1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9// UNSUPPORTED: c++03, c++11, c++14, c++17
10
11// XFAIL: availability-fp_to_chars-missing
12
13// The sample code is based on the bug report
14// https://github.com/llvm/llvm-project/issues/81590
15//
16// Tests whether this formatter does not fail to compile due to nested concept
17// evaluation.
18
19#include <format>
20#include <variant>
21
22struct X : std::variant<X*> {
23 X* p = nullptr;
24 constexpr const std::variant<X*>& decay() const noexcept { return *this; }
25};
26
27template <>
28struct std::formatter<X, char> : std::formatter<std::string, char> {
29 static constexpr auto format(const X& x, auto& ctx) {
30 if (!x.p)
31 return ctx.out();
32 auto m = [&](const X* t) { return std::format_to(ctx.out(), "{}", *t); };
33 return std::visit(m, x.decay());
34 }
35};
36
37void bug_81590() { (void)std::format("{}", X{}); }
38

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