexp)
+                  (setq expr-end (point)))
+              (scan-error nil))
+            (forward-char)
+            (if (>= (point) end) (throw 'stop nil))
+            (skip-chars-forward " \t\n" end)
+            (if (>= (point) end) (throw 'stop nil))
+            (if (= (char-after) ?}) (forward-char) ; Shortcut to bail.
+              ;; Recursively propertize the JSXExpressionContainer’s
+              ;; expression.
+              (js-syntax-propertize (point) (if expr-end (min (1- expr-end) end) end))
+              ;; Exit the JSXExpressionContainer if that’s possible,
+              ;; else move to the end of the propertized area.
+              (goto-char (if expr-end (min expr-end end) end)))))
+         ((= (char-after) ?/)
+          ;; Assume a tag is an open tag until a slash is found, then
+          ;; figure out what type it actually is.
+          (if (eq type 'open) (setq type (if name-beg 'self-closing 'close)))
+          (forward-char))
+         ((and (not name-beg) (looking-at js--dotted-name-re))
+          ;; Don’t match code like “if (i < await foo)”
+          (if (js--unary-keyword-p (match-string 0)) (throw 'stop nil))
+          ;; Save boundaries for later fontification after
+          ;; unambiguously determining the code is JSX.
+          (setq name-beg (match-beginning 0)
+                name-match-data (match-data))
+          (goto-char (match-end 0)))
+         ((and name-beg (looking-at js-jsx--attribute-name-re))
+          (setq unambiguous t) ; Non-unary name followed by 2nd name ⇒ JSX
+          ;; Save JSXAttribute’s name’s match data for font-locking later.
+          (put-text-property (match-beginning 0) (1+ (match-beginning 0))
+                             'js-jsx-attribute-name (match-data))
+          (goto-char (match-end 0))
+          (if (>= (point) end) (throw 'stop nil))
+          (skip-chars-forward " \t\n" end)
+          (if (>= (point) end) (throw 'stop nil))
+          ;; “=” is optional for null-valued JSXAttributes.
+          (when (= (char-after) ?=)
+            (forward-char)
+            (if (>= (point) end) (throw 'stop nil))
+            (skip-chars-forward " \t\n" end)
+            (if (>= (point) end) (throw 'stop nil))
+            ;; Skip over strings (if possible).  Any
+            ;; JSXExpressionContainer here will be parsed in the
+            ;; next iteration of the loop.
+            (if (memq (char-after) '(?\" ?\' ?\`))
+                (progn
+                  ;; Record the string’s position so derived modes
+                  ;; applying syntactic fontification atypically
+                  ;; (e.g. js2-mode) can recognize it as part of JSX.
+                  (put-text-property (point) (1+ (point)) 'js-jsx-string t)
+                  (condition-case nil
+                      (forward-sexp)
+                    (scan-error (throw 'stop nil))))
+              ;; Save JSXAttribute’s beginning in case we find a
+              ;; JSXExpressionContainer as the JSXAttribute’s value which
+              ;; we should associate with the JSXAttribute.
+              (setq expr-attribute-beg (match-beginning 0)))))
+         ;; There is nothing more to check; this either isn’t JSX, or
+         ;; the tag is incomplete.
+         (t (throw 'stop nil)))))
+    (when unambiguous
+      ;; Save JSXBoundaryElement’s name’s match data for font-locking.
+      (if name-beg (put-text-property name-beg (1+ name-beg) 'js-jsx-tag-name name-match-data))
+      ;; Make the opening “<” an open parenthesis.
+      (js-jsx--put-syntax-table tag-beg (1+ tag-beg) '(4))
+      ;; Prevent “out of range” errors when typing at the end of a buffer.
+      (setq tag-end (if (eobp) (1- (point)) (point)))
+      ;; Mark beginning and end of tag for font-locking.
+      (put-text-property tag-beg (1+ tag-beg) 'js-jsx-tag-beg (cons type tag-end))
+      (put-text-property tag-end (1+ tag-end) 'js-jsx-tag-end tag-beg)
+      ;; Use text properties to extend the syntax-propertize region
+      ;; backward to the beginning of the JSXBoundaryElement in the
+      ;; future.  Typically the closing angle bracket could suggest
+      ;; extending backward, but that would also involve more rigorous
+      ;; parsing, and the closing angle bracket may not even exist yet
+      ;; if the JSXBoundaryElement is still being typed.
+      (put-text-property tag-beg (1+ tag-end) 'syntax-multiline t))
+    (if (js-jsx--at-enclosing-tag-child-p) (js-jsx--syntax-propertize-tag-text end))))
+
+(defconst js-jsx--text-properties
+  (list
+   'js-jsx-tag-beg nil 'js-jsx-tag-end nil 'js-jsx-close-tag-pos nil
+   'js-jsx-tag-name nil 'js-jsx-attribute-name nil 'js-jsx-string nil
+   'js-jsx-text nil 'js-jsx-expr nil 'js-jsx-expr-attribute nil
+   'js-jsx-syntax-table nil)
+  "Plist of text properties added by `js-syntax-propertize'.")
+
 (defun js-syntax-propertize (start end)
   ;; JavaScript allows immediate regular expression objects, written /.../.
   (goto-char start)
+  (if js-jsx-syntax (remove-text-properties start end js-jsx--text-properties))
   (js-syntax-propertize-regexp end)
   (funcall
    (syntax-propertize-rules
@@ -1748,7 +2364,8 @@ This performs fontification according to `js--class-styles'."
            (put-text-property (match-beginning 1) (match-end 1)
                               'syntax-table (string-to-syntax "\"/"))
            (js-syntax-propertize-regexp end)))))
-    ("\\`\\(#\\)!" (1 "< b")))
+    ("\\`\\(#\\)!" (1 "< b"))
+    ("<" (0 (ignore (if js-jsx-syntax (js-jsx--syntax-propertize-tag end))))))
    (point) end))
 
