[RELAY] Add occurs check before unification (#2012)

src/relay/pass/type_solver.cc

@@ -61,6 +61,11 @@ Type TypeSolver::Unify(const Type& dst, const Type& src) {
   // - handle shape pattern matching
   TypeNode* lhs = GetTypeNode(dst);
   TypeNode* rhs = GetTypeNode(src);
+
+  // do occur check so we don't create self-referencing structure
+  if (lhs->FindRoot() == rhs->FindRoot()) {
+    return lhs->resolved_type;
+  }
   if (lhs->resolved_type.as<IncompleteTypeNode>()) {
     MergeFromTo(lhs, rhs);
     return rhs->resolved_type;

tests/cpp/relay_pass_type_infer_test.cc

+#include <gtest/gtest.h>
+#include <tvm/tvm.h>
+#include <tvm/relay/expr.h>
+#include <tvm/relay/type.h>
+#include <tvm/relay/pass.h>
+
+TEST(Relay, SelfReference) {
+  using namespace tvm;
+  auto type_a = relay::TypeVarNode::make("a", relay::TypeVarNode::kType);
+  auto type_b = relay::TypeVarNode::make("b", relay::TypeVarNode::kType);
+  auto x = relay::VarNode::make("x", type_a);
+  auto f = relay::FunctionNode::make(tvm::Array<relay::Var>{ x }, x, type_b, Array<relay::TypeVar>{});
+  auto fx = relay::CallNode::make(f, Array<relay::Expr>{ x });
+  auto type_fx = relay::InferType(fx, relay::EnvironmentNode::make(Map<relay::GlobalVar, relay::Function>{}));
+  CHECK_EQ(type_fx->checked_type(), type_a);
+}
+
+int main(int argc, char ** argv) {
+  testing::InitGoogleTest(&argc, argv);
+  testing::FLAGS_gtest_death_test_style = "threadsafe";
+  return RUN_ALL_TESTS();
+}

tests/python/relay/test_type_infer.py

@@ -107,6 +107,22 @@ def test_type_args():
     assert sh2[0].value == 1
     assert sh2[1].value == 10
 
+def test_self_reference():
+    """
+    Program:
+       def f(x) {
+           return x;
+       }
+    """
+    a = relay.TypeVar("a")
+    x = relay.var("x", a)
+    sb = relay.ScopeBuilder()
+    f = relay.Function([x], x)
+    fx = relay.Call(f, [x])
+    assert relay.ir_pass.infer_type(x).checked_type == a
+    assert relay.ir_pass.infer_type(f).checked_type == relay.FuncType([a], a)
+    assert relay.ir_pass.infer_type(fx).checked_type == a
+
 if __name__ == "__main__":
     test_free_expr()
     test_dual_op()
@@ -117,3 +133,4 @@ if __name__ == "__main__":
     test_tuple()
     test_free_expr()
     test_type_args()
+    test_self_reference()