[committed] libstdc++: Improve diagnostics for invalid std::format calls

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux. Pushed to trunk.

-- >8 --

Add a static_assert and a comment so that calling std::format for
unformattable argument types will now show:

/home/jwakely/gcc/13/include/c++/13.0.1/format:3563:22: error: static assertion 
failed: std::formatter must be specialized for each format arg
 3563 |       static_assert((is_default_constructible_v<formatter<_Args, 
_CharT>> && ...),
      |                      
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and:

  140 |       formatter() = delete; // No std::formatter specialization for 
this type.

libstdc++-v3/ChangeLog:

        * include/std/format (formatter): Add comment to deleted default
        constructor of primary template.
        (_Checking_scanner): Add static_assert.
---
 libstdc++-v3/include/std/format | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 72b6b450ad1..e4ef4f9b6d9 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -137,7 +137,7 @@ namespace __format
   template<typename _Tp, typename _CharT = char>
     struct formatter
     {
-      formatter() = delete;
+      formatter() = delete; // No std::formatter specialization for this type.
       formatter(const formatter&) = delete;
       formatter& operator=(const formatter&) = delete;
     };
@@ -3560,6 +3560,10 @@ namespace __format
   template<typename _CharT, typename... _Args>
     class _Checking_scanner : public _Scanner<_CharT>
     {
+      static_assert(
+       (is_default_constructible_v<formatter<_Args, _CharT>> && ...),
+       "std::formatter must be specialized for each type being formatted");
+
     public:
       constexpr
       _Checking_scanner(basic_string_view<_CharT> __str)
@@ -3581,17 +3585,17 @@ namespace __format
        __builtin_unreachable();
       }
 
-      template<typename _Head, typename... _Tail>
+      template<typename _Tp, typename... _OtherArgs>
        constexpr void
        _M_parse_format_spec(size_t __id)
        {
          if (__id == 0)
            {
-             formatter<_Head, _CharT> __f;
+             formatter<_Tp, _CharT> __f;
              this->_M_pc.advance_to(__f.parse(this->_M_pc));
            }
-         else if constexpr (sizeof...(_Tail) != 0)
-           _M_parse_format_spec<_Tail...>(__id - 1);
+         else if constexpr (sizeof...(_OtherArgs) != 0)
+           _M_parse_format_spec<_OtherArgs...>(__id - 1);
          else
            __builtin_unreachable();
        }
-- 
2.39.2