 (defconst js--prettify-symbols-alist
@@ -1774,6 +2391,11 @@ This performs fontification according to `js--class-styles'."
           (js--regexp-opt-symbol '("in" "instanceof")))
   "Regexp matching operators that affect indentation of continued expressions.")
 
+(defun js-jsx--looking-at-start-tag-p ()
+  "Non-nil if a JSXOpeningElement immediately follows point."
+  (let ((tag-beg (get-text-property (point) 'js-jsx-tag-beg)))
+    (and tag-beg (memq (car tag-beg) '(open self-closing)))))
+
 (defun js--looking-at-operator-p ()
   "Return non-nil if point is on a JavaScript operator, other than a comma."
   (save-match-data
@@ -1796,7 +2418,9 @@ This performs fontification according to `js--class-styles'."
                  (js--backward-syntactic-ws)
                  ;; We might misindent some expressions that would
                  ;; return NaN anyway.  Shouldn't be a problem.
-                 (memq (char-before) '(?, ?} ?{))))))))
+                 (memq (char-before) '(?, ?} ?{)))))
+         ;; “<” isn’t necessarily an operator in JSX.
+         (not (and js-jsx-syntax (js-jsx--looking-at-start-tag-p))))))
 
 (defun js--find-newline-backward ()
   "Move backward to the nearest newline that is not in a block comment."
@@ -1816,6 +2440,10 @@ This performs fontification according to `js--class-styles'."
         (setq result nil)))
     result))
 
+(defun js-jsx--looking-back-at-end-tag-p ()
+  "Non-nil if a JSXClosingElement immediately precedes point."
+  (get-text-property (point) 'js-jsx-tag-end))
+
 (defun js--continued-expression-p ()
   "Return non-nil if the current line continues an expression."
   (save-excursion
@@ -1833,12 +2461,19 @@ This performs fontification according to `js--class-styles'."
       (and (js--find-newline-backward)
            (progn
              (skip-chars-backward " \t")
-             (or (bobp) (backward-char))
-             (and (> (point) (point-min))
-                  (save-excursion (backward-char) (not (looking-at "[/*]/\\|=>")))
-                  (js--looking-at-operator-p)
-                  (and (progn (backward-char)
-                              (not (looking-at "\\+\\+\\|--\\|/[/*]"))))))))))
+             (and
+              ;; The “>” at the end of any JSXBoundaryElement isn’t
+              ;; part of a continued expression.
+              (not (and js-jsx-syntax (js-jsx--looking-back-at-end-tag-p)))
+              (progn
+                (or (bobp) (backward-char))
+                (and (> (point) (point-min))
+                     (save-excursion
+                       (backward-char)
+                       (not (looking-at "[/*]/\\|=>")))
+                     (js--looking-at-operator-p)
+                     (and (progn (backward-char)
+                                 (not (looking-at "\\+\\+\\|--\\|/[/*]"))))))))))))
 
 (defun js--skip-term-backward ()
   "Skip a term before point; return t if a term was skipped."
@@ -2064,23 +2699,182 @@ indentation is aligned to that column."
         (when comma-p
           (goto-char (1+ declaration-keyword-end))))))))
 
-(defconst js--line-terminating-arrow-re "\\s-*=>\\s-*\\(/[/*]\\|$\\)"
+(defconst js--line-terminating-arrow-re "=>\\s-*\\(/[/*]\\|$\\)"
   "Regexp matching the last \"=>\" (arrow) token on a line.
 Whitespace and comments around the arrow are ignored.")
 
