try_compile: Propagate platform variables in project-mode too

Add policy CMP0137 to propagate both our builtin variables and those
listed by `CMAKE_TRY_COMPILE_PLATFORM_VARIABLES` to `try_compile`
whole-project builds.

Inspired-by: Alexander Neumann <Alexander.Neumann@hamburg.de>
Fixes: #23219

Help/command/try_compile.rst

@@ -7,6 +7,8 @@ try_compile
 
 Try building some code.
 
+.. _`Try Compiling Whole Projects`:
+
 Try Compiling Whole Projects
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -25,6 +27,15 @@ will not be deleted after this command is run.  Specify ``<targetName>`` to
 build a specific target instead of the ``all`` or ``ALL_BUILD`` target.  See
 below for the meaning of other options.
 
+.. versionchanged:: 3.24
+  CMake variables describing platform settings, and those listed by the
+  :variable:`CMAKE_TRY_COMPILE_PLATFORM_VARIABLES` variable, are propagated
+  into the project's build configuration.  See policy :policy:`CMP0137`.
+  Previously this was only done by the
+  :ref:`source file <Try Compiling Source Files>` signature.
+
+.. _`Try Compiling Source Files`:
+
 Try Compiling Source Files
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 

Help/manual/cmake-policies.7.rst

@@ -58,6 +58,7 @@ Policies Introduced by CMake 3.24
 .. toctree::
    :maxdepth: 1
 
+   CMP0137: try_compile() passes platform variables in project mode. </policy/CMP0137>
    CMP0136: Watcom runtime library flags are selected by an abstraction. </policy/CMP0136>
    CMP0135: ExternalProject ignores timestamps in archives by default for the URL download method. </policy/CMP0135>
    CMP0134: Fallback to \"HOST\" Windows registry view when \"TARGET\" view is not usable. </policy/CMP0134>

Help/policy/CMP0137.rst

@@ -0,0 +1,29 @@
+CMP0137
+-------
+
+.. versionadded:: 3.24
+
+:command:`try_compile` passes platform variables in project mode.
+
+The :command:`try_compile` command :ref:`source file <Try Compiling Source
+Files>` signature propagates CMake variables containing platform settings,
+and those specified by the :variable:`CMAKE_TRY_COMPILE_PLATFORM_VARIABLES`
+variable, into the generated test project.  This helps the test project drive
+the toolchain the same way the calling project will.  In CMake 3.23 and below,
+the :ref:`whole-project <Try Compiling Whole Projects>` signature does not
+propagate platform variables automatically.  CMake 3.24 and above prefer to
+propagate platform variables in the :ref:`whole-project <Try Compiling Whole
+Projects>` signature.  This policy provides compatibility with projects that
+have not been updated to expect the behavior.
+
+The ``OLD`` behavior for this policy is to not pass any additional variables to
+the :ref:`whole-project <Try Compiling Whole Projects>` signature.
+The ``NEW`` behavior for this policy is to pass the same variables that the
+:ref:`source file <Try Compiling Source Files>` signature does.
+
+This policy was introduced in CMake version 3.24.  Use the
+:command:`cmake_policy` command to set this policy to ``OLD`` or ``NEW``
+explicitly. Unlike many policies, CMake version |release| does *not* warn
+by default when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt

Help/release/dev/try_compile-project-platform-vars.rst

@@ -0,0 +1,6 @@
+try_compile-project-platform-vars
+---------------------------------
+
+* The :command:`try_compile` command
+  :ref:`whole-project <Try Compiling Whole Projects>` signature
+  now propagates platform variables.  See policy :policy:`CMP0137`.

Help/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.rst

@@ -26,3 +26,8 @@ If a user passes ``-DMY_CUSTOM_VARIABLE=SomeValue`` to CMake then this
 setting will be made visible to the toolchain file both for the main
 project and for test projects generated by the :command:`try_compile`
 command source file signature.
+
+.. versionchanged:: 3.24
+  Listed variables are propagated to the :command:`try_compile`
+  :ref:`whole-project <Try Compiling Whole Projects>` signature too.
+  See :policy:`CMP0137`.

Source/cmCoreTryCompile.cxx

@@ -872,7 +872,9 @@ int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv,
   }
 
   // Forward a set of variables to the inner project cache.
-  if (this->SrcFileSignature) {
+  if (this->SrcFileSignature ||
+      this->Makefile->GetPolicyStatus(cmPolicies::CMP0137) ==
+        cmPolicies::NEW) {
     std::set<std::string> vars;
     vars.insert(&c_properties[lang_property_start],
                 &c_properties[lang_property_start + lang_property_size]);

Source/cmPolicies.h

@@ -411,7 +411,10 @@ class cmMakefile;
          3, 24, 0, cmPolicies::WARN)                                          \
   SELECT(POLICY, CMP0136,                                                     \
          "Watcom runtime library flags are selected by an abstraction.", 3,   \
-         24, 0, cmPolicies::WARN)
+         24, 0, cmPolicies::WARN)                                             \
+  SELECT(POLICY, CMP0137,                                                     \
+         "try_compile() passes platform variables in project mode", 3, 24, 0, \
+         cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
 #define CM_FOR_EACH_POLICY_ID(POLICY)                                         \

Tests/RunCMake/try_compile/CMP0137-Common.cmake

@@ -0,0 +1,8 @@
+set(CMAKE_ENABLE_EXPORTS 1)
+set(FOO 2)
+set(CMAKE_TRY_COMPILE_PLATFORM_VARIABLES FOO)
+try_compile(RESULT_VAR
+  ${CMAKE_CURRENT_BINARY_DIR}/CMP0137-build
+  ${CMAKE_CURRENT_SOURCE_DIR}/CMP0137
+  TestCMP0137)
+include(${CMAKE_CURRENT_BINARY_DIR}/CMP0137-build/info.cmake OPTIONAL)

Tests/RunCMake/try_compile/CMP0137-NEW-stdout.txt

@@ -0,0 +1,2 @@
+-- CMAKE_ENABLE_EXPORTS='1'
+-- FOO='2'

Tests/RunCMake/try_compile/CMP0137-NEW.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0137 NEW)
+include(CMP0137-Common.cmake)

Tests/RunCMake/try_compile/CMP0137-WARN-stdout.txt

@@ -0,0 +1,2 @@
+-- CMAKE_ENABLE_EXPORTS=''
+-- FOO=''

Tests/RunCMake/try_compile/CMP0137-WARN.cmake

@@ -0,0 +1,2 @@
+# policy CMP0137 not set
+include(CMP0137-Common.cmake)

Tests/RunCMake/try_compile/CMP0137/CMakeLists.txt

@@ -0,0 +1,6 @@
+cmake_minimum_required(VERSION 3.23)
+project(TestCMP0137 NONE)
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/info.cmake" "
+message(STATUS \"CMAKE_ENABLE_EXPORTS='${CMAKE_ENABLE_EXPORTS}'\")
+message(STATUS \"FOO='${FOO}'\")
+")

Tests/RunCMake/try_compile/RunCMakeTest.cmake

@@ -72,6 +72,8 @@ endif()
 run_cmake(CMP0056)
 run_cmake(CMP0066)
 run_cmake(CMP0067)
+run_cmake(CMP0137-WARN)
+run_cmake(CMP0137-NEW)
 
 if(RunCMake_GENERATOR MATCHES "Make|Ninja")
   # Use a single build tree for a few tests without cleaning.