Allow optional to tolerate being used with a nested class.

When Clang tries to complete a type containing `std::optional` it
considers the `in_place_t` constructor with no arguments which checks
if the value type is default constructible. If the value type is a
nested class type, then this check occurs too early and poisons the
is_default_constructible trait.

This patch makes optional deduce `in_place_t` so we can prevent
this early SFINAE evaluation. Technically this could break people
doing weird things with the in_place_t tag, but that seems less
important than making the nested class case work.

llvm-svn: 355877

libcxx/include/optional

@@ -687,11 +687,15 @@ public:
     _LIBCPP_INLINE_VISIBILITY constexpr optional(optional&&) = default;
     _LIBCPP_INLINE_VISIBILITY constexpr optional(nullopt_t) noexcept {}
 
-    template <class... _Args, class = enable_if_t<
-        is_constructible_v<value_type, _Args...>>
+    template <class _InPlaceT, class... _Args, class = enable_if_t<
+        __lazy_and<
+            is_same<_InPlaceT, in_place_t>,
+            is_constructible<value_type, _Args...>
+            >::value
+        >
     >
     _LIBCPP_INLINE_VISIBILITY
-    constexpr explicit optional(in_place_t, _Args&&... __args)
+    constexpr explicit optional(_InPlaceT, _Args&&... __args)
         : __base(in_place, _VSTD::forward<_Args>(__args)...) {}
 
     template <class _Up, class... _Args, class = enable_if_t<

libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/empty_in_place_t_does_not_clobber.pass.cpp

@@ -0,0 +1,41 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+// <optional>
+
+// constexpr optional(in_place_t);
+
+// Test that the SFINAE "is_constructible<value_type>" isn't evaluated by the
+// in_place_t constructor with no arguments when the Clang is trying to check
+// copy constructor.
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+#include "archetypes.hpp"
+
+using std::optional;
+
+struct Wrapped {
+  struct Inner {
+    bool Dummy = true;
+  };
+  std::optional<Inner> inner;
+};
+
+int main(int, char**) {
+  static_assert(std::is_default_constructible<Wrapped::Inner>::value, "");
+  Wrapped w;
+  w.inner.emplace();
+  assert(w.inner.has_value());
+
+  return 0;
+}