try_compile: Honor CMP0128 setting in test project

Some projects pass a raw `-std=` flag to the compiler in the
`try_compile` project.  If they do not set CMP0128 to NEW,
we should not append a `-std=` flag where we did not before
the policy was added.

Fixes: #24063

5 files changed, 66 insertions, 0 deletions

diff --git a/Tests/RunCMake/try_compile/CMP0128-NEW.cmake b/Tests/RunCMake/try_compile/CMP0128-NEW.cmake
new file mode 100644
index 0000000000..20e389a7d8
--- /dev/null
+++ b/Tests/RunCMake/try_compile/CMP0128-NEW.cmake
@@ -0,0 +1,7 @@
+cmake_policy(SET CMP0128 NEW)
+set(check_cxx_std "
+#if __cplusplus > 199711L && __cplusplus <= 201103L
+#  error Compiler is incorrectly in C++11 mode.
+#endif
+")
+include(CMP0128-common.cmake)
diff --git a/Tests/RunCMake/try_compile/CMP0128-WARN.cmake b/Tests/RunCMake/try_compile/CMP0128-WARN.cmake
new file mode 100644
index 0000000000..266bd22aee
--- /dev/null
+++ b/Tests/RunCMake/try_compile/CMP0128-WARN.cmake
@@ -0,0 +1,7 @@
+
+set(check_cxx_std "
+#if __cplusplus <= 199711L || __cplusplus > 201103L
+#  error Compiler is incorrectly not in C++11 mode.
+#endif
+")
+include(CMP0128-common.cmake)
diff --git a/Tests/RunCMake/try_compile/CMP0128-common.cmake b/Tests/RunCMake/try_compile/CMP0128-common.cmake
new file mode 100644
index 0000000000..0b8a12bc54
--- /dev/null
+++ b/Tests/RunCMake/try_compile/CMP0128-common.cmake
@@ -0,0 +1,31 @@
+cmake_policy(SET CMP0067 NEW)
+enable_language(CXX)
+
+# Isolate the one try_compile below in the error log.
+set(CMakeError_log "${CMAKE_BINARY_DIR}/CMakeFiles/CMakeError.log")
+file(REMOVE "${CMakeError_log}")
+
+# Add our own -std= flag to the try_compile check.
+set(CMAKE_REQUIRED_FLAGS -std=c++11)
+
+# Tell CMP0128 NEW behavior to append a -std= flag (after ours).
+if(CMAKE_CXX_EXTENSIONS_DEFAULT)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+else()
+  set(CMAKE_CXX_EXTENSIONS ON)
+endif()
+
+include(CheckSourceCompiles)
+check_source_compiles(CXX "
+${check_cxx_std}
+int main()
+{
+  return 0;
+}
+" SRC_COMPILED)
+if(NOT SRC_COMPILED)
+  if(EXISTS "${CMakeError_log}")
+    file(READ "${CMakeError_log}" err_log)
+  endif()
+  message("${err_log}")
+endif()
diff --git a/Tests/RunCMake/try_compile/Inspect.cmake b/Tests/RunCMake/try_compile/Inspect.cmake
new file mode 100644
index 0000000000..added410b1
--- /dev/null
+++ b/Tests/RunCMake/try_compile/Inspect.cmake
@@ -0,0 +1,4 @@
+enable_language(CXX)
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/info.cmake" "
+set(CMAKE_CXX_EXTENSIONS_DEFAULT \"${CMAKE_CXX_EXTENSIONS_DEFAULT}\")
+")
diff --git a/Tests/RunCMake/try_compile/RunCMakeTest.cmake b/Tests/RunCMake/try_compile/RunCMakeTest.cmake
index dcd37996e5..4301cf7ee3 100644
--- a/Tests/RunCMake/try_compile/RunCMakeTest.cmake
+++ b/Tests/RunCMake/try_compile/RunCMakeTest.cmake
@@ -114,6 +114,23 @@ if(RunCMake_GENERATOR MATCHES "Make|Ninja")
   unset(RunCMake_TEST_NO_CLEAN)
 endif()
 
+# Lookup CMAKE_CXX_EXTENSIONS_DEFAULT.
+# FIXME: Someday we could move this to the top of the file and use it in
+# place of some of the values passed by 'Tests/RunCMake/CMakeLists.txt'.
+run_cmake(Inspect)
+include("${RunCMake_BINARY_DIR}/Inspect-build/info.cmake")
+
+# FIXME: Support more compilers and default standard levels.
+if (CMAKE_CXX_COMPILER_ID MATCHES "^(GNU|AppleClang)$"
+    AND DEFINED CMAKE_CXX_STANDARD_DEFAULT
+    AND DEFINED CMAKE_CXX_EXTENSIONS_DEFAULT
+    )
+  run_cmake(CMP0128-WARN)
+  if(NOT CMAKE_CXX_STANDARD_DEFAULT EQUAL 11)
+    run_cmake(CMP0128-NEW)
+  endif()
+endif()
+
 if(UNIX)
   run_cmake(CleanupNoFollowSymlink)
 endif()