[libc++] Assume that compilers support extended constexpr in C++14 mode

[ldionne]

Summary

We don't support any compiler that doesn't support C++14 constexpr when
compiling in C++14 mode anymore, so we can just assume that we have C++14
extended constexpr when compiling in C++14 mode. This allows us to remove
some workarounds for older compilers.

[ldionne]

Fix issues with wrap_iter constexpr in C++11 and C++03 mode

[• Quuxplusone]

LGTM % if there's anything to do about my comment.

libcxx/include/__config

565–567

Should this number be bumped upward? Or does _MSC_VER==1900 actually support C++14 constexpr, and we just didn't notice it until now?

IIUC, this is a "major NFC" change ;) — we used to never use C++14 constexpr on _LIBCPP_COMPILER_MSVC, and now we're going to always use it. Right? (Not a "problem" per se, but noteworthy.)

[ldionne]

libcxx/include/__config

565–567

@mstorsjo Do you know what's the status of our support on MSVC? Officially, we don't support it, but does it actually work even a bit?

[ldionne]

Fix embarrassingly obvious logic error in _LIBCPP_CONSTEXPR_IF_NODEBUG

[mstorsjo]

libcxx/include/__config

565–567

I remember seeing a mention somewhere, that the biggest blocker is that MSVC doesn't support #include_next, which pretty much blocks it from even trying to use our headers.

In this case, the MSVC version number here corresponds to the version of MSVC that clang-cl impersonates, and also implying what version of the MSVC runtime is being used.

libcxx/include/__config

 #if __has_extension(blocks)
 #  define _LIBCPP_HAS_EXTENSION_BLOCKS
 #endif
 
 #if defined(_LIBCPP_HAS_EXTENSION_BLOCKS) && defined(__APPLE__)
 #  define _LIBCPP_HAS_BLOCKS_RUNTIME
 #endif
 
-#if !(__has_feature(cxx_relaxed_constexpr))
-#define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
-#endif
-
 #if !(__has_feature(cxx_variable_templates))
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #endif
 
 #if !(__has_feature(cxx_noexcept))
 #define _LIBCPP_HAS_NO_NOEXCEPT
 #endif
 

 #define _ALIGNAS_TYPE(x) __attribute__((__aligned__(_LIBCPP_ALIGNOF(x))))
 
 #define _LIBCPP_NORETURN __attribute__((noreturn))
 
 #if !defined(__EXCEPTIONS)
 #  define _LIBCPP_NO_EXCEPTIONS
 #endif
 
-// Determine if GCC supports relaxed constexpr
-#if !defined(__cpp_constexpr) || __cpp_constexpr < 201304L
-#define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
-#endif
-
 // GCC 5 supports variable templates
 #if !defined(__cpp_variable_templates) || __cpp_variable_templates < 201304L
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #endif
 
 #if !defined(__SANITIZE_ADDRESS__)
 #define _LIBCPP_HAS_NO_ASAN
 #endif
 
 #if _GNUC_VER >= 700
 #define _LIBCPP_COMPILER_HAS_BUILTIN_LAUNDER
 #define _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
 #endif
 
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
 #define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
 
 #elif defined(_LIBCPP_COMPILER_MSVC)
 
 #define _LIBCPP_TOSTRING2(x) #x
 #define _LIBCPP_TOSTRING(x) _LIBCPP_TOSTRING2(x)
 #define _LIBCPP_WARNING(x) __pragma(message(__FILE__ "(" _LIBCPP_TOSTRING(__LINE__) ") : warning note: " x))
 
 #if _MSC_VER < 1900
 #error "MSVC versions prior to Visual Studio 2015 are not supported"
 #endif
 
-#define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #define __alignof__ __alignof
 #define _LIBCPP_NORETURN __declspec(noreturn)
 #define _ALIGNAS(x) __declspec(align(x))
 #define _ALIGNAS_TYPE(x) alignas(x)
 
 #define _LIBCPP_WEAK
 

 #endif
 
 #if _LIBCPP_STD_VER <= 11
 #  define _LIBCPP_EXPLICIT_AFTER_CXX11
 #else
 #  define _LIBCPP_EXPLICIT_AFTER_CXX11 explicit
 #endif
 
-#if _LIBCPP_STD_VER > 11 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
+#if _LIBCPP_STD_VER > 11
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX11 constexpr
 #else
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX11
 #endif
 
-#if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
+#if _LIBCPP_STD_VER > 14
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX14 constexpr
 #else
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX14
 #endif
 
-#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
+#if _LIBCPP_STD_VER > 17
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17 constexpr
 #else
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17
 #endif
 
 // The _LIBCPP_NODISCARD_ATTRIBUTE should only be used to define other
 // NODISCARD macros to the correct attribute.
 #if __has_cpp_attribute(nodiscard) || defined(_LIBCPP_COMPILER_MSVC)

 #endif
 
 #if _LIBCPP_STD_VER > 14 && defined(__cpp_inline_variables) && (__cpp_inline_variables >= 201606L)
 #  define _LIBCPP_INLINE_VAR inline
 #else
 #  define _LIBCPP_INLINE_VAR
 #endif
 
-#if defined(_LIBCPP_DEBUG) || defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
-#   define _LIBCPP_CONSTEXPR_IF_NODEBUG
-#else
+#if !defined(_LIBCPP_DEBUG) && _LIBCPP_STD_VER > 11
 #   define _LIBCPP_CONSTEXPR_IF_NODEBUG constexpr
+#else
+#   define _LIBCPP_CONSTEXPR_IF_NODEBUG
 #endif
 
 #if __has_attribute(no_destroy)
 #  define _LIBCPP_NO_DESTROY __attribute__((__no_destroy__))
 #else
 #  define _LIBCPP_NO_DESTROY
 #endif
 

libcxx/test/libcxx/selftest/test_macros.pass.cpp

 
 void test_noexcept() TEST_NOEXCEPT
 {
 }
 
 void test_libcxx_macros()
 {
 //  ===== C++14 features =====
-//  defined(TEST_HAS_EXTENDED_CONSTEXPR)  != defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
-#ifdef TEST_HAS_EXTENDED_CONSTEXPR
-# ifdef _LIBCPP_HAS_NO_CXX14_CONSTEXPR
-#  error "TEST_EXTENDED_CONSTEXPR mismatch (1)"
-# endif
-#else
-# ifndef _LIBCPP_HAS_NO_CXX14_CONSTEXPR
-#  error "TEST_EXTENDED_CONSTEXPR mismatch (2)"
-# endif
-#endif
 
 //  defined(TEST_HAS_VARIABLE_TEMPLATES) != defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)
 #ifdef TEST_HAS_VARIABLE_TEMPLATES
 # ifdef _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #  error "TEST_VARIABLE_TEMPLATES mismatch (1)"
 # endif
 #else
 # ifndef _LIBCPP_HAS_NO_VARIABLE_TEMPLATES

libcxx/test/support/test_macros.h

 #if TEST_STD_VER >= 20
 # define TEST_CONSTEXPR_CXX20 constexpr
 #else
 # define TEST_CONSTEXPR_CXX20
 #endif
 
 /* Features that were introduced in C++14 */
 #if TEST_STD_VER >= 14
-#define TEST_HAS_EXTENDED_CONSTEXPR
 #define TEST_HAS_VARIABLE_TEMPLATES
 #endif
 
 /* Features that were introduced in C++17 */
 #if TEST_STD_VER >= 17
 #endif
 
 /* Features that were introduced after C++17 */