[libcxx] Remove the "reduced-arity-initialization" extension from the uses-allocator constructors

Summary:
A default uses-allocator constructor has been added since that overload was previously provided by the extended constructor.

Since Clang does implicit conversion checking after substitution this constructor has to deduce the allocator_arg_t parameter so that it can prevent the evaluation of "is_default_constructible" if the first argument doesn't match. See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1391 for more information.

This patch fixes PR24779 (https://llvm.org/bugs/show_bug.cgi?id=24779)

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19006

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@266409 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/alloc.pass.cpp

@@ -24,11 +24,27 @@
 #include "../alloc_first.h"
 #include "../alloc_last.h"
 
+template <class T = void>
+struct NonDefaultConstructible {
+  constexpr NonDefaultConstructible() {
+      static_assert(!std::is_same<T, T>::value, "Default Ctor instantiated");
+  }
+
+  explicit constexpr NonDefaultConstructible(int) {}
+};
+
+
+struct DerivedFromAllocArgT : std::allocator_arg_t {};
+
 int main()
 {
     {
         std::tuple<> t(std::allocator_arg, A1<int>());
     }
+    {
+        DerivedFromAllocArgT tag;
+        std::tuple<> t(tag, A1<int>());
+    }
     {
         std::tuple<int> t(std::allocator_arg, A1<int>());
         assert(std::get<0>(t) == 0);
@@ -78,4 +94,29 @@ int main()
         assert(!alloc_last::allocator_constructed);
         assert(std::get<2>(t) == alloc_last());
     }
+    {
+        // Test that allocator construction is selected when the user provides
+        // a custom tag type which derives from allocator_arg_t.
+        DerivedFromAllocArgT tag;
+        alloc_first::allocator_constructed = false;
+        alloc_last::allocator_constructed = false;
+
+        std::tuple<DefaultOnly, alloc_first, alloc_last> t(tag, A1<int>(5));
+
+        assert(std::get<0>(t) == DefaultOnly());
+        assert(alloc_first::allocator_constructed);
+        assert(std::get<1>(t) == alloc_first());
+        assert(alloc_last::allocator_constructed);
+        assert(std::get<2>(t) == alloc_last());
+    }
+    {
+        // Test that the uses-allocator default constructor does not evaluate
+        // it's SFINAE when it otherwise shouldn't be selected. Do this by
+        // using 'NonDefaultConstructible' which will cause a compile error
+        // if std::is_default_constructible is evaluated on it.
+        using T = NonDefaultConstructible<>;
+        T v(42);
+        std::tuple<T, T> t(v, v);
+        std::tuple<T, T> t2(42, 42);
+    }
 }