-(defun js--looking-at-broken-arrow-function-p ()
+(defun js--broken-arrow-terminates-line-p ()
   "Helper function for `js--proper-indentation'.
-Return t if point is at the start of a (possibly async) arrow
-function and the last non-comment, non-whitespace token of the
-current line is the \"=>\" token."
-  (when (looking-at "\\s-*async\\s-*")
-    (goto-char (match-end 0)))
-  (cond
-   ((eq (char-after) ?\()
-    (forward-list)
-    (looking-at-p js--line-terminating-arrow-re))
-   (t (looking-at-p
-       (concat js--name-re js--line-terminating-arrow-re)))))
+Return t if the last non-comment, non-whitespace token of the
+current line is the \"=>\" token (of an arrow function)."
+  (let ((from (point)))
+    (end-of-line)
+    (re-search-backward js--line-terminating-arrow-re from t)))
+
+;; When indenting, we want to know if the line is…
+;;
+;;   - within a multiline JSXElement, or
+;;   - within a string in a JSXBoundaryElement, or
+;;   - within JSXText, or
+;;   - within a JSXAttribute’s multiline JSXExpressionContainer.
+;;
+;; In these cases, special XML-like indentation rules for JSX apply.
+;; If JS is nested within JSX, then indentation calculations may be
+;; combined, such that JS indentation is “relative” to the JSX’s.
+;;
+;; Therefore, functions below provide such contextual information, and
+;; `js--proper-indentation' may call itself once recursively in order
+;; to finish calculating that “relative” JS+JSX indentation.
+
+(defun js-jsx--context ()
+  "Determine JSX context and move to enclosing JSX."
+  (let ((pos (point))
+        (parse-status (syntax-ppss))
+        (enclosing-tag-pos (js-jsx--enclosing-tag-pos)))
+    (when enclosing-tag-pos
+      (if (< pos (nth 1 enclosing-tag-pos))
+          (if (nth 3 parse-status)
+              (list 'string (nth 8 parse-status))
+            (list 'tag (nth 0 enclosing-tag-pos) (nth 1 enclosing-tag-pos)))
+        (list 'text (nth 0 enclosing-tag-pos) (nth 2 enclosing-tag-pos))))))
+
+(defun js-jsx--contextual-indentation (line context)
+  "Calculate indentation column for LINE from CONTEXT.
+The column calculation is based off of `sgml-calculate-indent'."
+  (pcase (nth 0 context)
+
+    ('string
+     ;; Go back to previous non-empty line.
+     (while (and (> (point) (nth 1 context))
+                 (zerop (forward-line -1))
+                 (looking-at "[ \t]*$")))
+     (if (> (point) (nth 1 context))
+         ;; Previous line is inside the string.
+         (current-indentation)
+       (goto-char (nth 1 context))
+       (1+ (current-column))))
+
+    ('tag
+     ;; Special JSX indentation rule: a “dangling” closing angle
+     ;; bracket on its own line is indented at the same level as the
+     ;; opening angle bracket of the JSXElement.  Otherwise, indent
+     ;; JSXAttribute space like SGML.
+     (if (and
+          js-jsx-align->-with-<
+          (progn
+            (goto-char (nth 2 context))
+            (and (= line (line-number-at-pos))
+                 (looking-back "^\\s-*/?>" (line-beginning-position)))))
+         (progn
+           (goto-char (nth 1 context))
+           (current-column))
+       ;; Indent JSXAttribute space like SGML.
+       (goto-char (nth 1 context))
+       ;; Skip tag name:
+       (skip-chars-forward " \t")
+       (skip-chars-forward "^ \t\n")
+       (skip-chars-forward " \t")
+       (if (not (eolp))
+           (current-column)
+         ;; This is the first attribute: indent.
+         (goto-char (+ (nth 1 context) js-jsx-attribute-offset))
+         (+ (current-column) (or js-jsx-indent-level js-indent-level)))))
+
+    ('text
+     ;; Indent to reflect nesting.
+     (goto-char (nth 1 context))
+     (+ (current-column)
+        ;; The last line isn’t nested, but the rest are.
+        (if (or (not (nth 2 context)) ; Unclosed.
+                (< line (line-number-at-pos (nth 2 context))))
+            (or js-jsx-indent-level js-indent-level)
+          0)))
+
+    ))
+
+(defun js-jsx--enclosing-curly-pos ()
+  "Return position of enclosing “{” in a “{/}” pair about point."
+  (let ((parens (reverse (nth 9 (syntax-ppss)))) paren-pos curly-pos)
+    (while
+        (and
+         (setq paren-pos (car parens))
+         (not (when (= (char-after paren-pos) ?{)
+                (setq curly-pos paren-pos)))
+         (setq parens (cdr parens))))
+    curly-pos))
+
+(defun js-jsx--goto-outermost-enclosing-curly (limit)
+  "Set point to enclosing “{” at or closest after LIMIT."
+  (let (pos)
+    (while
+        (and
+         (setq pos (js-jsx--enclosing-curly-pos))
+         (if (>= pos limit) (goto-char pos))
+         (> pos limit)))))
+
+(defun js-jsx--expr-attribute-pos (start limit)
+  "Look back from START to LIMIT for a JSXAttribute."
+  (save-excursion
+    (goto-char start) ; Skip the first curly.
+    ;; Skip any remaining enclosing curlies until the JSXElement’s
+    ;; beginning position; the last curly ought to be one of a
+    ;; JSXExpressionContainer, which may refer to its JSXAttribute’s
+    ;; beginning position (if it has one).
+    (js-jsx--goto-outermost-enclosing-curly limit)
+    (get-text-property (point) 'js-jsx-expr-attribute)))
+
+(defvar js-jsx--indent-col nil
+  "Baseline column for JS indentation within JSX.")
+
+(defvar js-jsx--indent-attribute-line nil
+  "Line relative to which indentation uses JSX as a baseline.")
+
+(defun js-jsx--expr-indentation (parse-status pos col)
+  "Indent using PARSE-STATUS; relative to POS, use base COL.
+To indent a JSXExpressionContainer’s expression, calculate the JS
+indentation, using JSX indentation as the base column when
+indenting relative to the beginning line of the
+JSXExpressionContainer’s JSXAttribute (if any)."
+  (let* ((js-jsx--indent-col col)
+         (js-jsx--indent-attribute-line
+          (if pos (line-number-at-pos pos))))
+    (js--proper-indentation parse-status)))
+
+(defun js-jsx--indentation (parse-status)
+  "Helper function for `js--proper-indentation'.
+Return the proper indentation of the current line if it is part
+of a JSXElement expression spanning multiple lines; otherwise,
+return nil."
+  (let ((current-line (line-number-at-pos))
+        (curly-pos (js-jsx--enclosing-curly-pos))
+        nth-context context expr-p beg-line col
+        forward-sexp-function) ; Use the Lisp version.
+    ;; Find the immediate context for indentation information, but
+    ;; keep going to determine that point is at the N+1th line of
+    ;; multiline JSX.
+    (save-excursion
+      (while
+          (and
+           (setq nth-context (js-jsx--context))
+           (progn
+             (unless context
+               (setq context nth-context)
+               (setq expr-p (and curly-pos (< (point) curly-pos))))
+             (setq beg-line (line-number-at-pos))
+             (and
+              (= beg-line current-line)
+              (or (not curly-pos) (> (point) curly-pos)))))))
+    ;; When on the second or later line of JSX, indent as JSX,
+    ;; possibly switching back to JS indentation within
+    ;; JSXExpressionContainers, possibly using the JSX as a base
+    ;; column while switching back to JS indentation.
+    (when (and context (> current-line beg-line))
+      (save-excursion
+        (setq col (js-jsx--contextual-indentation current-line context)))
+      (if expr-p
+          (js-jsx--expr-indentation
+           parse-status (js-jsx--expr-attribute-pos curly-pos (nth 1 context)) col)
+        col))))
 
