check if impl_self is Some

compiler/rustc_resolve/src/late.rs

@@ -1932,7 +1932,7 @@ fn is_self_ty(&self, ty: &Ty) -> bool {
                         if let Some(Res::SelfTyParam { .. } | Res::SelfTyAlias { .. }) = path_res {
                             return true;
                         }
-                        path_res == self.impl_self
+                        self.impl_self.is_some() && path_res == self.impl_self
                     }
                     _ => false,
                 }