[libc++] Keep char_traits<T> for arbitrary T around until LLVM 18

[ldionne]

Summary

This is in response to failures seen after landing D138307.

[ldionne]

Fix more tests to address CI failures.

[ldionne]

libcxx/test/std/utilities/format/format.formattable/concept.formattable.compile.pass.cpp

84–87

@Mordante Did you really mean to test std::format with a character type of int? Is that meaningful?

[ldionne]

Note that I'll land this as soon as I get a green CI run to unbreak the downstreams that were impacted by D138307. We can make additional tweaks post-commit as needed.

[ldionne]

Upon looking at the Bootstrapping build issue, I'm pretty sure it's not related to this patch. I'll land this now.

[Mordante]

libcxx/test/std/utilities/format/format.formattable/concept.formattable.compile.pass.cpp

84–87

Yes that is intended. When you look at assert_is_formattable the only formattable CharT's are char and wchar_t. So char8_t, char16_t, char32_t and non character types like int are never formattable.

I agree with the change; I didn't noticed before since the basic_string and basic_string_view "worked" with ints. Thanks for the fix.

libcxx/docs/ReleaseNotes.rst

 
 - The contents of ``<experimental/memory_resource>`` are now deprecated since libc++ ships ``<memory_resource>`` now.
   Please migrate to ``<memory_resource>`` instead. Per libc++'s TS deprecation policy,
   ``<experimental/memory_resource>`` will be removed in LLVM 18.
 
 - The ``_LIBCPP_DEBUG`` macro is not honored anymore, and it is an error to try to use it. Please migrate to
   ``_LIBCPP_ENABLE_DEBUG_MODE`` instead.
 
-- A base template for ``std::char_traits`` is not provided anymore. The Standard mandates that the library
-  provides specializations for several types like ``char`` and ``wchar_t``, which libc++ does. However, libc++
-  used to additionally provide a default implementation for ``std::char_traits<T>`` for arbitrary ``T``. Not
-  only does the Standard not mandate that one is provided, but such an implementation is bound to be incorrect
-  for some types, so it has been removed. As an exception, ``std::char_traits<unsigned char>`` and
-  ``std::char_traits<signed char>`` are kept for a limited period of time and marked as deprecated to let people
-  move off of those, since we know there were some users of those. They will be removed in LLVM 18.
-
 Upcoming Deprecations and Removals
 ----------------------------------
-- The specializations of ``std::char_traits`` for ``unsigned char`` and ``signed char`` are provided until
-  LLVM 18. Those non-standard specializations are provided for a transition period and marked as deprecated
-  but will be removed in the future.
+- The base template for ``std::char_traits`` has been marked as deprecated and will be removed in LLVM 18. If
+  you are using ``std::char_traits`` with types other than ``char``, ``wchar_t``, ``char8_t``, ``char16_t``,
+  ``char32_t`` or a custom character type for which you specialized ``std::char_traits``, your code will stop
+  working when we remove the base template. The Standard does not mandate that a base template is provided,
+  and such a base template is bound to be incorrect for some types, which could currently cause unexpected
+  behavior while going undetected.
 
 API Changes
 -----------
 - The comparison operators on ``thread::id`` are now defined as free-standing
   functions instead of as hidden friends, in conformance with the C++ standard.
   Also see `issue 56187 <https://github.com/llvm/llvm-project/issues/56187>`_.
 
 - ``_LIBCPP_ENABLE_NODISCARD`` and ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17`` are no longer respected.

libcxx/include/__string/char_traits.h

exposition-only to document what members a char_traits specialization should provide:
     static void assign(char_type&, const char_type&);
     static bool eq(char_type, char_type);
     static bool lt(char_type, char_type);
 
     static int              compare(const char_type*, const char_type*, size_t);
     static size_t           length(const char_type*);
     static const char_type* find(const char_type*, size_t, const char_type&);
     static char_type*       move(char_type*, const char_type*, size_t);
-    static char_type*       copy(char_type*, const char_type* __s2, size_t);
+    static char_type*       copy(char_type*, const char_type*, size_t);
     static char_type*       assign(char_type*, size_t, char_type);
 
     static int_type  not_eof(int_type);
     static char_type to_char_type(int_type);
     static int_type  to_int_type(char_type);
     static bool      eq_int_type(int_type, int_type);
     static int_type  eof();
 };
 */
 
