Add {{=}} as a built-in magic word

This is a replay of 4bc0dc348a, which
was reverted in 9bd4fc0ae9 due to unexpected
use on Dutch Wiktionary.  In 1.36 deprecation warnings and a tracking
category were added if a wiki defined [[Template:=]] to expand to
anything other than `=` (see aeb3f45c20).
This patch follows up that deprecation by finally defining `{{=}}` as
a built-in, since the last usage on deployed wikis was cleaned up
sometime around February 2021 (list at
https://meta.wikimedia.org/wiki/Equals_sign_parser_function_template_conflicts
).

We've left the tracking category defined for now, so that any remaining
pages left in the tracking category on third-party wikis still retain
localized category documentation.  But it is expected that the next MW
release will also remove the tracking category.

Bug: T91154
Change-Id: I4717172f1d74d326212d51015a6cd87c3758f30d

RELEASE-NOTES-1.39

@@ -52,6 +52,11 @@ For notes on 1.38.x and older releases, see HISTORY.
 * Administrators now have the option to delete/undelete the associated "Talk"
   page when they are (un)deleting a given page. `deletetalk` and `undeletetalk`
   options were added to the 'delete' and 'undelete' action APIs in MW 1.38.
+* `{{=}}` is now a wikitext built-in magic word, expanding to `=`.  This is
+  conventionally used as an escape mechanism to allow the use of `=` in
+  unnamed template arguments.  Defining [[Template:=]] to expand to something
+  other than `=` has been deprecated since 1.36, with affected pages put into
+  a special tracking category for migration.
 * …
 
 === New developer features in 1.39 ===

includes/MagicWordFactory.php

@@ -43,6 +43,7 @@ class MagicWordFactory {
 	/** @var string[] */
 	private $mVariableIDs = [
 		'!',
+		'=',
 		'currentmonth',
 		'currentmonth1',
 		'currentmonthname',

includes/TrackingCategories.php

@@ -73,6 +73,9 @@ class TrackingCategories {
 		'post-expand-template-argument-category',
 		'post-expand-template-inclusion-category',
 		'restricted-displaytitle-ignored',
+		# template-equals-category is unused in MW>=1.39, but the category
+		# can be left around for a major release or so for an easier
+		# transition for anyone who didn't do the cleanup. T91154
 		'template-equals-category',
 		'template-loop-category',
 		'unstrip-depth-category',

includes/parser/CoreMagicVariables.php

@@ -64,6 +64,8 @@ class CoreMagicVariables {
 		switch ( $id ) {
 			case '!':
 				return '|';
+			case '=':
+				return '=';
 			case 'currentmonth':
 				return $pageLang->formatNumNoSeparators( $ts->format( 'm' ) );
 			case 'currentmonth1':

languages/messages/MessagesEn.php

@@ -210,6 +210,7 @@ $bookstoreList = [
 $magicWords = [
 #   ID                               CASE  SYNONYMS
 	'!'                       => [ 1, '!' ],
+	'='                       => [ 1, '=' ],
 	'anchorencode'            => [ 0, 'ANCHORENCODE' ],
 	'articlepath'             => [ 0, 'ARTICLEPATH' ],
 	'basepagename'            => [ 1, 'BASEPAGENAME' ],

tests/parser/pfeqParserTests.txt

@@ -5,26 +5,23 @@ Template:=
 no
 !! endarticle
 
-# {{=}} will be converted to a parser function which expands to `=`
-# If you have it defined as a Template that expands to something other
-# than `=` that will generate a warning and a tracking category, although
-# referencing the template explicitly as {{Template:=}} is fine
+# {{=}} is a parser function which expands to `=`, although
+# referencing a template explicitly as {{Template:=}} is fine
 # regardless of what it expands to. (T91154)
 !! test
-Using {{=}} when that doesn't expand to "=" should create a tracking category
-!! options
-cat
+Basic usage of {{=}}
 !! wikitext
 This uses {{=}}.
-!! html/php
-cat=Pages_which_use_=_as_a_template sort=
+!! html
+<p>This uses =.
+</p>
 !! end
 
 !! test
-Using {{Template:=}} shouldn't create tracking category regardless of definition
-!! options
-cat
+Using {{Template:=}} doesn't invoke the parser function
 !! wikitext
 This uses {{Template:=}}.
-!! html/php
+!! html
+<p>This uses no.
+</p>
 !! end