Fixes issue #336

examples/smart_contracts/assert_foo_none.py

@@ -0,0 +1,10 @@
+#!opshin
+from opshin.prelude import *
+
+
+def foo() -> None:
+    return None
+
+
+def validator(datum: int, redeemer: int, context: ScriptContext) -> None:
+    assert foo(), f"Always fails!"

examples/smart_contracts/assert_none.py

@@ -0,0 +1,6 @@
+#!opshin
+from opshin.prelude import *
+
+
+def validator(datum: int, redeemer: int, context: ScriptContext) -> None:
+    assert None, f"Always fails!"

opshin/type_inference.py

@@ -838,6 +838,24 @@ def visit_Attribute(self, node: Attribute) -> TypedAttribute:
     def visit_Assert(self, node: Assert) -> TypedAssert:
         ta = copy(node)
         ta.test = self.visit(node.test)
+        try:
+            warn_assert_msg = f" (see assert with message '{node.msg.values[0].value}')"
+        except AttributeError as err:
+            warn_assert_msg = ""
+        if isinstance(ta.test.args[0], Constant) and ta.test.args[0].value is None:
+            OPSHIN_LOGGER.warning(
+                "Asserting `None'"
+                + warn_assert_msg
+                + " is equivalent to asserting False, which always fails."
+            )
+        if isinstance(ta.test.args[0], Call) and isinstance(
+            ta.test.args[0].typ.typ, UnitType
+        ):
+            OPSHIN_LOGGER.warning(
+                "Asserting `None'"
+                + warn_assert_msg
+                + " is likely to stem from a procedure already doing internal assertions and returning `None'. Asserting this is equivalent to asserting False, which always fails (likely unintended)."
+            )
         assert (
             ta.test.typ == BoolInstanceType
         ), "Assertions must result in a boolean type"