[libc++][format] Fixes constexpr validation.

[Mordante]

Summary

The constexpr validation parsed parts of the format string that didn't
belong to the specific replacement field.

Fixes https://llvm.org/PR60536

[ldionne]

This would be a good candidate for a cherry-pick if you want, it's a bug fix.

[Mordante]

Thanks for the quick review.

In D143402#4108937, @ldionne wrote:

This would be a good candidate for a cherry-pick if you want, it's a bug fix.

Agreed, I wanted to discuss that during review.

libcxx/include/__format/format_functions.h

case _CharT('}'):
     __parse_ctx.advance_to(__r.__last);
     break;
   default:
     std::__throw_format_error("The replacement field arg-id should terminate at a ':' or '}'");
   }
 
   if constexpr (same_as<_Ctx, __compile_time_basic_format_context<_CharT>>) {
     __arg_t __type = __ctx.arg(__r.__value);
-    if (__type == __arg_t::__handle)
+    if (__type == __arg_t::__none)
+      std::__throw_format_error("Argument index out of bounds");
+    else if (__type == __arg_t::__handle)
       __ctx.__handle(__r.__value).__parse(__parse_ctx);
-    else
-        __format::__compile_time_visit_format_arg(__parse_ctx, __ctx, __type);
+    else if (__parse)
   } else
     _VSTD::__visit_format_arg(
         [&](auto __arg) {
           if constexpr (same_as<decltype(__arg), monostate>)
             std::__throw_format_error("Argument index out of bounds");
           else if constexpr (same_as<decltype(__arg), typename basic_format_arg<_Ctx>::handle>)
             __arg.format(__parse_ctx, __ctx);
           else {

libcxx/test/std/utilities/format/format.functions/format_tests.h

 }
 
 template <class CharT, execution_modus modus, class TestFunction, class ExceptionTest>
 void format_tests(TestFunction check, ExceptionTest check_exception) {
   // *** Test escaping  ***
 
   check(SV("{"), SV("{{"));
   check(SV("}"), SV("}}"));
+  check(SV("{:^}"), SV("{{:^}}"));
+  check(SV("{: ^}"), SV("{{:{}^}}"), CharT(' '));
+  check(SV("{:{}^}"), SV("{{:{{}}^}}"));
+  check(SV("{:{ }^}"), SV("{{:{{{}}}^}}"), CharT(' '));
 
   // *** Test argument ID ***
   check(SV("hello false true"), SV("hello {0:} {1:}"), false, true);
   check(SV("hello true false"), SV("hello {1:} {0:}"), false, true);
 
   // *** Test many arguments ***
 
   // [format.args]/1