[libc++] Fix ambiguity when using std::scoped_allocator constructor (#80261)

Fixes #78754

Author: Rajveer100

libcxx/include/scoped_allocator

@@ -476,9 +476,8 @@ public:
   }
 
 private:
-  template <class _OuterA2, __enable_if_t<is_constructible<outer_allocator_type, _OuterA2>::value, int> = 0>
-  _LIBCPP_HIDE_FROM_ABI scoped_allocator_adaptor(_OuterA2&& __o, const inner_allocator_type& __i) _NOEXCEPT
-      : base(std::forward<_OuterA2>(__o), __i) {}
+  _LIBCPP_HIDE_FROM_ABI explicit scoped_allocator_adaptor(outer_allocator_type&& __o, inner_allocator_type&& __i) _NOEXCEPT
+      : base(std::move(__o), std::move(__i)) {}
 
   template <class _Tp, class... _Args>
   _LIBCPP_HIDE_FROM_ABI void __construct(integral_constant<int, 0>, _Tp* __p, _Args&&... __args) {

libcxx/test/std/utilities/allocator.adaptor/allocator.adaptor.cnstr/allocs.pass.cpp

@@ -104,13 +104,20 @@ int main(int, char**) {
     assert(a.outer_allocator() == A1<int>(4));
     assert((a.inner_allocator() == std::scoped_allocator_adaptor<A2<int>, A3<int>>(A2<int>(5), A3<int>(6))));
   }
-  //  Test for LWG2782
   {
+    //  Test for LWG2782
     static_assert(!std::is_convertible<A1<int>, A2<int>>::value, "");
     static_assert(
         !std::is_convertible< std::scoped_allocator_adaptor<A1<int>>, std::scoped_allocator_adaptor<A2<int>>>::value,
         "");
   }
+  {
+    // Test construction from convertible-to-allocator types (https://github.com/llvm/llvm-project/issues/78754)
+    typedef std::scoped_allocator_adaptor<A1<int>, A1<int>> A;
+    A a(A1<char>(4), A1<char>(5));
+    assert(a.outer_allocator() == A1<int>(4));
+    assert(a.inner_allocator() == A1<int>(5));
+  }
 
   return 0;
 }