From a46edc8a471202eda70e3fa680a3a8750c0ed7ce Mon Sep 17 00:00:00 2001
From: pca006132 <jl@m-labs.hk>
Date: Wed, 23 Dec 2020 17:08:04 +0800
Subject: [PATCH] allow type guard on unbounded type variables

---
 toy-impl/parse_stmt.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/toy-impl/parse_stmt.py b/toy-impl/parse_stmt.py
index 864b21a..82f1222 100644
--- a/toy-impl/parse_stmt.py
+++ b/toy-impl/parse_stmt.py
@@ -126,12 +126,12 @@ def parse_if_stmt(ctx: Context,
             (isinstance(node.test.ops[0], ast.Eq) or\
              isinstance(node.test.ops[0], ast.NotEq)):
         t = parse_expr(ctx, sym_table, node.test.left.args[0])
-        if not isinstance(t, TypeVariable) or len(t.constraints) < 2:
+        if not isinstance(t, TypeVariable):
             raise CustomError(
-                'type guard only support basic bounded type variables',
+                'type guard only support type variables',
                 node.test)
         t1, _ = parse_type(ctx, node.test.comparators[0])
-        if t1 not in t.constraints:
+        if len(t.constraints) > 0 and t1 not in t.constraints:
             raise CustomError(
                 f'{t1} is not a possible instance of {t}',
                 node.test)