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

   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
  22 struct X : std::variant<X*> {
  23   X* p = nullptr;
  24   constexpr const std::variant<X*>& decay() const noexcept { return *this; }
  25 };
  26
  27 template <>
  28 struct 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
  37 void bug_81590() { (void)std::format("{}", X{}); }