[libc++][format] Fixes nested concept evaluation. (llvm#85548)

Before the __formattable concept depended on itself in a contrieved
example. By using the underlying concept directly the cycle is broken.

Fixes llvm#81590

Author: mordante

libcxx/include/__format/format_arg_store.h

@@ -151,7 +151,7 @@ consteval __arg_t __determine_arg_t() {
 // The overload for not formattable types allows triggering the static
 // assertion below.
 template <class _Context, class _Tp>
-  requires(!__formattable<_Tp, typename _Context::char_type>)
+  requires(!__formattable_with<_Tp, _Context>)
 consteval __arg_t __determine_arg_t() {
   return __arg_t::__none;
 }
@@ -165,7 +165,6 @@ _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __valu
   using _Dp               = remove_const_t<_Tp>;
   constexpr __arg_t __arg = __determine_arg_t<_Context, _Dp>();
   static_assert(__arg != __arg_t::__none, "the supplied type is not formattable");
-
   static_assert(__formattable_with<_Tp, _Context>);
 
   // Not all types can be used to directly initialize the

libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp

@@ -0,0 +1,36 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// XFAIL: availability-fp_to_chars-missing
+
+// The sample code is based on the bug report
+// https://github.com/llvm/llvm-project/issues/81590
+//
+// Tests whether this formatter does not fail to compile due to nested concept
+// evaluation.
+
+#include <format>
+#include <variant>
+
+struct X : std::variant<X*> {
+  X* p = nullptr;
+  constexpr const std::variant<X*>& decay() const noexcept { return *this; }
+};
+
+template <>
+struct std::formatter<X, char> : std::formatter<std::string, char> {
+  static constexpr auto format(const X& x, auto ctx) {
+    if (!x.p)
+      return ctx.out();
+    auto m = [&](const X* t) { return std::format_to(ctx.out(), "{}", *t); };
+    return std::visit(m, x.decay());
+  }
+};
+
+void bug_81590() { (void)std::format("{}", X{}); }