* lisp/erc/erc-common.el (erc--features-to-modules,
erc--modules-to-features, erc--module-name-migrations): Remove unused
internal functions.
(erc--normalize-module-symbol): Make aware of new migration scheme
based on symbol properties.
* lisp/erc/erc-page.el: Add autoload cookie for module migration.
* lisp/erc/erc-pcomplete.el: Add autoload cookies for module
migration.
* lisp/erc/erc-services.el: Add autoload cookie for module migration.
* lisp/erc/erc-sound.el: Add autoload cookie for module migration.
* lisp/erc/erc-stamp.el: Add autoload cookie for module migration.
* lisp/erc/erc.el (erc-modules): Reorder default value, sorted by
`string<' so that Customize does not consider the value to have been
edited.  Remove non-existent module `hecomplete' from lineup and swap
a couple more to maintain sorted order.  Change `:initialize' function
to tag all symbols for built-in modules with an `erc--module'
property.  In the `:set' function, ensure third-party modules appear
after the sorted and normalized built-ins, but in user-defined order.
Do this to prevent all modules, built-ins included, from ending up as
populated form fields for the "other" checkbox in the Customize
interface.
(erc--find-mode): Add helper function for `erc--update-modules'.
(erc--update-modules): Always resolve module names and only
conditionally attempt to require corresponding features.
* test/lisp/erc/erc-tests.el (erc-tests--modules): Add manifest for
asserting built-in modules and features.  This is easier to verify
visually than looking at the custom-type set for `erc-modules'.
(erc-modules--initialize): New test.
(erc-modules--internal-property): Add test.
(erc--normalize-module-symbol): New test.
(erc--find-mode): New test.
(erc--update-modules) Adapt to new paradigm and make more
comprehensive.  (Bug#60954.)

* doc/misc/erc.texi: Mention in various places that ERC is also
available from GNU ELPA.
* etc/ERC-NEWS: Mention Compat dependency and shorten title for
auth-source section.
* lisp/erc/erc-backend.el: (erc-server-reconnect-function,
erc-tags-format): Update package version to 5.5.
(erc--parse-message-tags): Downcase warning "type" to remain
consistent with all other ERC warnings.
* lisp/erc/erc-button.el: (erc-button-alist): Change package-version
to 5.5.
* lisp/erc/erc-match.el (erc-match-quote-when-adding): Update package
version to 5.5.
* lisp/erc/erc-sasl.el: Mention actual info node in Commentary.
(erc-sasl): Update package version to 5.5.
(erc-sasl-password): Reword doc string.
(erc-sasl-auth-source-function): Capitalize "info" in doc string.
* lisp/erc/erc-services.el (erc-auth-source-services-function): Update
package version to 5.5.  Capitalize "info" in doc string.  Change
choice type from const to function-item.
* lisp/erc/erc.el (erc-password): Capitalize "info" in doc string.
(erc-inhibit-multiline-input, erc-ask-about-multiline-input,
erc-prompt-hidden, erc-hide-prompt, erc-unhide-query-prompt,
erc-join-buffer, erc-reconnect-display, erc-kill-server-hook,
erc-kill-channel-hook, erc-kill-buffer-hook,
erc-url-connect-function): Update package version to 5.5.
(erc-auth-source-server-function, erc-auth-source-join-function):
Update package version to 5.5.  Change choice type from const to
function-item.  Capitalize "info" in doc string.
(erc-tls): Capitalize "info" in doc string.

* doc/misc/erc.texi: Move auth-source description from the Password
subheading of the Advanced chapter's Connecting section to the new
Integrations section as a new node, Auth-Source, and give it a bit
more structure.  Fix various misuses of xref vs. pxref.  Convert URL
subheading to subsection and add anchor.  Prefer "backend" as a single
word, based on usage in other manuals.  Also replace loud "warning" in
SASL troubleshooting section.
* etc/ERC-NEWS: Re-link auth-source mention.
* lisp/erc/erc-sasl.el (erc-sasl-auth-source-function): Update info
node in doc string.
* lisp/erc/erc-services.el (erc-auth-source-services-function):
Re-link auth-source info node in doc string.
* lisp/erc/erc.el (erc-password, erc-auth-source-server-function,
erc-auth-source-join-function): Re-link auth-source info node in doc
strings.

* lisp/erc/erc-backend.el (erc-session-password): Add comment
explaining type is now string, nil, or function.
* lisp/erc/erc-compat.el (erc-compat--29-auth-source-pass-search):
Use obfuscation from auth-source function when available.
* lisp/erc/erc-sasl.el (erc-sasl--read-password,
erc-server-AUTHENTICATE): Use `erc--unfun'.
* lisp/erc/erc-services.el (erc-nickserv-get-password,
erc-nickserv-send-identify): Use `erc--unfun'.
* lisp/erc/erc.el (erc--unfun): New function for unwrapping a
password couched in a getter.
(erc--debug-irc-protocol-mask-secrets): Add variable to indicate
whether to mask passwords in debug logs.
(erc--mask-secrets): New function to swap masked secret with question
marks in debug logs.
(erc-log-irc-protocol): Conditionally mask secrets when
`erc--debug-irc-protocol-mask-secrets' is non-nil.
(erc--auth-source-search): Don't unwrap secret from function before
returning.
(erc-server-join-channel, erc-login): Use `erc--unfun'.

