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// REQUIRES: c++11 || c++14 || c++17 || c++20
10
11// This test makes sure that we don't apply P1951 before C++23, since that is
12// a breaking change. The examples in this test are taken from Richard Smith's
13// comments on https://llvm.org/D109066.
14
15#include <cassert>
16#include <utility>
17#include <vector>
18
19struct A {
20 int *p_;
21 A(int *p) : p_(p) { *p_ += 1; }
22 A(const A& a) : p_(a.p_) { *p_ += 1; }
23 ~A() { *p_ -= 1; }
24};
25
26int main(int, char**) {
27 // Example 1:
28 // Without P1951, we call the `pair(int, const A&)` constructor (the converting constructor is not usable because
29 // we can't deduce from an initializer list), which creates the A temporary as part of the call to f. With P1951,
30 // we call the `pair(U&&, V&&)` constructor, which creates a A temporary inside the pair constructor, and that
31 // temporary doesn't live long enough any more.
32 {
33 int i = 0;
34 auto f = [&](std::pair<std::vector<int>, const A&>) { assert(i >= 1); };
35 f({{42, 43}, &i});
36 }
37
38 // Example 2:
39 // Here, n doesn't need to be captured if we call the `pair(const int&, const long&)` constructor, because
40 // the lvalue-to-rvalue conversion happens in the lambda. But if we call the `pair(U&&, V&&)` constructor
41 // (deducing V = int), then n does need to be captured.
42 {
43 const int n = 5;
44 (void) []{ std::pair<int, long>({1}, n); };
45 }
46
47 return 0;
48}
49

source code of libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.brace-init.P1951.pass.cpp