try_compile: Add policy CMP0067 to honor language standards

Projects use `try_compile` to check if they will be able to compile some
particular source code.  When a language standard variable like
`CMAKE_CXX_STANDARD` is set, then the project intends to compile source
code using a compiler mode for that standard.  Therefore it makes sense
for `try_compile` to use that standard in the test project too.

Unfortunately this was not done when support for the
`CMAKE_CXX_STANDARD` variable was first implemented.  Add a policy to
introduce the improved behavior in a compatible way.

Closes: #16456

1 files changed, 40 insertions, 0 deletions

diff --git a/Tests/RunCMake/try_compile/CMP0067.cmake b/Tests/RunCMake/try_compile/CMP0067.cmake
new file mode 100644
index 0000000..dd05d96
--- /dev/null
+++ b/Tests/RunCMake/try_compile/CMP0067.cmake
@@ -0,0 +1,40 @@
+enable_language(C)
+
+set(CMAKE_POLICY_WARNING_CMP0067 ON)
+message("before try_compile with CMP0067 WARN-enabled but no variables")
+try_compile(RESULT ${CMAKE_CURRENT_BINARY_DIR}
+  ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  )
+message("after try_compile with CMP0067 WARN-enabled but no variables")
+set(CMAKE_POLICY_WARNING_CMP0067 OFF)
+
+#-----------------------------------------------------------------------------
+
+set(CMAKE_C_STANDARD 90)
+
+message("before try_compile with CMP0067 WARN-default")
+try_compile(RESULT ${CMAKE_CURRENT_BINARY_DIR}
+  ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  )
+message("after try_compile with CMP0067 WARN-default")
+
+set(CMAKE_POLICY_WARNING_CMP0067 ON)
+message("before try_compile with CMP0067 WARN-enabled")
+try_compile(RESULT ${CMAKE_CURRENT_BINARY_DIR}
+  ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  )
+message("after try_compile with CMP0067 WARN-enabled")
+
+cmake_policy(SET CMP0067 OLD)
+message("before try_compile with CMP0067 OLD")
+try_compile(RESULT ${CMAKE_CURRENT_BINARY_DIR}
+  ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  )
+message("after try_compile with CMP0067 OLD")
+
+cmake_policy(SET CMP0066 NEW)
+message("before try_compile with CMP0067 NEW")
+try_compile(RESULT ${CMAKE_CURRENT_BINARY_DIR}
+  ${CMAKE_CURRENT_SOURCE_DIR}/src.c
+  )
+message("after try_compile with CMP0067 NEW")