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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294612 91177308-0d34-0410-b5e6-96231b3b80d8

include/variant

@@ -578,7 +578,7 @@ private:
     constexpr decltype(auto) operator()(_Alts&&... __alts) const {
       __std_visit_exhaustive_visitor_check<
           _Visitor,
-          decltype(_VSTD::forward<_Alts>(__alts).__value)...>();
+          decltype((_VSTD::forward<_Alts>(__alts).__value))...>();
       return __invoke_constexpr(_VSTD::forward<_Visitor>(__visitor),
                                 _VSTD::forward<_Alts>(__alts).__value...);
     }

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();
 }