CMP0053: document that `$` is a valid literal variable character

This was overlooked in the initial implementation of CMP0053. However,
an additional policy to reject it again is not worth it. Instead, add
tests and document the behavior.

Fixes: #17883

Help/manual/cmake-language.7.rst

@@ -391,8 +391,9 @@ inside out, e.g. ``${outer_${inner_variable}_variable}``.
 
 Literal variable references may consist of alphanumeric characters,
 the characters ``/_.+-``, and `Escape Sequences`_.  Nested references
-may be used to evaluate variables of any name.  (See also policy
-:policy:`CMP0053` documentation for historical considerations.)
+may be used to evaluate variables of any name.  See also policy
+:policy:`CMP0053` documentation for historical considerations and reasons why
+the ``$`` is also technically permitted but is discouraged.
 
 The `Variables`_ section documents the scope of variable names
 and how their values are set.

Help/policy/CMP0053.rst

@@ -16,6 +16,10 @@ cleaned up to simplify the behavior.  Specifically:
 * Literal ``${VAR}`` reference syntax may contain only
   alphanumeric characters (``A-Z``, ``a-z``, ``0-9``) and
   the characters ``_``, ``.``, ``/``, ``-``, and ``+``.
+  Note that ``$`` is technically allowed in the ``NEW`` behavior, but is
+  invalid for ``OLD`` behavior.  This is due to an oversight during the
+  implementation of :policy:`CMP0053` and its use as a literal variable
+  reference is discouraged for this reason.
   Variables with other characters in their name may still
   be referenced indirectly, e.g.
 

Tests/RunCMake/Syntax/CMP0053-Dollar-NEW-stderr.txt

@@ -0,0 +1,2 @@
+^-->value<--
+-->value<--$

Tests/RunCMake/Syntax/CMP0053-Dollar-NEW.cmake

@@ -0,0 +1,6 @@
+cmake_policy(SET CMP0053 NEW)
+
+set($ value)
+set(dollar $)
+message("-->${${dollar}}<--")
+message("-->${$}<--")

Tests/RunCMake/Syntax/CMP0053-Dollar-OLD-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/Syntax/CMP0053-Dollar-OLD-stderr.txt

@@ -0,0 +1,24 @@
+^CMake Deprecation Warning at CMP0053-Dollar-OLD.cmake:1 \(cmake_policy\):
+  The OLD behavior for policy CMP0053 will be removed from a future version
+  of CMake.
+
+  The cmake-policies\(7\) manual explains that the OLD behaviors of all
+  policies are deprecated and that a policy should be set to OLD only under
+  specific short-term circumstances.  Projects should be ported to the NEW
+  behavior and not rely on setting a policy to OLD.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
++
+-->value<--
+CMake Error at CMP0053-Dollar-OLD.cmake:6 \(message\):
+  Syntax error in cmake code at
+
+    .*CMP0053-Dollar-OLD.cmake:6
+
+  when parsing string
+
+    -->\${\$}<--
+
+  syntax error, unexpected \$, expecting } \(7\)
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$

Tests/RunCMake/Syntax/CMP0053-Dollar-OLD.cmake

@@ -0,0 +1,6 @@
+cmake_policy(SET CMP0053 OLD)
+
+set($ value)
+set(dollar $)
+message("-->${${dollar}}<--")
+message("-->${$}<--")

Tests/RunCMake/Syntax/RunCMakeTest.cmake

@@ -111,6 +111,8 @@ run_cmake(CMP0053-NameWithNewlineQuoted)
 run_cmake(CMP0053-NameWithCarriageReturnQuoted)
 run_cmake(CMP0053-NameWithEscapedSpacesQuoted)
 run_cmake(CMP0053-NameWithEscapedTabsQuoted)
+run_cmake(CMP0053-Dollar-OLD)
+run_cmake(CMP0053-Dollar-NEW)
 
 # Variable special types
 run_cmake(QueryCache)