Merge topic 'WCDH_allow_unsupported'

1679fecb CompileFeatures Test: make sure the target "CompileFeatures" is always defined
98e6d1e5 Tests/Module/WCDH: make it work with only C features defined
c8703e9d WCDH: optionally omit error code for unknown compilers or compiler versions
0de9c398 WCDH: add macro to write simple replacement defines

4 files changed, 137 insertions, 139 deletions

diff --git a/Help/release/dev/WCDH_allow_unsupported.rst b/Help/release/dev/WCDH_allow_unsupported.rst
new file mode 100644
index 0000000..c0b7377
--- /dev/null
+++ b/Help/release/dev/WCDH_allow_unsupported.rst
@@ -0,0 +1,8 @@
+WCDH_allow_unsupported
+----------------------
+
+* The :module:`WriteCompilerDetectionHeader` module gained the
+  ``ALLOW_UNKNOWN_COMPILERS`` and ``ALLOW_UNKNOWN_COMPILER_VERSIONS`` options
+  that allow creation of headers that will work also with unknown or old
+  compilers by simply assuming they do not support any of the requested
+  features.
diff --git a/Modules/WriteCompilerDetectionHeader.cmake b/Modules/WriteCompilerDetectionHeader.cmake
index ba249e1..0b16aa4 100644
--- a/Modules/WriteCompilerDetectionHeader.cmake
+++ b/Modules/WriteCompilerDetectionHeader.cmake
@@ -20,6 +20,8 @@
 #              [VERSION <version>]
 #              [PROLOG <prolog>]
 #              [EPILOG <epilog>]
+#              [ALLOW_UNKNOWN_COMPILERS]
+#              [ALLOW_UNKNOWN_COMPILER_VERSIONS]
 #    )
 #
 # The ``write_compiler_detection_header`` function generates the
@@ -81,6 +83,11 @@
 # See the :manual:`cmake-compile-features(7)` manual for information on
 # compile features.
 #
+# ``ALLOW_UNKNOWN_COMPILERS`` and ``ALLOW_UNKNOWN_COMPILER_VERSIONS`` cause
+# the module to generate conditions that treat unknown compilers as simply
+# lacking all features.  Without these options the default behavior is to
+# generate a ``#error`` for unknown compilers.
+#
 # Feature Test Macros
 # ===================
 #
@@ -232,6 +239,19 @@ function(_load_compiler_variables CompilerId lang)
   set(_compiler_id_version_compute_${CompilerId} ${_compiler_id_version_compute} PARENT_SCOPE)
 endfunction()
 
+macro(_simpledefine FEATURE_NAME FEATURE_TESTNAME FEATURE_STRING FEATURE_DEFAULT_STRING)
+  if (feature STREQUAL "${FEATURE_NAME}")
+        set(def_value "${prefix_arg}_${FEATURE_TESTNAME}")
+        string(APPEND file_content "
+#  if defined(${def_name}) && ${def_name}
+#    define ${def_value} ${FEATURE_STRING}
+#  else
+#    define ${def_value} ${FEATURE_DEFAULT_STRING}
+#  endif
+\n")
+  endif()
+endmacro()
+
 function(write_compiler_detection_header
     file_keyword file_arg
     prefix_keyword prefix_arg
@@ -242,7 +262,7 @@ function(write_compiler_detection_header
   if (NOT "x${prefix_keyword}" STREQUAL "xPREFIX")
     message(FATAL_ERROR "write_compiler_detection_header: PREFIX parameter missing.")
   endif()
-  set(options)
+  set(options ALLOW_UNKNOWN_COMPILERS ALLOW_UNKNOWN_COMPILER_VERSIONS)
   set(oneValueArgs VERSION EPILOG PROLOG OUTPUT_FILES_VAR OUTPUT_DIR)
   set(multiValueArgs COMPILERS FEATURES)
   cmake_parse_arguments(_WCD "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
@@ -426,10 +446,12 @@ function(write_compiler_detection_header
         set(compiler_file_content file_content)
       endif()
 
