Add the WriteCompilerDetectionHeader module.

Provide a function to write a portable header to detect compiler
features.  Generate a preprocessor #error for unknown compilers
and compiler versions whose features are not yet recorded.  This
error condition might be relaxed in the future, but for now it
is useful for verification of expectations.

2 files changed, 98 insertions, 0 deletions

diff --git a/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt b/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
new file mode 100644
index 0000000..6c5e0be
--- /dev/null
+++ b/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
@@ -0,0 +1,79 @@
+cmake_minimum_required(VERSION 3.0.0)
+project(WriteCompilerDetectionHeader)
+
+set(CMAKE_INCLUDE_CURRENT_DIR ON)
+
+include(WriteCompilerDetectionHeader)
+
+get_property(cxx_known_features GLOBAL PROPERTY CMAKE_CXX_KNOWN_FEATURES)
+
+write_compiler_detection_header(
+  FILE "${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection.h"
+  PREFIX TEST
+  COMPILERS GNU
+  VERSION 3.1
+  PROLOG "// something"
+  EPILOG "// more"
+  FEATURES
+    ${cxx_known_features}
+)
+
+if (NOT CMAKE_CXX_COMPILE_FEATURES)
+  file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp"
+    "int main(int,char**) { return 0; }\n"
+  )
+  add_executable(WriteCompilerDetectionHeader "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp")
+
+  include(CheckCXXSourceCompiles)
+  check_cxx_source_compiles("#include \"${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection.h\"\nint main() { return 0; }\n"
+    file_include_works
+  )
+  if (file_include_works)
+    message(SEND_ERROR "Inclusion of ${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection.h was expected to cause an error, but did not.")
+  endif()
+  return()
+endif()
+
+macro(set_defines target true_defs false_defs)
+  set(defines)
+  foreach(def ${true_defs})
+    list(APPEND defines ${def}=1)
+  endforeach()
+  foreach(def ${false_defs})
+    list(APPEND defines ${def}=0)
+  endforeach()
+  target_compile_definitions(${target}
+    PRIVATE
+      ${defines}
+  )
+endmacro()
+
+if (CMAKE_CXX_COMPILER_ID STREQUAL GNU)
+  # False for C++98 mode.
+  list(APPEND false_defs EXPECTED_COMPILER_CXX_DELEGATING_CONSTRUCTORS)
+  list(APPEND false_defs EXPECTED_COMPILER_CXX_VARIADIC_TEMPLATES)
+endif()
+
+add_executable(WriteCompilerDetectionHeader main.cpp)
+set_property(TARGET WriteCompilerDetectionHeader PROPERTY CXX_STANDARD 98)
+set_defines(WriteCompilerDetectionHeader "${true_defs}" "${false_defs}")
+
+if(MSVC)
+  return() # MSVC has only one mode.
+endif()
+
+# Since GNU 4.7
+if (";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_delegating_constructors;")
+  list(APPEND true_defs EXPECTED_COMPILER_CXX_DELEGATING_CONSTRUCTORS)
+  list(REMOVE_ITEM false_defs EXPECTED_COMPILER_CXX_DELEGATING_CONSTRUCTORS)
+endif()
+
+# Since GNU 4.4
+if (";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_variadic_templates;")
+  list(APPEND true_defs EXPECTED_COMPILER_CXX_VARIADIC_TEMPLATES)
+  list(REMOVE_ITEM false_defs EXPECTED_COMPILER_CXX_VARIADIC_TEMPLATES)
+endif()
+
+add_executable(WriteCompilerDetectionHeader_11 main.cpp)
+set_property(TARGET WriteCompilerDetectionHeader_11 PROPERTY CXX_STANDARD 11)
+set_defines(WriteCompilerDetectionHeader_11 "${true_defs}" "${false_defs}")
diff --git a/Tests/Module/WriteCompilerDetectionHeader/main.cpp b/Tests/Module/WriteCompilerDetectionHeader/main.cpp
new file mode 100644
index 0000000..8b4ea52
--- /dev/null
+++ b/Tests/Module/WriteCompilerDetectionHeader/main.cpp
@@ -0,0 +1,19 @@
+
+#include "test_compiler_detection.h"
+
+#define JOIN_IMPL(A, B) A ## B
+#define JOIN(A, B) JOIN_IMPL(A, B)
+#define CHECK(FEATURE) (JOIN(TEST_COMPILER_, FEATURE) == JOIN(EXPECTED_COMPILER_, FEATURE))
+
+#if !CHECK(CXX_DELEGATING_CONSTRUCTORS)
+#error cxx_delegating_constructors expected availability did not match.
+#endif
+
+#if !CHECK(CXX_VARIADIC_TEMPLATES)
+#error cxx_variadic_templates expected availability did not match.
+#endif
+
+int main()
+{
+  return 0;
+}