Introduce a configure-time check for C++17 filesystem

Various compilers have various fun ways of failing to compile when it
is used so let's check if they will work properly during configure
rather than much later.

Change-Id: Ia93d4b91b3d269b4cab2a5f677c3c89e06b44ce3
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/corelib/configure.json

@@ -366,6 +366,15 @@
                 "main": "std::mt19937 mt(0);"
             }
         },
+        "cxx17_filesystem": {
+            "label": "C++17 <filesystem>",
+            "type": "compile",
+            "test": {
+                "include": "filesystem",
+                "main": "std::filesystem::path p(\"./file\");",
+                "qmake": "CONFIG += c++17"
+            }
+        },
         "eventfd": {
             "label": "eventfd",
             "type": "compile",
@@ -613,6 +622,13 @@
             "condition": "tests.cxx11_future",
             "output": [ "publicFeature" ]
         },
+        "cxx17_filesystem": {
+            "label": "C++17 <filesystem>",
+            "condition": "tests.cxx17_filesystem",
+            "output": [
+                "publicFeature"
+            ]
+        },
         "eventfd": {
             "label": "eventfd",
             "condition": "!config.wasm && tests.eventfd",

src/corelib/global/qconfig-bootstrapped.h

@@ -79,6 +79,7 @@
 #define QT_FEATURE_cborstreamwriter 1
 #define QT_CRYPTOGRAPHICHASH_ONLY_SHA1
 #define QT_FEATURE_cxx11_random (__has_include(<random>) ? 1 : -1)
+#define QT_FEATURE_cxx17_filesystem -1
 #define QT_NO_DATASTREAM
 #define QT_FEATURE_datestring 1
 #define QT_FEATURE_datetimeparser -1

tests/benchmarks/corelib/io/qdiriterator/main.cpp

@@ -44,11 +44,7 @@
 
 #include "qfilesystemiterator.h"
 
-#if QT_HAS_INCLUDE(<filesystem>) && defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L
-#define HAS_STD_FILESYSTEM
-#endif
-
-#ifdef HAS_STD_FILESYSTEM
+#if QT_CONFIG(cxx17_filesystem)
 #include <filesystem>
 #endif
 
@@ -248,7 +244,7 @@ void tst_qdiriterator::fsiterator()
 
 void tst_qdiriterator::stdRecursiveDirectoryIterator()
 {
-#ifdef HAS_STD_FILESYSTEM
+#if QT_CONFIG(cxx17_filesystem)
     QFETCH(QByteArray, dirpath);
 
     int count = 0;

tests/benchmarks/corelib/io/qdiriterator/qdiriterator.pro

@@ -4,7 +4,11 @@ QT = core testlib
 
 CONFIG += release
 # Enable c++17 support for std::filesystem
-qtConfig(c++1z): CONFIG += c++17
+qtConfig(cxx17_filesystem) {
+    CONFIG += c++17
+    gcc:lessThan(QMAKE_GCC_MAJOR_VERSION, 9): \
+        QMAKE_LFLAGS += -lstdc++fs
+}
 
 SOURCES += main.cpp qfilesystemiterator.cpp
 HEADERS += qfilesystemiterator.h