 (defun js--proper-indentation (parse-status)
   "Return the proper indentation for the current line."
@@ -2089,6 +2883,8 @@ current line is the \"=>\" token."
     (cond ((nth 4 parse-status)    ; inside comment
            (js--get-c-offset 'c (nth 8 parse-status)))
           ((nth 3 parse-status) 0) ; inside string
+          ((when (and js-jsx-syntax (not js-jsx--indent-col))
+             (save-excursion (js-jsx--indentation parse-status))))
           ((eq (char-after) ?#) 0)
           ((save-excursion (js--beginning-of-macro)) 4)
           ;; Indent array comprehension continuation lines specially.
@@ -2113,7 +2909,7 @@ current line is the \"=>\" token."
              (goto-char (nth 1 parse-status)) ; go to the opening char
              (if (or (not js-indent-align-list-continuation)
                      (looking-at "[({[]\\s-*\\(/[/*]\\|$\\)")
-                     (save-excursion (forward-char) (js--looking-at-broken-arrow-function-p)))
+                     (save-excursion (forward-char) (js--broken-arrow-terminates-line-p)))
                  (progn ; nothing following the opening paren/bracket
                    (skip-syntax-backward " ")
                    (when (eq (char-before) ?\)) (backward-list))
@@ -2125,17 +2921,24 @@ current line is the \"=>\" token."
                                              (and switch-keyword-p
                                                   in-switch-p)))
                           (indent
-                           (cond (same-indent-p
-                                  (current-column))
-                                 (continued-expr-p
-                                  (+ (current-column) (* 2 js-indent-level)
-                                     js-expr-indent-offset))
-                                 (t
-                                  (+ (current-column) js-indent-level
-                                     (pcase (char-after (nth 1 parse-status))
-                                       (?\( js-paren-indent-offset)
-                                       (?\[ js-square-indent-offset)
-                                       (?\{ js-curly-indent-offset)))))))
+                           (+
+                            (cond
+                             ((and js-jsx--indent-attribute-line
+                                   (eq js-jsx--indent-attribute-line
+                                       (line-number-at-pos)))
+                              js-jsx--indent-col)
+                             (t
+                              (current-column)))
+                            (cond (same-indent-p 0)
+                                  (continued-expr-p
+                                   (+ (* 2 js-indent-level)
+                                      js-expr-indent-offset))
+                                  (t
+                                   (+ js-indent-level
+                                      (pcase (char-after (nth 1 parse-status))
+                                        (?\( js-paren-indent-offset)
+                                        (?\[ js-square-indent-offset)
+                                        (?\{ js-curly-indent-offset))))))))
                      (if in-switch-p
                          (+ indent js-switch-indent-offset)
                        indent)))
@@ -2151,193 +2954,6 @@ current line is the \"=>\" token."
            (+ js-indent-level js-expr-indent-offset))
           (t (prog-first-column)))))
 
