Fix C++ standard detection

We cannot use a generator expression in an if statement, it does not
work. Instead, we could inspect the CMAKE_C/CXX_COMPILE_FEATURES list,
but unfortunately that's not reliable. For example it detects that ICPC
supports C++17 when in fact that depends on the installed libstdc++.
Therefore this patch revives our own configure tests.

Change-Id: Ic3bc5762fbe81837722523e3881ac16e84628519
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtFeature.cmake

@@ -472,7 +472,7 @@ function(qt_feature_copy_global_config_features_to_core target)
 endfunction()
 
 function(qt_config_compile_test name)
-    cmake_parse_arguments(arg "" "LABEL;PROJECT_PATH" "LIBRARIES;CODE" ${ARGN})
+    cmake_parse_arguments(arg "" "LABEL;PROJECT_PATH;C_STANDARD;CXX_STANDARD" "LIBRARIES;CODE" ${ARGN})
 
     if(arg_PROJECT_PATH)
         message(STATUS "Performing Test ${arg_LABEL}")
@@ -499,10 +499,24 @@ function(qt_config_compile_test name)
         endforeach()
 
         if(NOT DEFINED HAVE_${name})
+            set(_save_CMAKE_C_STANDARD "${CMAKE_C_STANDARD}")
+            set(_save_CMAKE_CXX_STANDARD "${CMAKE_CXX_STANDARD}")
+
+            if(arg_C_STANDARD)
+               set(CMAKE_C_STANDARD "${arg_C_STANDARD}")
+            endif()
+
+            if(arg_CXX_STANDARD)
+               set(CMAKE_CXX_STANDARD "${arg_CXX_STANDARD}")
+            endif()
+
             set(_save_CMAKE_REQUIRED_LIBRARIES "${CMAKE_REQUIRED_LIBRARIES}")
             set(CMAKE_REQUIRED_LIBRARIES "${arg_LIBRARIES}")
             check_cxx_source_compiles("${arg_UNPARSED_ARGUMENTS} ${arg_CODE}" HAVE_${name})
             set(CMAKE_REQUIRED_LIBRARIES "${_save_CMAKE_REQUIRED_LIBRARIES}")
+
+            set(CMAKE_C_STANDARD "${_save_CMAKE_C_STANDARD}")
+            set(CMAKE_CXX_STANDARD "${_save_CMAKE_CXX_STANDARD}")
         endif()
     endif()
 

configure.cmake

@@ -15,6 +15,54 @@ qt_find_package(Libudev PROVIDED_TARGETS PkgConfig::Libudev)
 
 #### Tests
 
+# c++14
+qt_config_compile_test(cxx14
+    LABEL "C++14 support"
+"#if __cplusplus > 201103L
+// Compiler claims to support C++14, trust it
+#else
+#  error __cplusplus must be > 201103L (the value of C++11)
+#endif
+
+
+int main(int argc, char **argv)
+{
+    (void)argc; (void)argv;
+    /* BEGIN TEST: */
+
+    /* END TEST: */
+    return 0;
+}
+"
+    CXX_STANDARD 14
+)
+
+# c++17
+qt_config_compile_test(cxx17
+    LABEL "C++17 support"
+"#if __cplusplus > 201402L
+// Compiler claims to support C++17, trust it
+#else
+#  error __cplusplus must be > 201402L (the value for C++14)
+#endif
+#include <map>  // https://bugs.llvm.org//show_bug.cgi?id=33117
+#include <variant>
+
+
+int main(int argc, char **argv)
+{
+    (void)argc; (void)argv;
+    /* BEGIN TEST: */
+std::variant<int> v(42);
+int i = std::get<int>(v);
+std::visit([](const auto &) { return 1; }, v);
+    /* END TEST: */
+    return 0;
+}
+"
+    CXX_STANDARD 17
+)
+
 # c++2a
 qt_config_compile_test(cxx2a
     LABEL "C++2a support"
@@ -33,7 +81,8 @@ int main(int argc, char **argv)
     /* END TEST: */
     return 0;
 }
-"# FIXME: qmake: CONFIG += c++11 c++14 c++17 c++2a
+"
+    CXX_STANDARD 20
 )
 
 # precompile_header
@@ -294,11 +343,11 @@ qt_feature("cxx11" PUBLIC
 )
 qt_feature("cxx14" PUBLIC
     LABEL "C++14"
-    CONDITION QT_FEATURE_cxx11 AND $<COMPILE_FEATURES:cxx_std_14>
+    CONDITION QT_FEATURE_cxx11 AND TEST_cxx14
 )
 qt_feature("cxx17" PUBLIC
     LABEL "C++17"
-    CONDITION QT_FEATURE_cxx14 AND $<COMPILE_FEATURES:cxx_std_17>
+    CONDITION QT_FEATURE_cxx14 AND TEST_cxx17
 )
 qt_feature("cxx1z" PUBLIC
     LABEL "C++17"

util/cmake/configurejson2cmake.py

@@ -54,13 +54,8 @@ class LibraryMapping:
 
 def map_tests(test: str) -> Optional[str]:
     testmap = {
-        "c++11": "$<COMPILE_FEATURES:cxx_std_11>",
-        "c++14": "$<COMPILE_FEATURES:cxx_std_14>",
-        "c++1z": "$<COMPILE_FEATURES:cxx_std_17>",
-        "c++17": "$<COMPILE_FEATURES:cxx_std_17>",
-        "c++20": "$<COMPILE_FEATURES:cxx_std_20>",
-        "c99": "$<COMPILE_FEATURES:c_std_99>",
-        "c11": "$<COMPILE_FEATURES:c_std_11>",
+        "c99": "c_std_99 IN_LIST CMAKE_C_COMPILE_FEATURES",
+        "c11": "c_std_11 IN_LIST CMAKE_C_COMPILE_FEATURES",
         "x86SimdAlways": "ON",  # FIXME: Make this actually do a compile test.
         "aesni": "TEST_subarch_aes",
         "avx": "TEST_subarch_avx",
@@ -489,12 +484,6 @@ def parseInput(ctx, sinput, data, cm_fh):
 #        },
 def parseTest(ctx, test, data, cm_fh):
     skip_tests = {
-        "c++11",
-        "c++14",
-        "c++17",
-        "c++20",
-        "c++1y",
-        "c++1z",
         "c11",
         "c99",
         "gc_binaries",
@@ -572,6 +561,7 @@ endif()
         sourceCode = sourceCode.replace('"', '\\"')
 
         librariesCmakeName = ""
+        languageStandard = ""
         qmakeFixme = ""
 
         cm_fh.write(f"# {test}\n")
@@ -594,6 +584,12 @@ endif()
             elif details["qmake"] == "CONFIG += c++11":
                 # do nothing we're always in c++11 mode
                 pass
+            elif details["qmake"] == "CONFIG += c++11 c++14":
+                languageStandard = "CXX_STANDARD 14"
+            elif details["qmake"] == "CONFIG += c++11 c++14 c++17":
+                languageStandard = "CXX_STANDARD 17"
+            elif details["qmake"] == "CONFIG += c++11 c++14 c++17 c++2a":
+                languageStandard = "CXX_STANDARD 20"
             else:
                 qmakeFixme = f"# FIXME: qmake: {details['qmake']}\n"
 
@@ -614,6 +610,8 @@ endif()
         cm_fh.write('"' + sourceCode + '"')
         if qmakeFixme != "":
             cm_fh.write(qmakeFixme)
+        if languageStandard != "":
+            cm_fh.write(f"\n    {languageStandard}\n")
         cm_fh.write(")\n\n")
 
     elif data["type"] == "libclang":