libcxx/test/std/utilities/variant/variant.variant/implicit_ctad.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
  10
  11 // <variant>
  12
  13 // template <class ...Types> class variant;
  14
  15 // Make sure that the implicitly-generated CTAD works.
  16
  17 // We make sure that it is not ill-formed, however we still produce a warning for
  18 // this one because explicit construction from a variant using CTAD is ambiguous
  19 // (in the sense that the programmer intent is not clear).
  20 // ADDITIONAL_COMPILE_FLAGS(gcc-style-warnings): -Wno-ctad-maybe-unsupported
  21
  22 #include <variant>
  23
  24 #include "test_macros.h"
  25
  26 int main(int, char**) {
  27   // This is the motivating example from P0739R0
  28   {
  29     std::variant<int, double> v1(3);
  30     std::variant v2 = v1;
  31     ASSERT_SAME_TYPE(decltype(v2), std::variant<int, double>);
  32   }
  33
  34   {
  35     std::variant<int, double> v1(3);
  36     std::variant v2 = std::variant(v1); // Technically valid, but intent is ambiguous!
  37     ASSERT_SAME_TYPE(decltype(v2), std::variant<int, double>);
  38   }
  39
  40   return 0;
  41 }