-      set(${compiler_file_content} "${${compiler_file_content}}
+      if(NOT _WCD_ALLOW_UNKNOWN_COMPILER_VERSIONS)
+        set(${compiler_file_content} "${${compiler_file_content}}
 #    if !(${_cmake_oldestSupported_${compiler}})
 #      error Unsupported compiler version
 #    endif\n")
+      endif()
 
       set(PREFIX ${prefix_arg}_)
       if (_need_hex_conversion)
@@ -460,67 +482,36 @@ function(write_compiler_detection_header
       endforeach()
     endforeach()
     if(pp_if STREQUAL "elif")
-      string(APPEND file_content "
+      if(_WCD_ALLOW_UNKNOWN_COMPILERS)
+        string(APPEND file_content "
+#  endif\n")
+      else()
+        string(APPEND file_content "
 #  else
 #    error Unsupported compiler
 #  endif\n")
+      endif()
     endif()
     foreach(feature ${${_lang}_features})
       string(TOUPPER ${feature} feature_upper)
       set(feature_PP "COMPILER_${feature_upper}")
       set(def_name ${prefix_arg}_${feature_PP})
-      if (feature STREQUAL c_restrict)
-        set(def_value "${prefix_arg}_RESTRICT")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} restrict
-#  else
-#    define ${def_value}
-#  endif
-\n")
-      endif()
-      if (feature STREQUAL cxx_constexpr)
-        set(def_value "${prefix_arg}_CONSTEXPR")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} constexpr
-#  else
-#    define ${def_value}
-#  endif
-\n")
-      endif()
-      if (feature STREQUAL cxx_final)
-        set(def_value "${prefix_arg}_FINAL")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} final
-#  else
-#    define ${def_value}
-#  endif
-\n")
-      endif()
-      if (feature STREQUAL cxx_override)
-        set(def_value "${prefix_arg}_OVERRIDE")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} override
-#  else
-#    define ${def_value}
-#  endif
-\n")
-      endif()
+      _simpledefine(c_restrict RESTRICT restrict "")
+      _simpledefine(cxx_constexpr CONSTEXPR constexpr "")
+      _simpledefine(cxx_final FINAL final "")
+      _simpledefine(cxx_override OVERRIDE override "")
       if (feature STREQUAL cxx_static_assert)
         set(def_value "${prefix_arg}_STATIC_ASSERT(X)")
         set(def_value_msg "${prefix_arg}_STATIC_ASSERT_MSG(X, MSG)")
         set(static_assert_struct "template<bool> struct ${prefix_arg}StaticAssert;\ntemplate<> struct ${prefix_arg}StaticAssert<true>{};\n")
         set(def_standard "#    define ${def_value} static_assert(X, #X)\n#    define ${def_value_msg} static_assert(X, MSG)")
         set(def_alternative "${static_assert_struct}#    define ${def_value} sizeof(${prefix_arg}StaticAssert<X>)\n#    define ${def_value_msg} sizeof(${prefix_arg}StaticAssert<X>)")
-        string(APPEND file_content "#  if ${def_name}\n${def_standard}\n#  else\n${def_alternative}\n#  endif\n\n")
+        string(APPEND file_content "#  if defined(${def_name}) && ${def_name}\n${def_standard}\n#  else\n${def_alternative}\n#  endif\n\n")
       endif()
       if (feature STREQUAL cxx_alignas)
         set(def_value "${prefix_arg}_ALIGNAS(X)")
         string(APPEND file_content "
-#  if ${def_name}
+#  if defined(${def_name}) && ${def_name}
 #    define ${def_value} alignas(X)
 #  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
 #    define ${def_value} __attribute__ ((__aligned__(X)))
@@ -534,7 +525,7 @@ function(write_compiler_detection_header
       if (feature STREQUAL cxx_alignof)
         set(def_value "${prefix_arg}_ALIGNOF(X)")
         string(APPEND file_content "
-#  if ${def_name}
+#  if defined(${def_name}) && ${def_name}
 #    define ${def_value} alignof(X)
 #  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
 #    define ${def_value} __alignof__(X)
@@ -543,30 +534,12 @@ function(write_compiler_detection_header
 #  endif
 \n")
       endif()
-      if (feature STREQUAL cxx_deleted_functions)
-        set(def_value "${prefix_arg}_DELETED_FUNCTION")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} = delete
-#  else
-#    define ${def_value}
-#  endif
-\n")
-      endif()
-      if (feature STREQUAL cxx_extern_templates)
-        set(def_value "${prefix_arg}_EXTERN_TEMPLATE")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} extern
-#  else
-#    define ${def_value}
-#  endif
-\n")
-      endif()
+      _simpledefine(cxx_deleted_functions DELETED_FUNCTION "= delete" "")
+      _simpledefine(cxx_extern_templates EXTERN_TEMPLATE extern "")
       if (feature STREQUAL cxx_noexcept)
         set(def_value "${prefix_arg}_NOEXCEPT")
         string(APPEND file_content "
-#  if ${def_name}
+#  if defined(${def_name}) && ${def_name}
 #    define ${def_value} noexcept
 #    define ${def_value}_EXPR(X) noexcept(X)
 #  else
@@ -575,20 +548,11 @@ function(write_compiler_detection_header
 #  endif
 \n")
       endif()
-      if (feature STREQUAL cxx_nullptr)
-        set(def_value "${prefix_arg}_NULLPTR")
-        string(APPEND file_content "
-#  if ${def_name}
-#    define ${def_value} nullptr
-#  else
-#    define ${def_value} 0
-#  endif
-\n")
-      endif()
+      _simpledefine(cxx_nullptr NULLPTR nullptr 0)
       if (feature STREQUAL cxx_thread_local)
         set(def_value "${prefix_arg}_THREAD_LOCAL")
         string(APPEND file_content "
-#  if ${def_name}
+#  if defined(${def_name}) && ${def_name}
 #    define ${def_value} thread_local
 #  elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang || ${prefix_arg}_COMPILER_IS_AppleClang
 #    define ${def_value} __thread
@@ -604,7 +568,7 @@ function(write_compiler_detection_header
         set(def_value "${prefix_arg}_DEPRECATED")
         string(APPEND file_content "
 #  ifndef ${def_value}
-#    if ${def_name}
+#    if defined(${def_name}) && ${def_name}
 #      define ${def_value} [[deprecated]]
 #      define ${def_value}_MSG(MSG) [[deprecated(MSG)]]
 #    elif ${prefix_arg}_COMPILER_IS_GNU || ${prefix_arg}_COMPILER_IS_Clang
diff --git a/Tests/CompileFeatures/CMakeLists.txt b/Tests/CompileFeatures/CMakeLists.txt
index 13aa86e..2a307d0 100644
--- a/Tests/CompileFeatures/CMakeLists.txt
+++ b/Tests/CompileFeatures/CMakeLists.txt
@@ -3,14 +3,6 @@ cmake_minimum_required(VERSION 3.1)
 
