P1423R3 char8_t compatibility remedies (#470)

Deletes stream insertion operators for `ostream` with non-`char` character types, and for `wostream` with `charX_t` character types. The `char8_t` operators are deleted in all language modes, but the others are C++20-only to avoid gratuitous breakage (with escape hatch `_HAS_STREAM_INSERTIONS_REMOVED_IN_CXX20`).

Skips libc++ tests that expect the pre-P1423R3 value of `__cpp_lib_char8_t`.

Resolves #59.

stl/inc/ostream

@@ -912,14 +912,48 @@ basic_ostream<char, _Traits>& operator<<(
 }
 
 #ifdef __cpp_char8_t // These deleted overloads are specified in P1423.
+// don't insert a UTF-8 NTBS
 template <class _Traits>
-basic_ostream<char, _Traits>& operator<<(
-    basic_ostream<char, _Traits>&, const char8_t*) = delete; // don't insert a UTF-8 NTBS
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, const char8_t*) = delete;
+template <class _Traits>
+basic_ostream<wchar_t, _Traits>& operator<<(basic_ostream<wchar_t, _Traits>&, const char8_t*) = delete;
+
+// don't insert a UTF-8 code unit
+template <class _Traits>
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, char8_t) = delete;
+template <class _Traits>
+basic_ostream<wchar_t, _Traits>& operator<<(basic_ostream<wchar_t, _Traits>&, char8_t) = delete;
+#endif // __cpp_char8_t
+
+#if !_HAS_STREAM_INSERTION_OPERATORS_DELETED_IN_CXX20
+#ifdef _NATIVE_WCHAR_T_DEFINED
+template <class _Traits>
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, wchar_t) = delete;
 
 template <class _Traits>
-basic_ostream<char, _Traits>& operator<<(
-    basic_ostream<char, _Traits>&, char8_t) = delete; // don't insert a UTF-8 code unit
-#endif // __cpp_char8_t
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, const wchar_t*) = delete;
+#endif // _NATIVE_WCHAR_T_DEFINED
+
+template <class _Traits>
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, char16_t) = delete;
+template <class _Traits>
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, char32_t) = delete;
+
+template <class _Traits>
+basic_ostream<wchar_t, _Traits>& operator<<(basic_ostream<wchar_t, _Traits>&, char16_t) = delete;
+template <class _Traits>
+basic_ostream<wchar_t, _Traits>& operator<<(basic_ostream<wchar_t, _Traits>&, char32_t) = delete;
+
+template <class _Traits>
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, const char16_t*) = delete;
+template <class _Traits>
+basic_ostream<char, _Traits>& operator<<(basic_ostream<char, _Traits>&, const char32_t*) = delete;
+
+template <class _Traits>
+basic_ostream<wchar_t, _Traits>& operator<<(basic_ostream<wchar_t, _Traits>&, const char16_t*) = delete;
+template <class _Traits>
+basic_ostream<wchar_t, _Traits>& operator<<(basic_ostream<wchar_t, _Traits>&, const char32_t*) = delete;
+#endif // !_HAS_STREAM_INSERTION_OPERATORS_DELETED_IN_CXX20
 
 template <class _Ostr, class _Ty, class = void>
 struct _Can_stream_out : false_type {};

stl/inc/yvals_core.h

@@ -173,6 +173,7 @@
 // P1227R2 Signed std::ssize(), Unsigned span::size()
 // P1357R1 is_bounded_array, is_unbounded_array
 // P1394R4 Range Constructor For span
+// P1423R3 char8_t Backward Compatibility Remediation
 // P1456R1 Move-Only Views
 // P1612R1 Relocating endian To <bit>
 // P1645R1 constexpr For <numeric> Algorithms
@@ -586,6 +587,12 @@
 #endif // _M_FP_EXCEPT
 #endif // _STD_VECTORIZE_WITH_FLOAT_CONTROL
 
+// P1423R3 char8_t Backward Compatibility Remediation
+// Controls whether we allow the stream insertions this proposal forbids
+#ifndef _HAS_STREAM_INSERTION_OPERATORS_DELETED_IN_CXX20
+#define _HAS_STREAM_INSERTION_OPERATORS_DELETED_IN_CXX20 (!_HAS_CXX20)
+#endif // _HAS_STREAM_INSERTION_OPERATORS_DELETED_IN_CXX20
+
 // P0174R2 Deprecating Vestigial Library Parts
 // P0521R0 Deprecating shared_ptr::unique()
 // Other C++17 deprecation warnings
@@ -1008,7 +1015,7 @@
 #define __cpp_lib_bounded_array_traits 201902L
 
 #ifdef __cpp_char8_t
-#define __cpp_lib_char8_t 201811L
+#define __cpp_lib_char8_t 201907L
 #endif // __cpp_char8_t
 
 #if defined(__cpp_concepts) && __cpp_concepts > 201507L

tests/libcxx/skipped_tests.txt

@@ -51,6 +51,13 @@ containers\unord\unord.set\insert_and_emplace_allocator_requirements.pass.cpp
 # See https://reviews.llvm.org/D73138
 containers\views\span.sub\subspan.pass.cpp
 
+# libcxx doesn't yet implement P1423R3, so it expects an "old" value for __cpp_lib_char8_t
+language.support\support.limits\support.limits.general\filesystem.version.pass.cpp
+language.support\support.limits\support.limits.general\istream.version.pass.cpp
+language.support\support.limits\support.limits.general\limits.version.pass.cpp
+language.support\support.limits\support.limits.general\locale.version.pass.cpp
+language.support\support.limits\support.limits.general\ostream.version.pass.cpp
+
 
 # *** INTERACTIONS WITH CONTEST / C1XX THAT UPSTREAM LIKELY WON'T FIX ***
 # Tracked by VSO-593630 "<filesystem> Enable libcxx filesystem tests"