* test/lisp/erc/erc-services-tests.el
(erc-services-tests--wrap-search): Add helper for `erc--unfun'.
(erc-services-tests--auth-source-standard,
erc-services-tests--auth-source-announced,
erc-services-tests--auth-source-overrides, erc-nickserv-get-password):
Use `erc--unfun'.
* test/lisp/erc/erc-tests.el (erc--debug-irc-protocol-mask-secrets):
Add test for masking secrets with `erc--unfun' and friends.

* lisp/erc/erc.el (erc-password): Deprecate variable only used by
`erc-select-read-args'.  Server passwords are primarily used as
surrogates for other forms of authentication.  Such use is common but
nonstandard and often discouraged in favor of the de facto standard,
SASL.  Folks in the habit of invoking `erc(-tls)' interactively should
be encouraged to use auth-source instead.
(erc-select-read-args): Before this change, `erc-select-read-args'
offered to use the value of a non-nil `erc-password' as the :password
argument for `erc' and `erc-tls', referring to it as the "default"
password.  And when `erc-prompt-for-password' was nil and
`erc-password' wasn't, the latter was passed along unconditionally.
This only further complicated an already confusing situation for new
users, who in most cases shouldn't be worried about sending a PASS
command at all.  Until SASL arrives, they should provide server
passwords manually or learn to use auth-source.
(erc-auth-source-server-function, erc-auth-source-join-function): New
user options for retrieving a password externally, ostensibly by
calling `auth-source-search'.
(erc--auth-source-determine-params-defaults): New helper for
`erc--auth-source-search' with potential for exporting publicly in the
future.  Favors :host and :port fields above others.  Prioritizes
network IDs over announced servers and dialed endpoints.
(erc--auth-source-determine-params-merge): Add new function for
merging contextual and default parameters.  This is another contender
for possible exporting.
(erc--auth-source-search): New function for consulting auth-source and
sorting the result as filtered and prioritized by the previously
mentioned helpers.
(erc-auth-source-search): New function to serve as default
value for auth-source query-function options.
(erc-server-join-channel): Use user option for consulting auth-source
facility.  Also accept nil for first argument (instead of server).
(erc-cmd-JOIN): Use above-mentioned facilities when joining new
channel.  Omit server when calling `erc-server-join-channel'.  Don't
filter target buffers twice.  Don't call `switch-to-buffer', which
would create phantom buffers with names like target/server that were
never used.  IOW, only switch to existing target buffers.
(erc--compute-server-password): Add new helper function for
determining password.
(erc-open, erc-determine-parameters): Move password figuring from the
first to the latter.

* lisp/erc/erc-services.el
(erc-auth-source-services-function): Add new option for consulting
auth-source in a NickServ context.
(erc-nickserv-get-password): Pass network-context ID, when looking up
password in `erc-nickserv-passwords' and when formatting prompt for
user input.
(erc-nickserv-passwords): Add comment to custom option definition type
tag.

* test/lisp/erc/erc-services-tests.el: Add new test file for above
changes.  For now, stash auth-source-related tests here until a
suitable home can be found.

* lisp/erc/erc-join.el (erc-autojoin--join): Don't pass session-like
entity from `erc-autojoin-channels-alist' match to
`erc-server-join-channel'.  Allow that function to decide for itself
which host to look up if necessary.

* test/lisp/erc/resources/base/auth-source/foonet.eld: New file.
* test/lisp/erc/resources/base/auth-source/nopass.eld: New file.
* test/lisp/erc/resources/erc-scenarios-common.el: New file.
* test/lisp/erc/resources/services/auth-source/libera.eld: New file.
* test/lisp/erc/erc-scenarios-auth-source.el: New file.
* test/lisp/erc/erc-scenarios-base-reuse-buffers.el: New file.
* test/lisp/erc/erc-scenarios-join-auth-source.el: New file.
* test/lisp/erc/resources/base/reuse-buffers/channel/barnet.eld: New file.
* test/lisp/erc/resources/base/reuse-buffers/channel/foonet.eld: New file.
* test/lisp/erc/resources/join/auth-source/foonet.eld: New file.
(Bug#48598)

* lisp/erc/erc*.el: Add J.P. as my comaintainer to headers.

* lisp/auth-source.el (auth-info-password):
  Extract from `auth-source-pick-first-password'.
(auth-source-pick-first-password, auth-source-secrets-create)
(auth-source-user-and-password): Use `auth-info-password'.
* lisp/erc/erc-services.el (erc-nickserv-get-password):
  Use `auth-source-pick-first-password'.
* lisp/erc/erc.el (erc-open, erc-server-join-channel): Likewise.
* lisp/gnus/mail-source.el (mail-source-set-1): Add a comment.
* lisp/gnus/nnimap.el (nnimap-credentials): Use `auth-info-password'.
* lisp/gnus/nntp.el (nntp-send-authinfo): Likewise.
* lisp/mail/rmail.el (rmail-get-remote-password): Likewise.
* lisp/mail/smtpmail.el (smtpmail-try-auth-methods): Likewise.
* lisp/net/sieve-manage.el (sieve-sasl-auth): Likewise.
* lisp/net/tramp.el (tramp-read-passwd): Likewise.
* lisp/net/rcirc.el (rcirc): Likewise (fixes a bug: the possibility
  that password might be a function was not handled).