Don't crash on undefined decorator involved in import cycle.

Fixes #1972.

mypy/checker.py

@@ -808,9 +808,13 @@ def check_method_override_for_base_with_name(
             # Map the overridden method type to subtype context so that
             # it can be checked for compatibility.
             original_type = base_attr.type
-            if original_type is None and isinstance(base_attr.node,
-                                                    FuncDef):
-                original_type = self.function_type(base_attr.node)
+            if original_type is None:
+                if isinstance(base_attr.node, FuncDef):
+                    original_type = self.function_type(base_attr.node)
+                elif isinstance(base_attr.node, Decorator):
+                    original_type = self.function_type(base_attr.node.func)
+                else:
+                    assert False, str(base_attr.node)
             if isinstance(original_type, FunctionLike):
                 original = map_type_from_supertype(
                     method_type(original_type),
@@ -824,7 +828,6 @@ def check_method_override_for_base_with_name(
                                     base.name(),
                                     defn)
             else:
-                assert original_type is not None
                 self.msg.signature_incompatible_with_supertype(
                     defn.name(), name, base.name(), defn)
 

test-data/unit/check-functions.test

@@ -1002,6 +1002,20 @@ main:1: note: In module imported here:
 tmp/a.py:3: error: Argument 1 to "dec" has incompatible type "int"; expected "str"
 tmp/a.py:5: error: "str" not callable
 
+[case testUndefinedDecoratorInImportCycle]
+# cmd: mypy -m foo.base
+[file foo/__init__.py]
+import foo.base
+class Derived(foo.base.Base):
+    def method(self) -> None: pass
+[file foo/base.py]
+import foo
+class Base:
+    @decorator
+    def method(self) -> None: pass
+[out]
+tmp/foo/base.py:3: error: Name 'decorator' is not defined
+
 
 -- Conditional function definition
 -- -------------------------------