libcxx/test/std/numerics/bit/bit.cast/bit_cast.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 // <bit>
  12 //
  13 // template<class To, class From>
  14 //   constexpr To bit_cast(const From& from) noexcept; // C++20
  15
  16 // This test makes sure that std::bit_cast fails when any of the following
  17 // constraints are violated:
  18 //
  19 //      (1.1) sizeof(To) == sizeof(From) is true;
  20 //      (1.2) is_trivially_copyable_v<To> is true;
  21 //      (1.3) is_trivially_copyable_v<From> is true.
  22 //
  23 // Also check that it's ill-formed when the return type would be
  24 // ill-formed, even though that is not explicitly mentioned in the
  25 // specification (but it can be inferred from the synopsis).
  26
  27 #include <bit>
  28 #include <concepts>
  29
  30 template<class To, class From>
  31 concept bit_cast_is_valid = requires(From from) {
  32     { std::bit_cast<To>(from) } -> std::same_as<To>;
  33 };
  34
  35 // Types are not the same size
  36 namespace ns1 {
  37     struct To { char a; };
  38     struct From { char a; char b; };
  39     static_assert(!bit_cast_is_valid<To, From>);
  40     static_assert(!bit_cast_is_valid<From&, From>);
  41 }
  42
  43 // To is not trivially copyable
  44 namespace ns2 {
  45     struct To { char a; To(To const&); };
  46     struct From { char a; };
  47     static_assert(!bit_cast_is_valid<To, From>);
  48 }
  49
  50 // From is not trivially copyable
  51 namespace ns3 {
  52     struct To { char a; };
  53     struct From { char a; From(From const&); };
  54     static_assert(!bit_cast_is_valid<To, From>);
  55 }
  56
  57 // The return type is ill-formed
  58 namespace ns4 {
  59     struct From { char a; char b; };
  60     static_assert(!bit_cast_is_valid<char[2], From>);
  61     static_assert(!bit_cast_is_valid<int(), From>);
  62 }