Rework fix for PR19460 - Use explicit bool as an extension instead.

In the previous fix I used a PMF type as a semi-safe bool type in C++03.
However immediately after committing I realized clang offered explicit
conversion operators as an extension. This patch removes the old fix and
enables _LIBCPP_EXPLICIT using __has_extension instead.

This change also affects the following other classes, which have
'_LIBCPP_EXPLICIT operator bool()'.

* shared_ptr
* unique_ptr
* error_condition
* basic_ios
* function (already C++11 only)
* istream::sentry
* experimental::string_view.

In all of the above cases I believe it is safe to enable the extension, except
in the experimental::string_view case. There seem to be some Clang bugs
affecting the experimental::string_view conversion to std::basic_string. To
work around that I manually disabled _LIBCPP_EXPLICIT in that case.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290831 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -727,7 +727,7 @@ template <unsigned> struct __static_assert_check {};
 #define _NOALIAS
 #endif
 
-#if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__) || \
+#if __has_extension(cxx_explicit_conversions) || defined(__IBMCPP__) || \
     (!defined(_LIBCPP_CXX03_LANG) && defined(__GNUC__)) // All supported GCC versions
 #   define _LIBCPP_EXPLICIT explicit
 #else

include/experimental/string_view

@@ -340,7 +340,12 @@ _LIBCPP_BEGIN_NAMESPACE_LFTS
         // [string.view.ops], string operations:
         template<class _Allocator>
         _LIBCPP_INLINE_VISIBILITY
-        _LIBCPP_EXPLICIT operator basic_string<_CharT, _Traits, _Allocator>() const
+        // Clang's extended C++11 explict conversions don't work with
+        // string_view in C++03.
+#ifndef _LIBCPP_CXX03_LANG
+        _LIBCPP_EXPLICIT
+#endif
+        operator basic_string<_CharT, _Traits, _Allocator>() const
         { return basic_string<_CharT, _Traits, _Allocator>( begin(), end()); }
 
         template<class _Allocator = allocator<_CharT> >

include/ios

@@ -572,13 +572,6 @@ ios_base::exceptions(iostate __iostate)
     clear(__rdstate_);
 }
 
-#if defined(_LIBCPP_CXX03_LANG)
-struct _LIBCPP_TYPE_VIS_ONLY __cxx03_bool {
-  typedef void (__cxx03_bool::*__bool_type)();
-  void __true_value() {}
-};
-#endif
-
 template <class _CharT, class _Traits>
 class _LIBCPP_TYPE_VIS_ONLY basic_ios
     : public ios_base
@@ -592,18 +585,8 @@ public:
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
 
-  // __true_value will generate undefined references when linking unless
-  // we give it internal linkage.
-
-#if defined(_LIBCPP_CXX03_LANG)
-    _LIBCPP_ALWAYS_INLINE
-    operator __cxx03_bool::__bool_type() const {
-        return !fail() ? &__cxx03_bool::__true_value : nullptr;
-    }
-#else
     _LIBCPP_ALWAYS_INLINE
     _LIBCPP_EXPLICIT operator bool() const {return !fail();}
-#endif
 
     _LIBCPP_ALWAYS_INLINE bool operator!() const    {return  fail();}
     _LIBCPP_ALWAYS_INLINE iostate rdstate() const   {return ios_base::rdstate();}