-;;; JSX Indentation
-
-(defsubst js--jsx-find-before-tag ()
-  "Find where JSX starts.
-
-Assume JSX appears in the following instances:
-- Inside parentheses, when returned or as the first argument
-  to a function, and after a newline
-- When assigned to variables or object properties, but only
-  on a single line
-- As the N+1th argument to a function
-
-This is an optimized version of (re-search-backward \"[(,]\n\"
-nil t), except set point to the end of the match.  This logic
-executes up to the number of lines in the file, so it should be
-really fast to reduce that impact."
-  (let (pos)
-    (while (and (> (point) (point-min))
-                (not (progn
-                       (end-of-line 0)
-                       (when (or (eq (char-before) 40)   ; (
-                                 (eq (char-before) 44))  ; ,
-                         (setq pos (1- (point))))))))
-    pos))
-
-(defconst js--jsx-end-tag-re
-  (concat "</" sgml-name-re ">\\|/>")
-  "Find the end of a JSX element.")
-
-(defconst js--jsx-after-tag-re "[),]"
-  "Find where JSX ends.
-This complements the assumption of where JSX appears from
-`js--jsx-before-tag-re', which see.")
-
-(defun js--jsx-indented-element-p ()
-  "Determine if/how the current line should be indented as JSX.
-
-Return `first' for the first JSXElement on its own line.
-Return `nth' for subsequent lines of the first JSXElement.
-Return `expression' for an embedded JS expression.
-Return `after' for anything after the last JSXElement.
-Return nil for non-JSX lines.
-
-Currently, JSX indentation supports the following styles:
-
-- Single-line elements (indented like normal JS):
-
-  var element = <div></div>;
-
-- Multi-line elements (enclosed in parentheses):
-
-  function () {
-    return (
-      <div>
-        <div></div>
-      </div>
-    );
- }
-
-- Function arguments:
-
-  React.render(
-    <div></div>,
-    document.querySelector('.root')
-  );"
-  (let ((current-pos (point))
-        (current-line (line-number-at-pos))
-        last-pos
-        before-tag-pos before-tag-line
-        tag-start-pos tag-start-line
-        tag-end-pos tag-end-line
-        after-tag-line
-        parens paren type)
-    (save-excursion
-      (and
-       ;; Determine if we're inside a jsx element
-       (progn
-         (end-of-line)
-         (while (and (not tag-start-pos)
-                     (setq last-pos (js--jsx-find-before-tag)))
-           (while (forward-comment 1))
-           (when (= (char-after) 60) ; <
-             (setq before-tag-pos last-pos
-                   tag-start-pos (point)))
-           (goto-char last-pos))
-         tag-start-pos)
-       (progn
-         (setq before-tag-line (line-number-at-pos before-tag-pos)
-               tag-start-line (line-number-at-pos tag-start-pos))
-         (and
-          ;; A "before" line which also starts an element begins with js, so
-          ;; indent it like js
-          (> current-line before-tag-line)
-          ;; Only indent the jsx lines like jsx
-          (>= current-line tag-start-line)))
-       (cond
-        ;; Analyze bounds if there are any
-        ((progn
-           (while (and (not tag-end-pos)
-                       (setq last-pos (re-search-forward js--jsx-end-tag-re nil t)))
-             (while (forward-comment 1))
-             (when (looking-at js--jsx-after-tag-re)
-               (setq tag-end-pos last-pos)))
-           tag-end-pos)
-         (setq tag-end-line (line-number-at-pos tag-end-pos)
-               after-tag-line (line-number-at-pos after-tag-line))
-         (or (and
-              ;; Ensure we're actually within the bounds of the jsx
-              (<= current-line tag-end-line)
-              ;; An "after" line which does not end an element begins with
-              ;; js, so indent it like js
-              (<= current-line after-tag-line))
-             (and
-              ;; Handle another case where there could be e.g. comments after
-              ;; the element
-              (> current-line tag-end-line)
-              (< current-line after-tag-line)
-              (setq type 'after))))
-        ;; They may not be any bounds (yet)
-        (t))
-       ;; Check if we're inside an embedded multi-line js expression
-       (cond
-        ((not type)
-         (goto-char current-pos)
-         (end-of-line)
-         (setq parens (nth 9 (syntax-ppss)))
-         (while (and parens (not type))
-           (setq paren (car parens))
-           (cond
-            ((and (>= paren tag-start-pos)
-                  ;; Curly bracket indicates the start of an embedded expression
-                  (= (char-after paren) 123) ; {
-                  ;; The first line of the expression is indented like sgml
-                  (> current-line (line-number-at-pos paren))
-                  ;; Check if within a closing curly bracket (if any)
-                  ;; (exclusive, as the closing bracket is indented like sgml)
-                  (cond
-                   ((progn
-                      (goto-char paren)
-                      (ignore-errors (let (forward-sexp-function)
-                                       (forward-sexp))))
-                    (< current-line (line-number-at-pos)))
-                   (t)))
-             ;; Indicate this guy will be indented specially
-             (setq type 'expression))
-            (t (setq parens (cdr parens)))))
-         t)
-        (t))
-       (cond
-        (type)
-        ;; Indent the first jsx thing like js so we can indent future jsx things
-        ;; like sgml relative to the first thing
-        ((= current-line tag-start-line) 'first)
-        ('nth))))))
-
-(defmacro js--as-sgml (&rest body)
-  "Execute BODY as if in sgml-mode."
-  `(with-syntax-table sgml-mode-syntax-table
-     (let (forward-sexp-function
-           parse-sexp-lookup-properties)
-       ,@body)))
-
-(defun js--expression-in-sgml-indent-line ()
-  "Indent the current line as JavaScript or SGML (whichever is farther)."
-  (let* (indent-col
-         (savep (point))
-         ;; Don't whine about errors/warnings when we're indenting.
-         ;; This has to be set before calling parse-partial-sexp below.
-         (inhibit-point-motion-hooks t)
-         (parse-status (save-excursion
-                         (syntax-ppss (point-at-bol)))))
-    ;; Don't touch multiline strings.
-    (unless (nth 3 parse-status)
-      (setq indent-col (save-excursion
-                         (back-to-indentation)
-                         (if (>= (point) savep) (setq savep nil))
-                         (js--as-sgml (sgml-calculate-indent))))
-      (if (null indent-col)
-          'noindent
-        ;; Use whichever indentation column is greater, such that the sgml
-        ;; column is effectively a minimum
-        (setq indent-col (max (js--proper-indentation parse-status)
-                              (+ indent-col js-indent-level)))
-        (if savep
-            (save-excursion (indent-line-to indent-col))
-          (indent-line-to indent-col))))))
-
 (defun js-indent-line ()
   "Indent the current line as JavaScript."
   (interactive)
@@ -2349,23 +2965,9 @@ Currently, JSX indentation supports the following styles:
       (when (> offset 0) (forward-char offset)))))
 
