[libcxx-commits] [libcxx] [libc++] Fix `std::pair`'s pair-like constructor's incorrect assumption (PR #66585)

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llvmbot wrote:


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<details>
<summary>Changes</summary>

GitHub Issue:
https://github.com/llvm/llvm-project/issues/65620

The helper function `__pair_like_explicit_wknd` is only SFINAE-ed with `tuple_size<remove_cvref_t<_PairLike>>::value == 2`, but its function body assumes `std::get` being valid
https://github.com/llvm/llvm-project/blame/main/libcxx/include/__utility/pair.h#L280

---
Full diff: https://github.com/llvm/llvm-project/pull/66585.diff


2 Files Affected:

- (modified) libcxx/include/__utility/pair.h (+1-1) 
- (modified) libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.pair_like.pass.cpp (+30) 


``````````diff
diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index 85d2f21af2420f3..99e4066c70fc483 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -276,7 +276,7 @@ struct _LIBCPP_TEMPLATE_VIS pair
     // This is a workaround for http://llvm.org/PR60710. We should be able to remove it once Clang is fixed.
     template <class _PairLike, bool _Enable = tuple_size<remove_cvref_t<_PairLike>>::value == 2>
     _LIBCPP_HIDE_FROM_ABI static constexpr bool __pair_like_explicit_wknd() {
-        if constexpr (tuple_size<remove_cvref_t<_PairLike>>::value == 2) {
+        if constexpr (__pair_like<_PairLike>) {
             return !is_convertible_v<decltype(std::get<0>(std::declval<_PairLike&&>())), first_type> ||
                    !is_convertible_v<decltype(std::get<1>(std::declval<_PairLike&&>())), second_type>;
         }
diff --git a/libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.pair_like.pass.cpp b/libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.pair_like.pass.cpp
index 3362a872a58579d..24e75563e045805 100644
--- a/libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.pair_like.pass.cpp
+++ b/libcxx/test/std/utilities/utility/pairs/pairs.pair/ctor.pair_like.pass.cpp
@@ -23,6 +23,36 @@
 #include <type_traits>
 #include <utility>
 
+namespace my_ns{
+struct MyPairLike {
+  
+template <std::size_t N>
+friend int get(MyPairLike const&)
+{
+  return 0;
+}
+
+};
+
+} // namespace my_ns
+
+namespace std {
+
+template <>
+struct tuple_size<my_ns::MyPairLike> : std::integral_constant<std::size_t, 2> {};
+
+template <std::size_t N>
+struct tuple_element<N, my_ns::MyPairLike> {
+  using type = int;
+};
+
+} // namespace std
+
+// https://github.com/llvm/llvm-project/issues/65620
+// This used to be a hard error 
+static_assert(!std::is_constructible_v<std::pair<int,int>, my_ns::MyPairLike const&>);
+
+
 constexpr bool test() {
   // Make sure construction works from array, tuple, and ranges::subrange
   {

``````````

</details>


https://github.com/llvm/llvm-project/pull/66585