+//
+// Temporary extension to provide a base template for std::char_traits.
+// TODO: Remove in LLVM 18.
+//
+template <class _CharT>
+struct _LIBCPP_DEPRECATED_("char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 18, so please migrate off of it.")
+    char_traits
+{
+    using char_type  = _CharT;
+    using int_type   = int;
+    using off_type   = streamoff;
+    using pos_type   = streampos;
+    using state_type = mbstate_t;
+
+    static inline void _LIBCPP_CONSTEXPR_SINCE_CXX17
+        assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
+    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
+        {return __c1 == __c2;}
+    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
+        {return __c1 < __c2;}
+
+    static _LIBCPP_CONSTEXPR_SINCE_CXX17
+    int compare(const char_type* __s1, const char_type* __s2, size_t __n) {
+        for (; __n; --__n, ++__s1, ++__s2)
+        {
+            if (lt(*__s1, *__s2))
+                return -1;
+            if (lt(*__s2, *__s1))
+                return 1;
+        }
+        return 0;
+    }
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_SINCE_CXX17
+    size_t length(const char_type* __s) {
+        size_t __len = 0;
+        for (; !eq(*__s, char_type(0)); ++__s)
+            ++__len;
+        return __len;
+    }
+    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_SINCE_CXX17
+    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) {
+        for (; __n; --__n)
+        {
+            if (eq(*__s, __a))
+                return __s;
+            ++__s;
+        }
+        return nullptr;
+    }
+    static _LIBCPP_CONSTEXPR_SINCE_CXX20
+    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) {
+        if (__n == 0) return __s1;
+        char_type* __r = __s1;
+        if (__s1 < __s2)
+        {
+            for (; __n; --__n, ++__s1, ++__s2)
+                assign(*__s1, *__s2);
+        }
+        else if (__s2 < __s1)
+        {
+            __s1 += __n;
+            __s2 += __n;
+            for (; __n; --__n)
+                assign(*--__s1, *--__s2);
+        }
+        return __r;
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    static _LIBCPP_CONSTEXPR_SINCE_CXX20
+    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) {
+        if (!__libcpp_is_constant_evaluated()) {
+            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
+        }
+        char_type* __r = __s1;
+        for (; __n; --__n, ++__s1, ++__s2)
+            assign(*__s1, *__s2);
+        return __r;
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    static _LIBCPP_CONSTEXPR_SINCE_CXX20
+    char_type*       assign(char_type* __s, size_t __n, char_type __a) {
+        char_type* __r = __s;
+        for (; __n; --__n, ++__s)
+            assign(*__s, __a);
+        return __r;
+    }
+
+    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
+        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
+    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
+        {return char_type(__c);}
+    static inline _LIBCPP_CONSTEXPR int_type  to_int_type(char_type __c) _NOEXCEPT
+        {return int_type(__c);}
+    static inline _LIBCPP_CONSTEXPR bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
+        {return __c1 == __c2;}
+    static inline _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
+        {return int_type(EOF);}
+};
+
 template <class _CharT>
 _LIBCPP_HIDE_FROM_ABI static inline _LIBCPP_CONSTEXPR_SINCE_CXX20
 _CharT* __char_traits_move(_CharT* __dest, const _CharT* __source, size_t __n) _NOEXCEPT
 {
 #ifdef _LIBCPP_COMPILER_GCC
   if (__libcpp_is_constant_evaluated()) {
     if (__n == 0)
       return __dest;
char_traits<char32_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
     {
         if (eq(*__s, __a))
             return __s;
         ++__s;
     }
     return nullptr;
 }
 