 (defun js-jsx-indent-line ()
-  "Indent the current line as JSX (with SGML offsets).
-i.e., customize JSX element indentation with `sgml-basic-offset',
-`sgml-attribute-offset' et al."
+  "Indent the current line as JavaScript+JSX."
   (interactive)
-  (let ((indentation-type (js--jsx-indented-element-p)))
-    (cond
-     ((eq indentation-type 'expression)
-      (js--expression-in-sgml-indent-line))
-     ((or (eq indentation-type 'first)
-          (eq indentation-type 'after))
-      ;; Don't treat this first thing as a continued expression (often a "<" or
-      ;; ">" causes this misinterpretation)
-      (cl-letf (((symbol-function #'js--continued-expression-p) 'ignore))
-        (js-indent-line)))
-     ((eq indentation-type 'nth)
-      (js--as-sgml (sgml-indent-line)))
-     (t (js-indent-line)))))
+  (let ((js-jsx-syntax t)) (js-indent-line)))
 
 ;;; Filling
 
@@ -3856,6 +4458,77 @@ If one hasn't been set, or if it's stale, prompt for a new one."
         (when temp-name
           (delete-file temp-name))))))
 
+;;; Syntax extensions
+
+(defvar js-syntactic-mode-name t
+  "If non-nil, print enabled syntaxes in the mode name.")
+
+(defun js--syntactic-mode-name-part ()
+  "Return a string like “[JSX]” when `js-jsx-syntax' is enabled."
+  (if js-syntactic-mode-name
+      (let (syntaxes)
+        (if js-jsx-syntax (push "JSX" syntaxes))
+        (if syntaxes
+            (concat "[" (mapconcat #'identity syntaxes ",") "]")
+          ""))
+    ""))
+
+(defun js-use-syntactic-mode-name ()
+  "Print enabled syntaxes if `js-syntactic-mode-name' is t.
+Modes deriving from `js-mode' should call this to ensure that
+their `mode-name' updates to show enabled syntax extensions."
+  (when (stringp mode-name)
+    (setq mode-name `(,mode-name (:eval (js--syntactic-mode-name-part))))))
+
+(defun js-jsx-enable ()
+  "Enable JSX in the current buffer."
+  (interactive)
+  (setq-local js-jsx-syntax t))
+
+;; To make discovering and using syntax extensions features easier for
+;; users (who might not read the docs), try to safely and
+;; automatically enable syntax extensions based on heuristics.
+
+(defvar js-jsx-regexps
+  (list "\\_<\\(?:var\\|let\\|const\\|import\\)\\_>.*?React")
+  "Regexps for detecting JSX in JavaScript buffers.
+When `js-jsx-detect-syntax' is non-nil and any of these regexps
+match text near the beginning of a JavaScript buffer,
+`js-jsx-syntax' (which see) will be made buffer-local and set to
+t.")
+
+(defun js-jsx--detect-and-enable (&optional arbitrarily)
+  "Detect if JSX is likely to be used, and enable it if so.
+Might make `js-jsx-syntax' buffer-local and set it to t.  Matches
+from the beginning of the buffer, unless optional arg ARBITRARILY
+is non-nil.  Return t after enabling, nil otherwise."
+  (when (or (and (buffer-file-name)
+                 (string-match-p "\\.jsx\\'" (buffer-file-name)))
+            (and js-jsx-detect-syntax
+                 (save-excursion
+                   (unless arbitrarily
+                     (goto-char (point-min)))
+                   (catch 'match
+                     (mapc
+                      (lambda (regexp)
+                        (if (re-search-forward regexp 4000 t) (throw 'match t)))
+                      js-jsx-regexps)
+                     nil))))
+    (js-jsx-enable)
+    t))
+
+(defun js-jsx--detect-after-change (beg end _len)
+  "Detect if JSX is likely to be used after a change.
+This function is intended for use in `after-change-functions'."
+  (when (<= end 4000)
+    (save-excursion
+      (goto-char beg)
+      (beginning-of-line)
+      (save-restriction
+        (narrow-to-region (point) end)
+        (when (js-jsx--detect-and-enable 'arbitrarily)
+          (remove-hook 'after-change-functions #'js-jsx--detect-after-change t))))))
+
 ;;; Main Function
 
