* lisp/erc/erc.el (erc--initialize-markers): Commit 0d6c8d41ab7 "Use
overlay instead of text prop to hide ERC's prompt" introduced a bug
that caused the prompt to remain hidden upon reconnecting because the
stashed overlay would get clobbered by ERC's major-mode setup.
Binding its old value while unhiding fixes the issue.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Don't permanently set
the default value of `erc-hide-prompt'.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--join-network-id): Add assertions for prompt
hiding and unhiding on reconnect.  (Bug#51082)

* lisp/erc/erc-fill.el (erc-fill--wrap-insert-merged-post): Use
correct variable.
* test/lisp/erc/erc-scenarios-display-message.el: Mute unrelated noisy
test.
* test/lisp/erc/erc-tests.el (erc--channel-modes): Bail out following
first group of assertions when running on a graphical display.
(erc--channel-modes/graphic-p): New test asserting different values
for graphical terminals.  Note that this too is tagged as :unstable.
See also: d2ce30afdd2 "; Mark test for erc--update-channel-modes as
:unstable".

* test/lisp/erc/erc-scenarios-auth-source.el
(erc-scenarios-common--auth-source): Extend timeout.
* test/lisp/erc/erc-scenarios-base-buffer-display.el
(erc-scenarios-base-buffer-display--count-reset-timeout): Await
initial condition.
* test/lisp/erc/erc-scenarios-base-renick.el
(erc-scenarios-base-renick-queries-solo): Extend timeout.
* test/lisp/erc/erc-scenarios-misc.el (erc-scenarios-base-flood):
Extend timeout.
* test/lisp/erc/erc-tests.el (erc--channel-modes): Mark test as
:unstable pending further investigation.  This test has been shown to
be unreliable, possibly because it expects Emacs to report characters
being a certain width in all environments.  Thanks to Mattias
Engdegård for reporting this failure.
* test/lisp/erc/resources/base/reuse-buffers/server/barnet.eld:
Extend timeouts.
* test/lisp/erc/resources/base/reuse-buffers/server/foonet.eld:
Extend timeouts.

* etc/ERC-NEWS: Add entry for `erc-prompt-format'.
* lisp/erc/erc-compat.el (erc-compat--defer-format-spec-in-buffer):
New macro to wrap `format-spec' specification values in functions that
run in the current buffer and fall back to the empty string.
* lisp/erc/erc.el (erc-prompt): Add predefined Custom choice for
function type in `erc-prompt-format'.
(erc--prompt-format-face-example): New "pre-propertized" value for
option `erc-prompt-format'.
(erc-prompt-format): New companion option for `erc-prompt' choice
`erc-prompt-format'.  New function of the same name to perform format
substitutions and serve as a Custom choice value for `erc-prompt'.
Based on work and ideas originally proposed by Stefan Kangas.
(erc--away-indicator, erc-away-status-indicator,
erc--format-away-indicator): New formatting function and helper
variables for displaying short away status.
(erc--user-modes-indicator): New variable.
(erc--format-user-modes): New function.
(erc--format-channel-status-prefix): New function.
(erc--format-modes): New function.
* test/lisp/erc/erc-scenarios-prompt-format.el: New file.  (Bug#51082)

Co-authored-by: Stefan Kangas <stefankangas@gmail.com>

* lisp/erc/erc-fill.el (erc-fill): Use `when-let' instead of
`when-let*'.
(erc-fill-wrap-merge): Mention companion options in doc string.
(erc-fill-wrap-merge-indicator): New option to display a
distinguishing "indicator" in the form of a one-character string
between messages from the same speaker.
(erc-fill-wrap-mode, erc-fill-wrap-disable): Mention
`erc-fill-wrap-merge-indicator' in doc string and kill related local
variables.
(erc-fill--wrap-merge-indicator-pre,
erc-fill--wrap-merge-indicator-post): New internal variables for
caching merge indicator.
(erc-fill--wrap-insert-merged-post, erc-fill--wrap-insert-merged-pre):
New functions for adding merge indicators either before or after a
message.
(erc-fill-wrap): Add logic for deferring to merge-indicator helpers
when needed.
* test/lisp/erc/erc-fill-tests.el (erc-fill-wrap-tests--merge-action,
erc-fill-wrap--merge-action): Move body of latter test into former, a
new fixture function.
(erc-fill-wrap--merge-action/indicator-pre,
erc-fill-wrap--merge-action/indicator-post): New tests.
* test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-post-01.eld:
New test data file.
* test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-pre-01.eld:
New test data file.  (Bug#60936)