-//
-// Temporary extensions for std::char_traits<unsigned char> and std::char_traits<signed char>.
-// TODO: Remove those in LLVM 18.
-//
-template <>
-struct _LIBCPP_TEMPLATE_VIS
-    _LIBCPP_DEPRECATED_("char_traits<unsigned char> is non-standard and is provided for a temporary period. It will be removed in LLVM 18, so please migrate off of it.")
-    char_traits<unsigned char>
-{
-    using char_type  = unsigned char;
-    using int_type   = int;
-    using off_type   = streamoff;
-    using pos_type   = streampos;
-    using state_type = mbstate_t;
-
-    static inline void _LIBCPP_CONSTEXPR_SINCE_CXX17
-        assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
-    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
-        {return __c1 == __c2;}
-    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
-        {return __c1 < __c2;}
-
-    static _LIBCPP_CONSTEXPR_SINCE_CXX17
-    int compare(const char_type* __s1, const char_type* __s2, size_t __n) {
-        for (; __n; --__n, ++__s1, ++__s2)
-        {
-            if (lt(*__s1, *__s2))
-                return -1;
-            if (lt(*__s2, *__s1))
-                return 1;
-        }
-        return 0;
-    }
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_SINCE_CXX17
-    size_t length(const char_type* __s) {
-        size_t __len = 0;
-        for (; !eq(*__s, char_type(0)); ++__s)
-            ++__len;
-        return __len;
-    }
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_SINCE_CXX17
-    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) {
-        for (; __n; --__n)
-        {
-            if (eq(*__s, __a))
-                return __s;
-            ++__s;
-        }
-        return nullptr;
-    }
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) {
-        if (__n == 0) return __s1;
-        char_type* __r = __s1;
-        if (__s1 < __s2)
-        {
-            for (; __n; --__n, ++__s1, ++__s2)
-                assign(*__s1, *__s2);
-        }
-        else if (__s2 < __s1)
-        {
-            __s1 += __n;
-            __s2 += __n;
-            for (; __n; --__n)
-                assign(*--__s1, *--__s2);
-        }
-        return __r;
-    }
-    _LIBCPP_INLINE_VISIBILITY
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) {
-        if (!__libcpp_is_constant_evaluated()) {
-            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
-        }
-        char_type* __r = __s1;
-        for (; __n; --__n, ++__s1, ++__s2)
-            assign(*__s1, *__s2);
-        return __r;
-    }
-    _LIBCPP_INLINE_VISIBILITY
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       assign(char_type* __s, size_t __n, char_type __a) {
-        char_type* __r = __s;
-        for (; __n; --__n, ++__s)
-            assign(*__s, __a);
-        return __r;
-    }
-
-    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
-        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
-    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
-        {return char_type(__c);}
-    static inline _LIBCPP_CONSTEXPR int_type  to_int_type(char_type __c) _NOEXCEPT
-        {return int_type(__c);}
-    static inline _LIBCPP_CONSTEXPR bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
-        {return __c1 == __c2;}
-    static inline _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
-        {return int_type(EOF);}
-};
-
-template <>
-struct _LIBCPP_TEMPLATE_VIS
-    _LIBCPP_DEPRECATED_("char_traits<signed char> is non-standard and is provided for a temporary period. It will be removed in LLVM 18, so please migrate off of it.")
-    char_traits<signed char>
-{
-    using char_type  = signed char;
-    using int_type   = int;
-    using off_type   = streamoff;
-    using pos_type   = streampos;
-    using state_type = mbstate_t;
-
-    static inline void _LIBCPP_CONSTEXPR_SINCE_CXX17
-        assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
-    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
-        {return __c1 == __c2;}
-    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
-        {return __c1 < __c2;}
-
-    static _LIBCPP_CONSTEXPR_SINCE_CXX17
-    int compare(const char_type* __s1, const char_type* __s2, size_t __n) {
-        for (; __n; --__n, ++__s1, ++__s2)
-        {
-            if (lt(*__s1, *__s2))
-                return -1;
-            if (lt(*__s2, *__s1))
-                return 1;
-        }
-        return 0;
-    }
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_SINCE_CXX17
-    size_t length(const char_type* __s) {
-        size_t __len = 0;
-        for (; !eq(*__s, char_type(0)); ++__s)
-            ++__len;
-        return __len;
-    }
-    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_SINCE_CXX17
-    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) {
-        for (; __n; --__n)
-        {
-            if (eq(*__s, __a))
-                return __s;
-            ++__s;
-        }
-        return nullptr;
-    }
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) {
-        if (__n == 0) return __s1;
-        char_type* __r = __s1;
-        if (__s1 < __s2)
-        {
-            for (; __n; --__n, ++__s1, ++__s2)
-                assign(*__s1, *__s2);
-        }
-        else if (__s2 < __s1)
-        {
-            __s1 += __n;
-            __s2 += __n;
-            for (; __n; --__n)
-                assign(*--__s1, *--__s2);
-        }
-        return __r;
-    }
-    _LIBCPP_INLINE_VISIBILITY
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) {
-        if (!__libcpp_is_constant_evaluated()) {
-            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
-        }
-        char_type* __r = __s1;
-        for (; __n; --__n, ++__s1, ++__s2)
-            assign(*__s1, *__s2);
-        return __r;
-    }
-    _LIBCPP_INLINE_VISIBILITY
-    static _LIBCPP_CONSTEXPR_SINCE_CXX20
-    char_type*       assign(char_type* __s, size_t __n, char_type __a) {
-        char_type* __r = __s;
-        for (; __n; --__n, ++__s)
-            assign(*__s, __a);
-        return __r;
-    }
-
-    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
-        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
-    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
-        {return char_type(__c);}
-    static inline _LIBCPP_CONSTEXPR int_type  to_int_type(char_type __c) _NOEXCEPT
-        {return int_type(__c);}
-    static inline _LIBCPP_CONSTEXPR bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
-        {return __c1 == __c2;}
-    static inline _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
-        {return int_type(EOF);}
-};
-
 // helper fns for basic_string and string_view
 