 project(CompileFeatures)
 
-if (NOT CMAKE_C_COMPILE_FEATURES AND NOT CMAKE_CXX_COMPILE_FEATURES)
-  file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp"
-    "int main(int,char**) { return 0; }\n"
-  )
-  add_executable(CompileFeatures "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp")
-  return()
-endif()
-
 macro(run_test feature lang)
   if (";${CMAKE_${lang}_COMPILE_FEATURES};" MATCHES ${feature})
     add_library(test_${feature} OBJECT ${feature})
@@ -277,8 +269,14 @@ if (CMAKE_CXX_COMPILE_FEATURES)
   endif()
 endif ()
 
-# these tests only work if at least cxx_auto_type is available
-if (";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_auto_type;")
+# always add a target "CompileFeatures"
+if (NOT ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_auto_type;")
+  file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp"
+    "int main(int,char**) { return 0; }\n"
+  )
+  add_executable(CompileFeatures "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp")
+else()
+  # these tests only work if at least cxx_auto_type is available
   add_executable(CompileFeatures main.cpp)
   set_property(TARGET CompileFeatures
     PROPERTY COMPILE_FEATURES "cxx_auto_type"
diff --git a/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt b/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
index 8b251d7..7957ab8 100644
--- a/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
+++ b/Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt
@@ -4,6 +4,7 @@ project(WriteCompilerDetectionHeader)
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
 include(WriteCompilerDetectionHeader)
+include(CheckCXXSourceCompiles)
 
 get_property(cxx_known_features GLOBAL PROPERTY CMAKE_CXX_KNOWN_FEATURES)
 get_property(c_known_features GLOBAL PROPERTY CMAKE_C_KNOWN_FEATURES)
@@ -19,14 +20,63 @@ write_compiler_detection_header(
     ${cxx_known_features} ${c_known_features}
 )
 
