Merge topic 'if-sanity'

858d5a0b Fix if() checks of CMAKE_SYSTEM_NAME on Cygwin
e177e7af FPHSA: Avoid if() dereferencing of quoted variable
425acc52 cmcurl: Use if(DEFINED) to simplify conditions
cede5cbd libarchive: Avoid depending on if() to dereference a quoted variable
2d97178b FindGTK2: Avoid depending on if() to dereference a quoted variable
0b12815d Modules/Test*.cmake: Use if(DEFINED) to simplify conditions
188a1f23 If: Introduce policy CMP0054 - don't dereference quoted variables in if()
b900c1cc If: Extract cmConditionEvaluator from if() implementation

Help/command/if.rst

@@ -199,3 +199,9 @@ above-documented signature accepts ``<variable|string>``:
 * The left and right hand arguments to ``AND`` and ``OR`` are independently
   tested to see if they are boolean constants, if so they are used as
   such, otherwise they are assumed to be variables and are dereferenced.
+
+To prevent ambiguity, potential variable or keyword names can be
+specified in a :ref:`Quoted Argument` or a :ref:`Bracket Argument`.
+A quoted or bracketed variable or keyword will be interpreted as a
+string and not dereferenced or interpreted.
+See policy :policy:`CMP0054`.

Help/manual/cmake-policies.7.rst

@@ -105,3 +105,4 @@ All Policies
    /policy/CMP0051
    /policy/CMP0052
    /policy/CMP0053
+   /policy/CMP0054

Help/policy/CMP0054.rst

@@ -0,0 +1,18 @@
+CMP0054
+-------
+
+Only interpret :command:`if` arguments as variables or keywords when unquoted.
+
+CMake 3.1 and above no longer dereference variables or interpret keywords
+in an :command:`if` command argument when it is a :ref:`Quoted Argument`
+or a :ref:`Bracket Argument`.
+
+The ``OLD`` behavior for this policy is to dereference variables and
+interpret keywords even if they are quoted or bracketed.
+The ``NEW`` behavior is to not dereference variables or interpret keywords
+that have been quoted or bracketed.
+
+This policy was introduced in CMake version 3.1.
+CMake version |release| warns when the policy is not set and uses
+``OLD`` behavior.  Use the :command:`cmake_policy` command to set
+it to ``OLD`` or ``NEW`` explicitly.

Help/release/dev/if-sanity.rst

@@ -0,0 +1,6 @@
+if-sanity
+---------
+
+* The :command:`if` command no longer automatically dereferences
+  variables named in quoted or bracket arguments.  See policy
+  :policy:`CMP0054`.