 ;;;###autoload
@@ -3871,6 +4544,10 @@ If one hasn't been set, or if it's stale, prompt for a new one."
                     '(font-lock-syntactic-face-function
                       . js-font-lock-syntactic-face-function)))
   (setq-local syntax-propertize-function #'js-syntax-propertize)
+  (add-hook 'syntax-propertize-extend-region-functions
+            #'syntax-propertize-multiline 'append 'local)
+  (add-hook 'syntax-propertize-extend-region-functions
+            #'js--syntax-propertize-extend-region 'append 'local)
   (setq-local prettify-symbols-alist js--prettify-symbols-alist)
 
   (setq-local parse-sexp-ignore-comments t)
@@ -3878,6 +4555,7 @@ If one hasn't been set, or if it's stale, prompt for a new one."
 
   ;; Comments
   (setq-local comment-start "// ")
+  (setq-local comment-start-skip "\\(//+\\|/\\*+\\)\\s *")
   (setq-local comment-end "")
   (setq-local fill-paragraph-function #'js-fill-paragraph)
   (setq-local normal-auto-fill-function #'js-do-auto-fill)
@@ -3888,6 +4566,11 @@ If one hasn't been set, or if it's stale, prompt for a new one."
   ;; Frameworks
   (js--update-quick-match-re)
 
+  ;; Syntax extensions
+  (unless (js-jsx--detect-and-enable)
+    (add-hook 'after-change-functions #'js-jsx--detect-after-change nil t))
+  (js-use-syntactic-mode-name)
+
   ;; Imenu
   (setq imenu-case-fold-search nil)
   (setq imenu-create-index-function #'js--imenu-create-index)
@@ -3898,8 +4581,7 @@ If one hasn't been set, or if it's stale, prompt for a new one."
         c-paragraph-separate "$"
         c-block-comment-prefix "* "
         c-line-comment-starter "//"
-        c-comment-start-regexp "/[*/]\\|\\s!"
-        comment-start-skip "\\(//+\\|/\\*+\\)\\s *")
+        c-comment-start-regexp "/[*/]\\|\\s!")
   (setq-local comment-line-break-function #'c-indent-new-comment-line)
   (setq-local c-block-comment-start-regexp "/\\*")
   (setq-local comment-multi-line t)
@@ -3932,19 +4614,33 @@ If one hasn't been set, or if it's stale, prompt for a new one."
   ;;(syntax-propertize (point-max))
   )
 
-;;;###autoload
-(define-derived-mode js-jsx-mode js-mode "JSX"
-  "Major mode for editing JSX.
-
-To customize the indentation for this mode, set the SGML offset
-variables (`sgml-basic-offset', `sgml-attribute-offset' et al.)
-locally, like so:
+;; Since we made JSX support available and automatically-enabled in
+;; the base `js-mode' (for ease of use), now `js-jsx-mode' simply
+;; serves as one other interface to unconditionally enable JSX in
+;; buffers, mostly for backwards-compatibility.
+;;
+;; Since it is probably more common for packages to integrate with
+;; `js-mode' than with `js-jsx-mode', it is therefore probably
+;; slightly better for users to use one of the many other methods for
+;; enabling JSX syntax.  But using `js-jsx-mode' can’t be that bad
+;; either, so we won’t bother users with an obsoletion warning.
 
