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

5 files changed, 52 insertions, 0 deletions

diff --git a/Help/command/try_compile.rst b/Help/command/try_compile.rst
index 4b2a63169e..06b236228c 100644
--- a/Help/command/try_compile.rst
+++ b/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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index 259cebbe64..788d086ce1 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/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>
diff --git a/Help/policy/CMP0137.rst b/Help/policy/CMP0137.rst
new file mode 100644
index 0000000000..2efb9060b0
--- /dev/null
+++ b/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
diff --git a/Help/release/dev/try_compile-project-platform-vars.rst b/Help/release/dev/try_compile-project-platform-vars.rst
new file mode 100644
index 0000000000..2f409a396e
--- /dev/null
+++ b/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`.
diff --git a/Help/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.rst b/Help/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.rst
index d17851304b..5e84fd0c80 100644
--- a/Help/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.rst
+++ b/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`.