Implicitly look through UncheckedOptional<T> when it's the

base of a member access or subscript.

Swift SVN r12345

lib/Sema/CSSimplify.cpp

@@ -1328,6 +1328,11 @@ ConstraintSystem::simplifyMemberConstraint(const Constraint &constraint) {
   Type baseTy = simplifyType(constraint.getFirstType());
   Type baseObjTy = baseTy->getRValueType();
 
+  // Try to look through UncheckedOptional<T>; the result is always an r-value.
+  if (auto objTy = lookThroughUncheckedOptionalType(baseObjTy)) {
+    baseTy = baseObjTy = objTy;
+  }
+
   // Dig out the instance type.
   bool isMetatype = false;
   Type instanceTy = baseObjTy;