-  (defun set-jsx-indentation ()
-    (setq-local sgml-basic-offset js-indent-level))
-  (add-hook \\='js-jsx-mode-hook #\\='set-jsx-indentation)"
+;;;###autoload
+(define-derived-mode js-jsx-mode js-mode "JavaScript"
+  "Major mode for editing JavaScript+JSX.
+
+Simply makes `js-jsx-syntax' buffer-local and sets it to t.
+
+`js-mode' may detect and enable support for JSX automatically if
+it appears to be used in a JavaScript file.  You could also
+customize `js-jsx-regexps' to improve that detection; or, you
+could set `js-jsx-syntax' to t in your init file, or in a
+.dir-locals.el file, or using file variables; or, you could call
+`js-jsx-enable' in `js-mode-hook'.  You may be better served by
+one of the aforementioned options instead of using this mode."
   :group 'js
-  (setq-local indent-line-function #'js-jsx-indent-line))
+  (js-jsx-enable)
+  (js-use-syntactic-mode-name))
 
 ;;;###autoload (defalias 'javascript-mode 'js-mode)
 
diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 5d0d03d5029..b05f9a33e90 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -675,7 +675,7 @@ Which one will be chosen depends on the value of
 
 (defconst python-syntax-propertize-function
   (syntax-propertize-rules
-   ((python-rx string-delimiter)
+   ((rx (or "\"\"\"" "'''"))
     (0 (ignore (python-syntax-stringify))))))
 
 (define-obsolete-variable-alias 'python--prettify-symbols-alist
@@ -701,35 +701,27 @@ is used to limit the scan."
 
 (defun python-syntax-stringify ()
   "Put `syntax-table' property correctly on single/triple quotes."
-  (let* ((num-quotes (length (match-string-no-properties 1)))
-         (ppss (prog2
-                   (backward-char num-quotes)
-                   (syntax-ppss)
-                 (forward-char num-quotes)))
-         (string-start (and (not (nth 4 ppss)) (nth 8 ppss)))
-         (quote-starting-pos (- (point) num-quotes))
-         (quote-ending-pos (point))
-         (num-closing-quotes
-          (and string-start
-               (python-syntax-count-quotes
-                (char-before) string-start quote-starting-pos))))
-    (cond ((and string-start (= num-closing-quotes 0))
-           ;; This set of quotes doesn't match the string starting
-           ;; kind. Do nothing.
+  (let* ((ppss (save-excursion (backward-char 3) (syntax-ppss)))
+         (string-start (and (eq t (nth 3 ppss)) (nth 8 ppss)))
+         (quote-starting-pos (- (point) 3))
+         (quote-ending-pos (point)))
+    (cond ((or (nth 4 ppss)             ;Inside a comment
+               (and string-start
+                    ;; Inside of a string quoted with different triple quotes.
+                    (not (eql (char-after string-start)
+                              (char-after quote-starting-pos)))))
+           ;; Do nothing.
            nil)
-          ((not string-start)
+          ((nth 5 ppss)
+           ;; The first quote is escaped, so it's not part of a triple quote!
+           (goto-char (1+ quote-starting-pos)))
+          ((null string-start)
            ;; This set of quotes delimit the start of a string.
            (put-text-property quote-starting-pos (1+ quote-starting-pos)
                               'syntax-table (string-to-syntax "|")))
-          ((= num-quotes num-closing-quotes)
+          (t
            ;; This set of quotes delimit the end of a string.
            (put-text-property (1- quote-ending-pos) quote-ending-pos
-                              'syntax-table (string-to-syntax "|")))
-          ((> num-quotes num-closing-quotes)
-           ;; This may only happen whenever a triple quote is closing
-           ;; a single quoted string. Add string delimiter syntax to
-           ;; all three quotes.
-           (put-text-property quote-starting-pos quote-ending-pos
                               'syntax-table (string-to-syntax "|"))))))
 
 (defvar python-mode-syntax-table
diff --git a/lisp/progmodes/sh-script.el b/lisp/progmodes/sh-script.el
index dd3a6fa411e..853a3500ee1 100644
--- a/lisp/progmodes/sh-script.el
+++ b/lisp/progmodes/sh-script.el
@@ -2905,8 +2905,7 @@ STRING	     This is ignored for the purposes of calculating
                       (setq align-point (point))))
                 (or (bobp)
                     (forward-char -1))
-                ;; FIXME: This charset looks too much like a regexp.  --Stef
-                (skip-chars-forward "[a-z0-9]*?")
+                (skip-chars-forward "*0-9?[]a-z")
                 )
                ((string-match "[])}]" x)
                 (setq x (sh-safe-forward-sexp -1))
diff --git a/lisp/progmodes/verilog-mode.el b/lisp/progmodes/verilog-mode.el
index 7b9c3921fba..9226291ffbb 100644
--- a/lisp/progmodes/verilog-mode.el
+++ b/lisp/progmodes/verilog-mode.el
@@ -14263,13 +14263,13 @@ and the case items."
 (defun verilog-sk-define-signal ()
   "Insert a definition of signal under point at top of module."
   (interactive "*")
-  (let* ((sig-re "[a-zA-Z0-9_]*")
+  (let* ((sig-chars "a-zA-Z0-9_")
          (v1 (buffer-substring
               (save-excursion
-                (skip-chars-backward sig-re)
+                (skip-chars-backward sig-chars)
                 (point))
               (save-excursion
-                (skip-chars-forward sig-re)
+                (skip-chars-forward sig-chars)
                 (point)))))
     (if (not (member v1 verilog-keywords))
         (save-excursion