 // __str_find
 template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
 inline _SizeT _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_INLINE_VISIBILITY
 __str_find(const _CharT *__p, _SizeT __sz,
              _CharT __c, _SizeT __pos) _NOEXCEPT
 {

libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.deprecated.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // <string>
 
-// template<> struct char_traits<unsigned char>
-// template<> struct char_traits<signed char>
+// template<> struct char_traits<T> for arbitrary T
 
 // Make sure we issue deprecation warnings.
 
 #include <string>
 
 void f() {
-    std::char_traits<unsigned char> uc; // expected-warning{{'char_traits<unsigned char>' is deprecated}}
-    std::char_traits<signed char> sc; // expected-warning{{'char_traits<signed char>' is deprecated}}
-
-    (void)uc;
-    (void)sc;
+    std::char_traits<unsigned char> t1; (void)t1; // expected-warning{{'char_traits<unsigned char>' is deprecated}}
+    std::char_traits<signed char> t2; (void)t2; // expected-warning{{'char_traits<signed char>' is deprecated}}
+    std::char_traits<unsigned long> t3; (void)t3; // expected-warning{{'char_traits<unsigned long>' is deprecated}}
 }

libcxx/test/libcxx/strings/char.traits/char.traits.specializations/arbitrary_char_type.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // <string>
 
-// template<> struct char_traits<unsigned char>
-// template<> struct char_traits<signed char>
+// template<> struct char_traits<T> for arbitrary T
 
 // Non-standard but provided temporarily for users to migrate.
 
 // ADDITIONAL_COMPILE_FLAGS: -Wno-deprecated
 
 #include <string>
 #include <cassert>
 #include <type_traits>
TEST_CONSTEXPR_CXX20 bool test() {
     }
 
     return true;
 }
 
 int main(int, char**) {
     test<unsigned char>();
     test<signed char>();
+    test<unsigned long>();
 
 #if TEST_STD_VER > 17
     static_assert(test<unsigned char>());
     static_assert(test<signed char>());
+    static_assert(test<unsigned long>());
 #endif
 
   return 0;
 }

libcxx/test/std/input.output/iostream.forward/iosfwd.pass.cpp

 }
 
 int main(int, char**)
 {
     test<std::char_traits<char>*          >();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     test<std::char_traits<wchar_t>*       >();
 #endif
-    test<std::char_traits<unsigned short>*>();
 
     test<std::basic_ios<char>*          >();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     test<std::basic_ios<wchar_t>*       >();
 #endif
     test<std::basic_ios<unsigned short>*>();
 
     test<std::basic_streambuf<char>*          >();

libcxx/test/std/strings/basic.string.hash/enabled_hashes.pass.cpp

 #include "poisoned_hash_helper.h"
 #include "test_allocator.h"
 #include "test_macros.h"
 
 struct MyChar {
   char c;
 };
 
