Improve C++ concept indentation.

This fixes bug #62386.

* lisp/progmodes/cc-engine.el (c-forward-over-compound-identifier): Don't
move forward over whitespace following the identifier.
(c-forward-primary-expression): Add parameter stop-at-end meaning don't move
forward over whitespace after the construct when non-nil.  Don't recognise a
primary expression when an open brace follows a parenthesized expression.
(c-forward-constraint-clause): Extracted from c-forward-c++-requires-clause.
Add parameter stop-at-end as above.
(c-forward-c++-requires-clause): New stop-at-end parameter as above.  Call the
new function c-forward-constraint-clause.
(c-forward-concept-fragment, c-looking-at-concept)
(c-in-requires-or-at-end-of-clause, c-c++-vsemi-p): New functions.
(c-guess-basic-syntax): New CASE 5A.7: "defun" open in a requires expression.
CASE 5F: Close of a non-class declaration level block: Move to earlier in the
function.
CASE 5D: Also check for being in or at end of a constraint.
New CASE 20: A C++ requires sub-clause.
New CASE 16G: The closing brace of a requires clause.
New CASE 17J: First "statement" inside a requires "function".
(c-forward-primary-expression, c-forward-declarator, c-forward-decl-or-cast-1)
(c-looking-at-or-maybe-in-bracelist): Amend the method of detecting end of
symbol "requires" (aka c-fun-name-substitute-key).

* lisp/progmodes/cc-fonts.el (c-get-fontification-context): Amend the method
of detecting end of symbol "requires".

* lisp/progmodes/cc-langs.el (c-at-vsemi-p-fn): Change the C++ entry to
c-c++-vsemi-p.
(c-fun-name-substitute-key): Change to an unadorned regexp.

* lisp/progmodes/cc-vars.el (c-offsets-alist): Add new syntactic symbol
constraint-cont.

* doc/misc/cc-mode.texi (Syntactic Symbols): Add an entry for contraint-cont.
(Constraint Symbols): New node under Syntactic Symbols.

1 files changed, 38 insertions, 0 deletions

diff --git a/doc/misc/cc-mode.texi b/doc/misc/cc-mode.texi
index 3a808619868..71bf3fcee4a 100644
--- a/doc/misc/cc-mode.texi
+++ b/doc/misc/cc-mode.texi
@@ -330,6 +330,7 @@ Syntactic Symbols
 * Multiline Macro Symbols::
 * Objective-C Method Symbols::
 * Java Symbols::
+* Constraint Symbols::
 * Statement Block Symbols::
 * K&R Symbols::
 
@@ -4234,6 +4235,9 @@ The first line in a ``topmost'' definition.  @ref{Function Symbols}.
 Topmost definition continuation lines.  This is only used in the parts
 that aren't covered by other symbols such as @code{func-decl-cont} and
 @code{knr-argdecl}.  @ref{Function Symbols}.
+@item constraint-cont
+Continuation line of a topmost C++20 concept or requires clause.
+@ref{Constraint Symbols}.
 @item annotation-top-cont
 Topmost definition continuation lines where all previous items are
 annotations.  @ref{Java Symbols}.
@@ -4397,6 +4401,7 @@ Java.  @ref{Java Symbols}.
 * Multiline Macro Symbols::
 * Objective-C Method Symbols::
 * Java Symbols::
+* Constraint Symbols::
 * Statement Block Symbols::
 * K&R Symbols::
 @end menu
@@ -5071,6 +5076,39 @@ syntax due to it being a continuation of a variable declaration where preceding
 the declaration is an annotation.
 
 @comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+@node    Constraint Symbols
+@subsection C++ Constraint Symbols
+@comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+The C++20 standard introduced the notion of @dfn{concepts} and
+@dfn{requirements}, a typical instance of which looks something like
+this:
+
+@example
+ 1: template <typename T>
+ 2: requires
+ 3:   requires (T t) @{
+ 4:     @{ ++t; @}
+ 5:   @}
+ 6:   && std::is_integral<T>
+ 7:   int foo();
+@end example
+
+@ssindex constraint-cont
+Line 1 is assigned the familiar @code{topmost-intro}.  Line 2 gets
+@code{topmost-intro-cont}, being the keyword which introduces a
+@dfn{requires clause}.  Lines 3, 6, and 7 are assigned the syntax
+@code{constraint-cont}, being continuations of the requires clause
+started on line 2.  Lines 4 and 5 get the syntaxes
+@code{defun-block-intro} and @code{defun-close}, being analyzed as
+though part of a function.
+
+Note that the @code{requires} on Line 3 begins a @dfn{requires
+expression}, not a a requires clause, hence its components are not
+assigned @code{constraint-cont}.  See
+@url{https://en.cppreference.com/w/cpp/language/requires}.
+
+@comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 @node    Statement Block Symbols
 @subsection Statement Block Symbols
 @comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!