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

5 files changed, 90 insertions, 0 deletions

diff --git a/Tests/RunCMake/try_compile/CMP0067-stderr.txt b/Tests/RunCMake/try_compile/CMP0067-stderr.txt
new file mode 100644
index 0000000..e2677ed
--- /dev/null
+++ b/Tests/RunCMake/try_compile/CMP0067-stderr.txt
@@ -0,0 +1,25 @@
+before try_compile with CMP0067 WARN-enabled but no variables
+after try_compile with CMP0067 WARN-enabled but no variables
+before try_compile with CMP0067 WARN-default
+after try_compile with CMP0067 WARN-default
+before try_compile with CMP0067 WARN-enabled
+CMake Warning \(dev\) at CMP0067.cmake:[0-9]+ \(try_compile\):
+  Policy CMP0067 is not set: Honor language standard in try_compile\(\)
+  source-file signature.  Run "cmake --help-policy CMP0067" for policy
+  details.  Use the cmake_policy command to set the policy and suppress this
+  warning.
+
+  For compatibility with older versions of CMake, try_compile is not honoring
+  language standard variables in the test project:
+
+    CMAKE_C_STANDARD
+
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+after try_compile with CMP0067 WARN-enabled
+before try_compile with CMP0067 OLD
+after try_compile with CMP0067 OLD
+before try_compile with CMP0067 NEW
+after try_compile with CMP0067 NEW
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")
diff --git a/Tests/RunCMake/try_compile/CStandardGNU.cmake b/Tests/RunCMake/try_compile/CStandardGNU.cmake
index 29ce315..79ae874 100644
--- a/Tests/RunCMake/try_compile/CStandardGNU.cmake
+++ b/Tests/RunCMake/try_compile/CStandardGNU.cmake
@@ -9,3 +9,15 @@ try_compile(result ${CMAKE_CURRENT_BINARY_DIR}
 if(NOT result)
   message(FATAL_ERROR "try_compile failed:\n${out}")
 endif()
+
+cmake_policy(SET CMP0067 NEW)
+set(CMAKE_C_STANDARD 99)
+set(CMAKE_C_STANDARD_REQUIRED 1)
+set(CMAKE_C_EXTENSIONS 0)
+try_compile(result ${CMAKE_CURRENT_BINARY_DIR}
+  SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/CStandardGNU.c
+  OUTPUT_VARIABLE out
+  )
+if(NOT result)
+  message(FATAL_ERROR "try_compile failed:\n${out}")
+endif()
diff --git a/Tests/RunCMake/try_compile/CxxStandardGNU.cmake b/Tests/RunCMake/try_compile/CxxStandardGNU.cmake
index cc16bea..e714fe4 100644
--- a/Tests/RunCMake/try_compile/CxxStandardGNU.cmake
+++ b/Tests/RunCMake/try_compile/CxxStandardGNU.cmake
@@ -9,3 +9,15 @@ try_compile(result ${CMAKE_CURRENT_BINARY_DIR}
 if(NOT result)
   message(FATAL_ERROR "try_compile failed:\n${out}")
 endif()
+
+cmake_policy(SET CMP0067 NEW)
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED 1)
+set(CMAKE_CXX_EXTENSIONS 0)
+try_compile(result ${CMAKE_CURRENT_BINARY_DIR}
+  SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/CxxStandardGNU.cxx
+  OUTPUT_VARIABLE out
+  )
+if(NOT result)
+  message(FATAL_ERROR "try_compile failed:\n${out}")
+endif()
diff --git a/Tests/RunCMake/try_compile/RunCMakeTest.cmake b/Tests/RunCMake/try_compile/RunCMakeTest.cmake
index dadcf35..d1b0217 100644
--- a/Tests/RunCMake/try_compile/RunCMakeTest.cmake
+++ b/Tests/RunCMake/try_compile/RunCMakeTest.cmake
@@ -44,6 +44,7 @@ endif()
 
 run_cmake(CMP0056)
 run_cmake(CMP0066)
+run_cmake(CMP0067)
 
 if(RunCMake_GENERATOR MATCHES "Make|Ninja")
   # Use a single build tree for a few tests without cleaning.