try_compile: Honor CMAKE_WARN_DEPRECATED in test project

This causes the `-Wno-deprecated` option to be honored even inside a
`try_compile` test project, which is needed to suppress all deprecation
warnings as the option documents.

Closes: #16446

Help/release/dev/try_compile-honor-CMAKE_WARN_DEPRECATED.rst

@@ -0,0 +1,6 @@
+try_compile-honor-CMAKE_WARN_DEPRECATED
+---------------------------------------
+
+* The :command:`try_compile` command source file signature now
+  honors the :variable:`CMAKE_WARN_DEPRECATED` variable value
+  in the generated test project.

Source/cmCoreTryCompile.cxx

@@ -44,6 +44,7 @@ static std::string const kCMAKE_TRY_COMPILE_OSX_ARCHITECTURES =
   "CMAKE_TRY_COMPILE_OSX_ARCHITECTURES";
 static std::string const kCMAKE_TRY_COMPILE_PLATFORM_VARIABLES =
   "CMAKE_TRY_COMPILE_PLATFORM_VARIABLES";
+static std::string const kCMAKE_WARN_DEPRECATED = "CMAKE_WARN_DEPRECATED";
 
 int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv,
                                      bool isTryRun)
@@ -453,6 +454,7 @@ int cmCoreTryCompile::TryCompileCode(std::vector<std::string> const& argv,
       vars.insert(kCMAKE_OSX_SYSROOT);
       vars.insert(kCMAKE_POSITION_INDEPENDENT_CODE);
       vars.insert(kCMAKE_SYSROOT);
+      vars.insert(kCMAKE_WARN_DEPRECATED);
 
       if (const char* varListStr = this->Makefile->GetDefinition(
             kCMAKE_TRY_COMPILE_PLATFORM_VARIABLES)) {

Tests/RunCMake/try_compile/RunCMakeTest.cmake

@@ -18,6 +18,7 @@ run_cmake(NonSourceCompileDefinitions)
 
 set(RunCMake_TEST_OPTIONS --debug-trycompile)
 run_cmake(PlatformVariables)
+run_cmake(WarnDeprecated)
 unset(RunCMake_TEST_OPTIONS)
 
 run_cmake(TargetTypeExe)

Tests/RunCMake/try_compile/WarnDeprecated.cmake

@@ -0,0 +1,19 @@
+enable_language(C)
+
+set(CMAKE_WARN_DEPRECATED SOME_VALUE)
+
+try_compile(result ${CMAKE_CURRENT_BINARY_DIR}
+  SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  OUTPUT_VARIABLE out
+  )
+if(NOT result)
+  message(FATAL_ERROR "try_compile failed:\n${out}")
+endif()
+
+# Check that the cache was populated with our custom variable.
+file(STRINGS ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeTmp/CMakeCache.txt entries
+  REGEX CMAKE_WARN_DEPRECATED:UNINITIALIZED=${CMAKE_WARN_DEPRECATED}
+  )
+if(NOT entries)
+  message(FATAL_ERROR "try_compile did not populate cache as expected")
+endif()