* lisp/erc/erc-backend.el (erc--hidden-prompt-overlay):
New variable, a buffer-local handle for the prompt overlay.
(erc--reveal-prompt): Delete overlay instead of text prop.
(erc--conceal-prompt): Add overlay instead of text prop.
(erc--unhide-prompt): Run `erc--refresh-prompt-hook' after revealing.
(erc--hide-prompt): Run `erc--refresh-prompt-hook' after hiding.
* lisp/erc/erc-stamp.el (erc-stamp--adjust-margin): Attempt a more
accurate estimate of the prompt's width in columns when initially
setting left-margin.
(erc-stamp--skip-left-margin-prompt-p): New variable to inhibit normal
behavior of displaying prompt in left margin.
(erc-stamp--display-margin-mode): Allow opting out of
prompt-in-left-margin behavior.
(erc--reveal-prompt): Delete unneeded method implementation.
(erc--conceal-prompt): Put overlay in margin.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Use
`get-char-property' instead of `get-text-property' in order to
accommodate overlay-based prompt hiding.  (Bug#51082)

* lisp/erc/erc.el (erc--merge-prop-behind-p): New variable to be
dynamically bound around rare calls to `erc--merge-props' when the
latter should append to the end of existing list-valued text
properties.
(erc--inhibit-prompt-display-property-p): New variable to be non-nil
in buffers where an active module needs to reserve all uses of the
`display' text property in the prompt region for itself.
(erc--prompt-properties): Collect all common prompt properties in one
place for code reuse and maintenance purposes.
(erc--refresh-prompt-continue, erc--refresh-prompt-continue-request):
New function and state variable for custom `erc-prompt' functions to
indicate to ERC that they need the prompt to be refreshed in all
buffers and not just the current one.
(erc--refresh-prompt): Merge `erc-prompt-face' behind any applied by a
customized `erc-prompt' function value.  Crucially, don't inherit
properties at the beginning of the prompt because doing so may clobber
any added by a custom `erc-prompt' function.  Instead, apply known
properties from `erc-display-prompt' manually.  Integrate
`erc--refresh-prompt-continue' logic.
(erc--merge-prop): Recognize flag to activate `append' behavior in
which new prop values are appended to the tail of existing ones rather
than consed in front.  This functionality could be extended to
arbitrary splices as well.
(erc-display-prompt): Use common text properties defined elsewhere.
* test/lisp/erc/erc-tests.el (erc--merge-prop): Add assertion for
`erc--merge-prop-behind-p' non-nil behavior.  (Bug#51082)

* etc/ERC-NEWS: Add entry for more expansive "%m" in header line.
* lisp/erc/erc-common.el (erc--channel-mode-types): New slot
`shortargs' for caching truncated mode args.
* lisp/erc/erc.el (erc--mode-line-chanmodes-arg-len): New internal
variable for adjusting the truncation length of channel-mode arguments
as they appear in the header line.
(erc--mode-line-mode-string): New variable for caching the relevant
"modestring", if any, in ERC buffers.
(erc--process-channel-modes): Don't associate args with group 4/D,
which are all nullary modes.  This fixes a bug in which arguments were
associated with the wrong letters.  Also, set cached mode string for
channel.
(erc--user-modes): Simplify slightly by removing likely useless
variant for overloaded arg AS-TYPE.  This function is new in ERC 5.6.
(erc--channel-modes):  New function.  A higher-level getter for
current channel mode representation to complement `erc--user-modes'.
(erc--parse-user-modes): Set `erc--mode-line-mode-string in server
buffers.
(erc--handle-channel-mode): Change model to associate modes of type A
with a running plus/minus tally of state changes since joining the
channel.
(erc-update-mode-line-buffer): Use cached verbose representation of
channel or user modes instead of calling `erc-format-channel-modes'.
* test/lisp/erc/erc-tests.el (erc--update-channel-modes): Update to
reflect new running tally associations for type A modes.
(erc--channel-modes): New test.
(erc--user-modes): Update to reflect parameter simplification.
(Bug#67220)

