libstdc++: Fix std::basic_format_arg::handle for BasicFormatters

std::basic_format_arg::handle is supposed to format its value as const
if that is valid, to reduce the number of instantiations of the
formatter's format function. I made a silly typo so that it checks
formattable_with<TD, Context> not formattable_with<const TD, Context>,
which breaks support for BasicFormatters i.e. ones that can only format
non-const types.

There's a static_assert in the handle constructor which is supposed to
improve diagnostics for trying to format a const argument with a
formatter that doesn't support it. That condition can't fail, because
the std::basic_format_arg constructor is already constrained to check
that the argument type is formattable. The static_assert can be removed.

libstdc++-v3/ChangeLog:

	* include/std/format (basic_format_arg::handle::__maybe_const_t):
	Fix condition to check if const type is formattable.
	(basic_format_arg::handle::handle(T&)): Remove redundant
	static_assert.
	* testsuite/std/format/formatter/basic.cc: New test.

1 files changed, 1 insertions, 5 deletions

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 961441e..ee189f9 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -3210,7 +3210,7 @@ namespace __format
 	// Format as const if possible, to reduce instantiations.
 	template<typename _Tp>
 	  using __maybe_const_t
-	    = __conditional_t<__formattable<_Tp>, const _Tp, _Tp>;
+	    = __conditional_t<__formattable<const _Tp>, const _Tp, _Tp>;
 
 	template<typename _Tq>
 	  static void
@@ -3228,10 +3228,6 @@ namespace __format
 	  explicit
 	  handle(_Tp& __val) noexcept
 	  {
-	    if constexpr (!__formattable<const _Tp>)
-	      static_assert(!is_const_v<_Tp>, "std::format argument must be "
-					      "non-const for this type");
-
 	    this->_M_ptr = __builtin_addressof(__val);
 	    auto __func = _S_format<__maybe_const_t<_Tp>>;
 	    this->_M_func = reinterpret_cast<void(*)()>(__func);