-if (NOT CMAKE_CXX_COMPILE_FEATURES AND NOT CMAKE_C_COMPILE_FEATURES)
+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
+      ${defines}
+      EXPECTED_COMPILER_VERSION_MAJOR=${COMPILER_VERSION_MAJOR}
+      EXPECTED_COMPILER_VERSION_MINOR=${COMPILER_VERSION_MINOR}
+      EXPECTED_COMPILER_VERSION_PATCH=${COMPILER_VERSION_PATCH}
+  )
+endmacro()
+
+if (CMAKE_C_COMPILE_FEATURES)
+  string(REGEX REPLACE "^([0-9]+)\\.[0-9]+\\.[0-9]+.*" "\\1" COMPILER_VERSION_MAJOR "${CMAKE_C_COMPILER_VERSION}")
+  string(REGEX REPLACE "^[0-9]+\\.([0-9]+)\\.[0-9]+.*" "\\1" COMPILER_VERSION_MINOR "${CMAKE_C_COMPILER_VERSION}")
+  string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" COMPILER_VERSION_PATCH "${CMAKE_C_COMPILER_VERSION}")
+
+  if (CMAKE_C_COMPILER_ID STREQUAL "GNU"
+      OR CMAKE_C_COMPILER_ID STREQUAL "Clang"
+      OR CMAKE_C_COMPILER_ID STREQUAL "AppleClang"
+      OR CMAKE_C_COMPILER_ID STREQUAL "Intel")
+    add_executable(WriteCompilerDetectionHeader_C11 main.c)
+    set_property(TARGET WriteCompilerDetectionHeader_C11 PROPERTY C_STANDARD 11)
+    set_defines(WriteCompilerDetectionHeader_C11 "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES;EXPECTED_COMPILER_C_RESTRICT" "")
+
+    add_executable(WriteCompilerDetectionHeader_C11_multi main_multi.c)
+    set_property(TARGET WriteCompilerDetectionHeader_C11_multi PROPERTY C_STANDARD 11)
+    set_defines(WriteCompilerDetectionHeader_C11_multi "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES;EXPECTED_COMPILER_C_RESTRICT" "")
+    target_include_directories(WriteCompilerDetectionHeader_C11_multi PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files)
+
+    add_executable(C_undefined c_undefined.c)
+    set_property(TARGET C_undefined PROPERTY C_STANDARD 90)
+    target_compile_options(C_undefined PRIVATE -Werror=undef)
+
+    add_executable(WriteCompilerDetectionHeader_C main.c)
+    set_property(TARGET WriteCompilerDetectionHeader_C PROPERTY C_STANDARD 90)
+    set_defines(WriteCompilerDetectionHeader_C "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES" "EXPECTED_COMPILER_C_RESTRICT")
+
+    add_executable(WriteCompilerDetectionHeader_C_multi main_multi.c)
+    set_property(TARGET WriteCompilerDetectionHeader_C_multi PROPERTY C_STANDARD 90)
+    set_defines(WriteCompilerDetectionHeader_C_multi "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES" "EXPECTED_COMPILER_C_RESTRICT")
+    target_include_directories(WriteCompilerDetectionHeader_C_multi PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files)
+  endif()
+endif()
+
+if (NOT CMAKE_CXX_COMPILE_FEATURES)
   file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp"
     "int main(int,char**) { return 0; }\n"
   )
   add_executable(WriteCompilerDetectionHeader "${CMAKE_CURRENT_BINARY_DIR}/dummy.cpp")
 
   if(UNIX OR NOT CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-    include(CheckCXXSourceCompiles)
     check_cxx_source_compiles("#include \"${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection.h\"\nint main() { return 0; }\n"
       file_include_works
     )
@@ -41,23 +91,6 @@ string(REGEX REPLACE "^([0-9]+)\\.[0-9]+\\.[0-9]+.*" "\\1" COMPILER_VERSION_MAJO
 string(REGEX REPLACE "^[0-9]+\\.([0-9]+)\\.[0-9]+.*" "\\1" COMPILER_VERSION_MINOR "${CMAKE_CXX_COMPILER_VERSION}")
 string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" COMPILER_VERSION_PATCH "${CMAKE_CXX_COMPILER_VERSION}")
 
-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
-      ${defines}
-      EXPECTED_COMPILER_VERSION_MAJOR=${COMPILER_VERSION_MAJOR}
-      EXPECTED_COMPILER_VERSION_MINOR=${COMPILER_VERSION_MINOR}
-      EXPECTED_COMPILER_VERSION_PATCH=${COMPILER_VERSION_PATCH}
-  )
-endmacro()
-
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
     OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang"
     OR CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang"
@@ -79,25 +112,6 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
   endif()
 endif()
 