+template <>
+struct std::char_traits<MyChar> {
+  using char_type  = MyChar;
+  using int_type   = std::char_traits<char>::int_type;
+  using off_type   = std::char_traits<char>::off_type;
+  using pos_type   = std::char_traits<char>::pos_type;
+  using state_type = std::char_traits<char>::state_type;
+
+  static void assign(char_type&, const char_type&);
+  static bool eq(char_type, char_type);
+  static bool lt(char_type, char_type);
+
+  static int              compare(const char_type*, const char_type*, size_t);
+  static size_t           length(const char_type*);
+  static const char_type* find(const char_type*, size_t, const char_type&);
+  static char_type*       move(char_type*, const char_type*, size_t);
+  static char_type*       copy(char_type*, const char_type*, size_t);
+  static char_type*       assign(char_type*, size_t, char_type);
+
+  static int_type  not_eof(int_type);
+  static char_type to_char_type(int_type);
+  static int_type  to_int_type(char_type);
+  static bool      eq_int_type(int_type, int_type);
+  static int_type  eof();
+};
+
 int main(int, char**) {
   test_library_hash_specializations_available();
   {
     test_hash_enabled_for_type<std::string>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     test_hash_enabled_for_type<std::wstring>();
 #endif
 #ifndef TEST_HAS_NO_CHAR8_T

libcxx/test/std/strings/string.view/string.view.hash/enabled_hashes.pass.cpp

 #include "constexpr_char_traits.h"
 #include "poisoned_hash_helper.h"
 #include "test_macros.h"
 
 struct MyChar {
   char c;
 };
 
+template <>
+struct std::char_traits<MyChar> {
+  using char_type  = MyChar;
+  using int_type   = std::char_traits<char>::int_type;
+  using off_type   = std::char_traits<char>::off_type;
+  using pos_type   = std::char_traits<char>::pos_type;
+  using state_type = std::char_traits<char>::state_type;
+
+  static void assign(char_type&, const char_type&);
+  static bool eq(char_type, char_type);
+  static bool lt(char_type, char_type);
+
+  static int              compare(const char_type*, const char_type*, size_t);
+  static size_t           length(const char_type*);
+  static const char_type* find(const char_type*, size_t, const char_type&);
+  static char_type*       move(char_type*, const char_type*, size_t);
+  static char_type*       copy(char_type*, const char_type*, size_t);
+  static char_type*       assign(char_type*, size_t, char_type);
+
+  static int_type  not_eof(int_type);
+  static char_type to_char_type(int_type);
+  static int_type  to_int_type(char_type);
+  static bool      eq_int_type(int_type, int_type);
+  static int_type  eof();
+};
+
 int main(int, char**) {
   test_library_hash_specializations_available();
   {
     test_hash_enabled_for_type<std::string_view>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
     test_hash_enabled_for_type<std::wstring_view>();
 #endif
 #ifndef TEST_HAS_NO_CHAR8_T

libcxx/test/std/utilities/format/format.formattable/concept.formattable.compile.pass.cpp

#ifndef TEST_HAS_NO_WIDE_CHARACTERS
   // Tests the special formatter that converts a char to a wchar_t.
   assert_is_formattable<char, wchar_t>();
 #endif
   assert_is_formattable<CharT, CharT>();
 
   assert_is_formattable<CharT*, CharT>();
   assert_is_formattable<const CharT*, CharT>();
   assert_is_formattable<CharT[42], CharT>();
+  if constexpr (!std::same_as<CharT, int>) { // string and string_view only work with proper character types
     assert_is_formattable<std::basic_string<CharT>, CharT>();
     assert_is_formattable<std::basic_string_view<CharT>, CharT>();
+  }
 
   assert_is_formattable<bool, CharT>();
 
   assert_is_formattable<signed char, CharT>();
   assert_is_formattable<signed short, CharT>();
   assert_is_formattable<signed int, CharT>();
   assert_is_formattable<signed long, CharT>();
   assert_is_formattable<signed long long, CharT>();