* etc/ERC-NEWS: Mention shift toward CHANMODES ISUPPORT parameter for
dictating MODE parsing behavior.
* lisp/erc/erc-backend.el (erc--init-channel-modes, erc--update-modes,
erc-set-modes, erc-update-modes): Forward declarations, the last two
being removals.
(erc-server-MODE, erc-server-221): Use `erc--update-modes' instead of
`erc-update-modes'.
(erc-server-324): Use `erc--init-channel-modes' instead of
`erc-set-modes'.
* lisp/erc/erc-common.el (erc--channel-mode-types): New struct for
stashing processed \"CHANMODES\" data for the current server.
* lisp/erc/erc.el (erc-channel-modes): Fix doc string.
(erc-set-initial-user-mode): Display a local notice when requesting
redundant user MODE operations.
(erc-set-modes, erc-parse-modes, erc-update-modes): Deprecate for
reasons explained in associated ERC-NEWS entry.
(erc--update-membership-prefix): New function, a helper for specifying
arguments to the rather unruly `erc-update-current-channel-member'.
(erc--channel-modes): New variable to record channel-mode state in a
hash table.
(erc--channel-mode-types): New variable and getter to stash
and retrieve server-local instance of the struct of the same name.
(erc--process-channel-modes): New function to parse channel-mode
changes, dispatch handlers for unary modes, and update the local
variables `erc-channel-modes' and `erc--channel-modes'.
(erc--user-modes): New local variable for remembering user modes per
server.  New function of the same name, a "getter" for the variable.
(erc--parse-user-modes): New function to parse user modes only.
(erc--update-user-modes): New function to update and sort
`erc--user-modes'.
(erc--update-channel-modes): New function to replace much of
`erc-update-modes', currently a thin wrapper around
`erc--process-channel-modes' to ensure it updates status prefixes.
(erc--update-modes): New function to call appropriate mode-updating
function for the current buffer.
(erc--init-channel-modes): New function to update channel mode letters
without status prefixes.
(erc--handle-channel-mode): New generic function, a placeholder for an
eventual API to handle specific "unary" mode letters, meaning those
that specify a single parameter for setting or unsetting.
(erc-update-channel-limit): Update doc string and answer question
posed by ancient comment.
(erc-message-english-user-mode-redundant-add,
erc-message-english-user-mode-redundant-drop): New English catalog
messages.
* test/lisp/erc/erc-scenarios-base-chan-modes.el: New file.
* test/lisp/erc/erc-tests.el (erc-parse-modes,
erc--update-channel-modes, erc--update-user-modes, erc--user-modes,
erc--parse-user-modes): New tests.
* test/lisp/erc/resources/base/modes/chan-changed.eld: New test data
file.  (Bug#67220)

* lisp/erc/erc-common.el (erc--parsed-prefix): New struct to help with
tasks that depends on the advertised "PREFIX" parameter.
* lisp/erc/erc.el (erc-parse-prefix): Rework slightly for readability.
(erc--parsed-prefix): New variable and function of the same name for
caching the reversed result of `erc-parse-prefix' locally per server.
(erc-channel-receive-names): Use value stored in `erc--parsed-prefix'.
* test/lisp/erc/erc-tests.el (erc-with-server-buffer): Only activate
spy around actual test case forms.
(erc--parse-prefix): New test.  (Bug#67220)