[ConstraintSystem] Add a new reason for invalid member reference

This new reason indicates that result type of a static member
doesn't conform to a specific protocol so it can't be referenced
on a protocol metatype.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -1460,7 +1460,11 @@ struct MemberLookupResult {
     UR_ReferenceWritableKeyPathOnMutatingMember,
 
     /// This is a KeyPath whose root type is AnyObject
-    UR_KeyPathWithAnyObjectRootType
+    UR_KeyPathWithAnyObjectRootType,
+
+    /// This is a static member being access through a protocol metatype
+    /// but its result type doesn't conform to this protocol.
+    UR_InvalidStaticMemberOnProtocolMetatype,
   };
 
   /// This is a list of considered (but rejected) candidates, along with a

lib/Sema/CSSimplify.cpp

@@ -7369,7 +7369,10 @@ fixMemberRef(ConstraintSystem &cs, Type baseTy,
       break;
     case MemberLookupResult::UR_KeyPathWithAnyObjectRootType:
       return AllowAnyObjectKeyPathRoot::create(cs, locator);
-   }
+
+    case MemberLookupResult::UR_InvalidStaticMemberOnProtocolMetatype:
+      return AllowInvalidStaticMemberRefOnProtocolMetatype::create(cs, locator);
+    }
   }
 
   return nullptr;