[gcc r13-7182] libstdc++: Improve diagnostics for invalid std::format calls

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:6a9547f3ca6c48f116478169d5fc62165dde7f43

commit r13-7182-g6a9547f3ca6c48f116478169d5fc62165dde7f43
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 13 16:34:51 2023 +0100

    libstdc++: Improve diagnostics for invalid std::format calls
    
    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.

Diff:
---
 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();
 	}