Add a missing '__uncvref_t' to the SFINAE constraints for optional's assignment operator. Fixes PR38638. Thanks to Jonathan Wakely for the report

llvm-svn: 364574

libcxx/test/std/utilities/optional/optional.object/optional.object.assign/assign_value.pass.cpp

@@ -241,6 +241,16 @@ enum MyEnum { Zero, One, Two, Three, FortyTwo = 42 };
 
 using Fn = void(*)();
 
+// https://bugs.llvm.org/show_bug.cgi?id=38638
+template <class T>
+constexpr T pr38638(T v)
+{
+  std::optional<T> o;
+  o = v;
+  return *o + 2;
+}
+
+
 int main(int, char**)
 {
     test_sfinae();
@@ -269,5 +279,7 @@ int main(int, char**)
     }
     test_throws();
 
+    static_assert(pr38638(3) == 5, "");
+
   return 0;
 }