Fix PR31916 - std::visit rejects visitors accepting lvalue arguments

A static assertion was misfiring since it checked
is_callable<Visitor, decltype(__variant_alt<T>.value)>. However
the decltype expression doesn't capture the value category as
required. This patch applies extra braces to decltype to fix
that.

llvm-svn: 294612

1 files changed, 11 insertions, 0 deletions

diff --git a/libcxx/test/std/utilities/variant/variant.visit/visit.pass.cpp b/libcxx/test/std/utilities/variant/variant.visit/visit.pass.cpp
index 46d225883ae..408a65c0903 100644
--- a/libcxx/test/std/utilities/variant/variant.visit/visit.pass.cpp
+++ b/libcxx/test/std/utilities/variant/variant.visit/visit.pass.cpp
@@ -283,9 +283,20 @@ void test_exceptions() {
 #endif
 }
 
+// See http://llvm.org/PR31916
+void test_caller_accepts_nonconst() {
+  struct A {};
+  struct Visitor {
+    void operator()(A&) {}
+  };
+  std::variant<A> v;
+  std::visit(Visitor{}, v);
+}
+
 int main() {
   test_call_operator_forwarding();
   test_argument_forwarding();
   test_constexpr();
   test_exceptions();
+  test_caller_accepts_nonconst();
 }