Don't wrap feature detection macros with QT_HAS_FOO() variants

Using wrappers for these macros is problematic when for example passing the
-frewrite-includes flag to preprocess sources before shipping off to distcc
or Icecream. It will also start producing warnings when compilers implement
http://eel.is/c++draft/cpp.cond#7.sentence-2. See for example
https://reviews.llvm.org/D49091

Both https://clang.llvm.org/docs/LanguageExtensions.html and the SD-6 document at
https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations
recommend defining '__has_foo(x) 0' as a fallback for compilers without the
macros, so that's what we go for.

Change-Id: I0298cd3b4a6ff6618821e34642a5ddd6728be767
Reviewed-by: Alex Richardson <arichardson.kde@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 37 insertions, 40 deletions

diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index e47f284a42..60dc7ce688 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -506,6 +506,39 @@
 #endif
 
 /*
+ * SG10's SD-6 feature detection and some useful extensions from Clang and GCC
+ * https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations
+ * http://clang.llvm.org/docs/LanguageExtensions.html#feature-checking-macros
+ * Not using wrapper macros, per http://eel.is/c++draft/cpp.cond#7.sentence-2
+ */
+#ifndef __has_builtin
+#  define __has_builtin(x)             0
+#endif
+#ifndef __has_feature
+#  define __has_feature(x)             0
+#endif
+#ifndef __has_attribute
+#  define __has_attribute(x)           0
+#endif
+#ifndef __has_cpp_attribute
+#  define __has_cpp_attribute(x)       0
+#endif
+#ifndef __has_include
+#  define __has_include(x)             0
+#endif
+#ifndef __has_include_next
+#  define __has_include_next(x)        0
+#endif
+
+// Kept around until all submodules have transitioned
+#define QT_HAS_BUILTIN(x)        __has_builtin(x)
+#define QT_HAS_FEATURE(x)        __has_feature(x)
+#define QT_HAS_ATTRIBUTE(x)      __has_attribute(x)
+#define QT_HAS_CPP_ATTRIBUTE(x)  __has_cpp_attribute(x)
+#define QT_HAS_INCLUDE(x)        __has_include(x)
+#define QT_HAS_INCLUDE_NEXT(x)   __has_include_next(x)
+
+/*
  * C++11 support
  *
  *  Paper           Macro                               SD-6 macro
@@ -1032,37 +1065,6 @@
 #endif
 
 /*
- * SG10's SD-6 feature detection and some useful extensions from Clang and GCC
- * https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations
- * http://clang.llvm.org/docs/LanguageExtensions.html#feature-checking-macros
- */
-#ifdef __has_builtin
-#  define QT_HAS_BUILTIN(x)             __has_builtin(x)
-#else
-#  define QT_HAS_BUILTIN(x)             0
-#endif
-#ifdef __has_attribute
-#  define QT_HAS_ATTRIBUTE(x)           __has_attribute(x)
-#else
-#  define QT_HAS_ATTRIBUTE(x)           0
-#endif
-#ifdef __has_cpp_attribute
-#  define QT_HAS_CPP_ATTRIBUTE(x)       __has_cpp_attribute(x)
-#else
-#  define QT_HAS_CPP_ATTRIBUTE(x)       0
-#endif
-#ifdef __has_include
-#  define QT_HAS_INCLUDE(x)             __has_include(x)
-#else
-#  define QT_HAS_INCLUDE(x)             0
-#endif
-#ifdef __has_include_next
-#  define QT_HAS_INCLUDE_NEXT(x)        __has_include_next(x)
-#else
-#  define QT_HAS_INCLUDE_NEXT(x)        0
-#endif
-
-/*
  * C++11 keywords and expressions
  */
 #ifdef Q_COMPILER_NULLPTR
@@ -1138,7 +1140,7 @@
 #  define Q_DECL_ALIGN(n)   alignas(n)
 #endif
 
-#if QT_HAS_CPP_ATTRIBUTE(nodiscard) && !defined(Q_CC_CLANG)         // P0188R1
+#if __has_cpp_attribute(nodiscard) && !defined(Q_CC_CLANG)         // P0188R1
 // Can't use [[nodiscard]] with Clang, see https://bugs.llvm.org/show_bug.cgi?id=33518
 #  undef Q_REQUIRED_RESULT
 #  define Q_REQUIRED_RESULT [[nodiscard]]
@@ -1240,11 +1242,6 @@
 #ifndef QT_MAKE_CHECKED_ARRAY_ITERATOR
 #  define QT_MAKE_CHECKED_ARRAY_ITERATOR(x, N) (x)
 #endif
-#ifdef __has_feature
-#  define QT_HAS_FEATURE(x)             __has_feature(x)
-#else
-#  define QT_HAS_FEATURE(x)             0
-#endif
 
 /*
  * Warning/diagnostic handling
@@ -1335,11 +1332,11 @@
     } while (false)
 
 #if defined(__cplusplus)
-#if QT_HAS_CPP_ATTRIBUTE(clang::fallthrough)
+#if __has_cpp_attribute(clang::fallthrough)
 #    define Q_FALLTHROUGH() [[clang::fallthrough]]
-#elif QT_HAS_CPP_ATTRIBUTE(gnu::fallthrough)
+#elif __has_cpp_attribute(gnu::fallthrough)
 #    define Q_FALLTHROUGH() [[gnu::fallthrough]]
-#elif QT_HAS_CPP_ATTRIBUTE(fallthrough)
+#elif __has_cpp_attribute(fallthrough)
 #  define Q_FALLTHROUGH() [[fallthrough]]
 #endif
 #endif