[gcc r11-4590] libstdc++: Define type traits for wchar_t even when libc support missing

[Jonathan Wakely]

https://gcc.gnu.org/g:29e418485848c4a6943d8561cd8fb0b1abf14015

commit r11-4590-g29e418485848c4a6943d8561cd8fb0b1abf14015
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sun Nov 1 10:56:36 2020 +0000

    libstdc++: Define type traits for wchar_t even when libc support missing
    
    This meets the requirement that std::is_integral_v<wchar_t> is true,
    even when full library support for wchar_t via specializations of
    char_traits etc. is not provided. This is done by checking
    __WCHAR_TYPE__ to see if the compiler knows about the type, rather than
    checking the library's own _GLIBCXX_USE_WCHAR_T autoconf macro.
    
    This assumes that the C++ compiler correctly defines wchar_t as a
    distinct type, not a typedef for one of the other integeral types. This
    is always true for G++ and should be true for any supported non-GNU
    compilers.
    
    Similarly, the std::make_unsigned and std::make_signed traits and the
    internal helpers std::__is_integer and std::__is_char are also changed
    to depend on the same macro.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (is_integral<wchar_t>)
            (make_unsigned<wchar_t>, make_signed<wchar_t>): Define based
            on #ifdef __WCHAR_TYPE__ instead of _GLIBCXX_USE_WCHAR_T.
            * include/bits/cpp_type_traits.h (__is_integer<wchar_t>)
            (__is_char<wchar_t>): Likewise.

Diff:
---
 libstdc++-v3/include/bits/cpp_type_traits.h | 4 ++--
 libstdc++-v3/include/std/type_traits        | 9 ++++++---
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
index b48d1adc63c..433a2d7d35b 100644
--- a/libstdc++-v3/include/bits/cpp_type_traits.h
+++ b/libstdc++-v3/include/bits/cpp_type_traits.h
@@ -162,7 +162,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef __true_type __type;
     };
 
-# ifdef _GLIBCXX_USE_WCHAR_T
+# ifdef __WCHAR_TYPE__
   template<>
     struct __is_integer<wchar_t>
     {
@@ -363,7 +363,7 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
       typedef __true_type __type;
     };
 
-#ifdef _GLIBCXX_USE_WCHAR_T
+#ifdef __WCHAR_TYPE__
   template<>
     struct __is_char<wchar_t>
     {
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index e9a0f55dd4a..34e068b5952 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -269,7 +269,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct __is_integral_helper<unsigned char>
     : public true_type { };
 
-#ifdef _GLIBCXX_USE_WCHAR_T
+  // We want is_integral<wchar_t> to be true (and make_signed/unsigned to work)
+  // even when libc doesn't provide working <wchar.h> and related functions,
+  // so check __WCHAR_TYPE__ instead of _GLIBCXX_USE_WCHAR_T.
+#ifdef __WCHAR_TYPE__
   template<>
     struct __is_integral_helper<wchar_t>
     : public true_type { };
@@ -1742,7 +1745,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // neither signed integer types nor unsigned integer types, so must be
   // transformed to the unsigned integer type with the smallest rank.
   // Use the partial specialization for enumeration types to do that.
-#if defined(_GLIBCXX_USE_WCHAR_T)
+#ifdef __WCHAR_TYPE__
   template<>
     struct __make_unsigned<wchar_t>
     {
@@ -1868,7 +1871,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // signed integer types nor unsigned integer types, so must be
   // transformed to the signed integer type with the smallest rank.
   // Use the partial specialization for enumeration types to do that.
-#if defined(_GLIBCXX_USE_WCHAR_T)
+#if defined(__WCHAR_TYPE__)
   template<>
     struct __make_signed<wchar_t>
     {