fix: distinguish different type parameters in subtyping

Author: coeff-aij

src/rty/subtyping.rs

@@ -71,8 +71,7 @@ where
             (Type::Int, Type::Int)
             | (Type::Bool, Type::Bool)
             | (Type::String, Type::String)
-            | (Type::Never, Type::Never)
-            | (Type::Param(_), Type::Param(_)) => {}
+            | (Type::Never, Type::Never) => {}
             (Type::Enum(got), Type::Enum(expected)) if got.symbol() == expected.symbol() => {
                 for (got_ty, expected_ty) in got.args.iter().zip(expected.args.iter()) {
                     let cs = self.relate_sub_refined_type(got_ty, expected_ty);
@@ -121,6 +120,7 @@ where
                 let cs2 = self.relate_sub_refined_type(&got.elem, &expected.elem);
                 clauses.extend(cs2);
             }
+            (Type::Param(got), Type::Param(expected)) if got.idx == expected.idx => {}
             _ => panic!(
                 "inconsistent types: got={}, expected={}",
                 got.display(),