-if (CMAKE_C_COMPILER_ID STREQUAL "GNU"
-    OR CMAKE_C_COMPILER_ID STREQUAL "Clang"
-    OR CMAKE_C_COMPILER_ID STREQUAL "AppleClang"
-    OR CMAKE_C_COMPILER_ID STREQUAL "Intel")
-
-  add_executable(C_undefined c_undefined.c)
-  set_property(TARGET C_undefined PROPERTY C_STANDARD 90)
-  target_compile_options(C_undefined PRIVATE -Werror=undef)
-
-  add_executable(WriteCompilerDetectionHeader_C main.c)
-  set_property(TARGET WriteCompilerDetectionHeader_C PROPERTY C_STANDARD 90)
-  set_defines(WriteCompilerDetectionHeader_C "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES" "EXPECTED_COMPILER_C_RESTRICT")
-
-  add_executable(WriteCompilerDetectionHeader_C_multi main_multi.c)
-  set_property(TARGET WriteCompilerDetectionHeader_C_multi PROPERTY C_STANDARD 90)
-  set_defines(WriteCompilerDetectionHeader_C_multi "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES" "EXPECTED_COMPILER_C_RESTRICT")
-  target_include_directories(WriteCompilerDetectionHeader_C_multi PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files)
-endif()
-
 add_executable(WriteCompilerDetectionHeader main.cpp)
 set_property(TARGET WriteCompilerDetectionHeader PROPERTY CXX_STANDARD 98)
 set_defines(WriteCompilerDetectionHeader "${true_defs}" "${false_defs}")
@@ -143,16 +157,30 @@ set_property(TARGET multi_files_11 PROPERTY CXX_STANDARD 11)
 target_include_directories(multi_files_11 PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files)
 set_defines(multi_files_11 "${true_defs}" "${false_defs}")
 
-if (CMAKE_C_COMPILER_ID STREQUAL "GNU"
-    OR CMAKE_C_COMPILER_ID STREQUAL "Clang"
-    OR CMAKE_C_COMPILER_ID STREQUAL "AppleClang"
-    OR CMAKE_C_COMPILER_ID STREQUAL "Intel")
-  add_executable(WriteCompilerDetectionHeader_C11 main.c)
-  set_property(TARGET WriteCompilerDetectionHeader_C11 PROPERTY C_STANDARD 11)
-  set_defines(WriteCompilerDetectionHeader_C11 "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES;EXPECTED_COMPILER_C_RESTRICT" "")
-
-  add_executable(WriteCompilerDetectionHeader_C11_multi main_multi.c)
-  set_property(TARGET WriteCompilerDetectionHeader_C11_multi PROPERTY C_STANDARD 11)
-  set_defines(WriteCompilerDetectionHeader_C11_multi "EXPECTED_COMPILER_C_FUNCTION_PROTOTYPES;EXPECTED_COMPILER_C_RESTRICT" "")
-  target_include_directories(WriteCompilerDetectionHeader_C11_multi PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files)
+# test for ALLOW_UNKNOWN_COMPILERS
+
+# use a compiler does not match the current one,
+# so one always hits the fallback code
+if (CMAKE_CXX_COMPILER_ID STREQUAL "SunPro")
+  set(OTHER_CXX "Intel")
+else()
+  set(OTHER_CXX "SunPro")
+endif()
+
+write_compiler_detection_header(
+  FILE "${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection_allow_unknown.h"
+  PREFIX TEST
+  COMPILERS ${OTHER_CXX}
+  FEATURES cxx_nullptr
+  ALLOW_UNKNOWN_COMPILERS
+)
+
+# intentionally abuse the TEST_NULLPR variable: this will only work
+# with the fallback code.
+check_cxx_source_compiles("#include \"${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection_allow_unknown.h\"
+int main() {\n int i = TEST_NULLPTR;\n return 0; }\n"
+  file_include_works_allow_unknown
+)
+if (NOT file_include_works_allow_unknown)
+  message(SEND_ERROR "Inclusion of ${CMAKE_CURRENT_BINARY_DIR}/test_compiler_detection_allow_unknown.h was expected to work, but did not.")
 endif()