Add an `__is_inplace_index` metafunction.

[mpark]

Summary

This is used to constrain variant's converting constructor correctly.

[EricWF]

include/utility

931 ↗

(On Diff #102219)

Missing __uncvref_t . Is this intentional?

[mpark]

include/utility

931 ↗

(On Diff #102219)

Oh, I can add that. As far as I understand, we're required to perform decay_t from variant's side anyway,
so uncvref would be a no-op for that case. But I guess that's fine?

[mpark]

Add __uncvref.

[EricWF]

include/utility

931 ↗

(On Diff #102219)

Should be fine. I guess it doesn't matter. but it makes me a bit uncomfortable to have two different metafunctions, one that does decay and another that doesn't.

[mclow.lists]

LGTM. Do we need any tests?

[mpark]

In D34111#778637, @mclow.lists wrote:

LGTM. Do we need any tests?

I could add some. Where do you think they should live?

[mclow.lists]

I could add some. Where do you think they should live?

libc++ specific tests (which this would definitely be) live in test/libcxx/.

How about test/libcxx/utilities/meta/meta.unary ?

[mpark]

Add tests.

[mpark]

Lined up the static_asserts.

[mpark]

Add more tests.

[mclow.lists]

LGTM!

libcxx/trunk/include/utility

 constexpr in_place_index_t<_Idx> in_place_index{};
 
 template <class _Tp> struct __is_inplace_type_imp : false_type {};
 template <class _Tp> struct __is_inplace_type_imp<in_place_type_t<_Tp>> : true_type {};
 
 template <class _Tp>
 using __is_inplace_type = __is_inplace_type_imp<__uncvref_t<_Tp>>;
 
+template <class _Tp> struct __is_inplace_index_imp : false_type {};
+template <size_t _Idx> struct __is_inplace_index_imp<in_place_index_t<_Idx>> : true_type {};
+
+template <class _Tp>
+using __is_inplace_index = __is_inplace_index_imp<__uncvref_t<_Tp>>;
+
 #endif // _LIBCPP_STD_VER > 14
 
 template <class _Arg, class _Result>
 struct _LIBCPP_TEMPLATE_VIS unary_function
 {
     typedef _Arg    argument_type;
     typedef _Result result_type;
 };

libcxx/trunk/test/libcxx/utilities/utility/__is_inplace_index.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// template <class _Tp> using __is_inplace_index
+
+#include <utility>
+
+struct S {};
+
+int main() {
+  using I = std::in_place_index_t<0>;
+  static_assert( std::__is_inplace_index<I>::value, "");
+  static_assert( std::__is_inplace_index<const I>::value, "");
+  static_assert( std::__is_inplace_index<const volatile I>::value, "");
+  static_assert( std::__is_inplace_index<I&>::value, "");
+  static_assert( std::__is_inplace_index<const I&>::value, "");
+  static_assert( std::__is_inplace_index<const volatile I&>::value, "");
+  static_assert( std::__is_inplace_index<I&&>::value, "");
+  static_assert( std::__is_inplace_index<const I&&>::value, "");
+  static_assert( std::__is_inplace_index<const volatile I&&>::value, "");
+  static_assert(!std::__is_inplace_index<std::in_place_type_t<int>>::value, "");
+  static_assert(!std::__is_inplace_index<std::in_place_t>::value, "");
+  static_assert(!std::__is_inplace_index<void>::value, "");
+  static_assert(!std::__is_inplace_index<int>::value, "");
+  static_assert(!std::__is_inplace_index<S>::value, "");
+}

libcxx/trunk/test/libcxx/utilities/utility/__is_inplace_type.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// template <class _Tp> using __is_inplace_type
+
+#include <utility>
+
+struct S {};
+
+int main() {
+  using T = std::in_place_type_t<int>;
+  static_assert( std::__is_inplace_type<T>::value, "");
+  static_assert( std::__is_inplace_type<const T>::value, "");
+  static_assert( std::__is_inplace_type<const volatile T>::value, "");
+  static_assert( std::__is_inplace_type<T&>::value, "");
+  static_assert( std::__is_inplace_type<const T&>::value, "");
+  static_assert( std::__is_inplace_type<const volatile T&>::value, "");
+  static_assert( std::__is_inplace_type<T&&>::value, "");
+  static_assert( std::__is_inplace_type<const T&&>::value, "");
+  static_assert( std::__is_inplace_type<const volatile T&&>::value, "");
+  static_assert(!std::__is_inplace_type<std::in_place_index_t<0>>::value, "");
+  static_assert(!std::__is_inplace_type<std::in_place_t>::value, "");
+  static_assert(!std::__is_inplace_type<void>::value, "");
+  static_assert(!std::__is_inplace_type<int>::value, "");
+  static_assert(!std::__is_inplace_type<S>::value, "");
+}