[libc++] ADL-proof <variant> by adding _VSTD:: qualification on calls.

Differential Revision: https://reviews.llvm.org/D92036

libcxx/include/variant

@@ -491,7 +491,7 @@ private:
     template <class _Fp, class... _Vs>
     inline _LIBCPP_INLINE_VISIBILITY
     static constexpr decltype(auto) __dispatch(_Fp __f, _Vs... __vs) {
-        return __invoke_constexpr(
+        return _VSTD::__invoke_constexpr(
             static_cast<_Fp>(__f),
             __access::__base::__get_alt<_Is>(static_cast<_Vs>(__vs))...);
     }
@@ -599,7 +599,7 @@ private:
       __std_visit_exhaustive_visitor_check<
           _Visitor,
           decltype((_VSTD::forward<_Alts>(__alts).__value))...>();
-      return __invoke_constexpr(_VSTD::forward<_Visitor>(__visitor),
+      return _VSTD::__invoke_constexpr(_VSTD::forward<_Visitor>(__visitor),
                                 _VSTD::forward<_Alts>(__alts).__value...);
     }
     _Visitor&& __visitor;

libcxx/test/std/utilities/variant/variant.visit/robust_against_adl.pass.cpp

@@ -0,0 +1,46 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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++03, c++11, c++14
+
+// Throwing bad_variant_access is supported starting in macosx10.13
+// XFAIL: with_system_cxx_lib=macosx10.12 && !no-exceptions
+// XFAIL: with_system_cxx_lib=macosx10.11 && !no-exceptions
+// XFAIL: with_system_cxx_lib=macosx10.10 && !no-exceptions
+// XFAIL: with_system_cxx_lib=macosx10.9 && !no-exceptions
+
+// <variant>
+// template <class Visitor, class... Variants>
+// constexpr see below visit(Visitor&& vis, Variants&&... vars);
+
+#include <variant>
+
+#include "test_macros.h"
+
+struct Incomplete;
+template<class T> struct Holder { T t; };
+
+constexpr bool test(bool do_it)
+{
+    if (do_it) {
+        std::variant<Holder<Incomplete>*, int> v = nullptr;
+        std::visit([](auto){}, v);
+        std::visit([](auto) -> Holder<Incomplete>* { return nullptr; }, v);
+    }
+    return true;
+}
+
+int main(int, char**)
+{
+    test(true);
+#if TEST_STD_VER > 17
+    static_assert(test(true));
+#endif
+    return 0;
+}