Fix type checker crash in multiple inheritance

This potentially addresses #998.

mypy/checker.py

@@ -914,15 +914,21 @@ def check_compatibility(self, name: str, base1: TypeInfo,
         second_type = second.type
         if second_type is None and isinstance(second.node, FuncDef):
             second_type = self.function_type(cast(FuncDef, second.node))
-        # TODO: What if first_type or second_type is None? What if some classes are generic?
+        # TODO: What if some classes are generic?
         if (isinstance(first_type, FunctionLike) and
                 isinstance(second_type, FunctionLike)):
             # Method override
             first_sig = method_type(cast(FunctionLike, first_type))
             second_sig = method_type(cast(FunctionLike, second_type))
             ok = is_subtype(first_sig, second_sig)
-        else:
+        elif first_type and second_type:
             ok = is_equivalent(first_type, second_type)
+        else:
+            if first_type is None:
+                self.msg.cannot_determine_type_in_base(name, base1.name(), ctx)
+            if second_type is None:
+                self.msg.cannot_determine_type_in_base(name, base2.name(), ctx)
+            ok = True
         if not ok:
             self.msg.base_class_definitions_incompatible(name, base1, base2,
                                                          ctx)

mypy/messages.py

@@ -679,6 +679,9 @@ def string_interpolation_mixing_key_and_non_keys(self, context: Context) -> None
     def cannot_determine_type(self, name: str, context: Context) -> None:
         self.fail("Cannot determine type of '%s'" % name, context)
 
+    def cannot_determine_type_in_base(self, name: str, base: str, context: Context) -> None:
+        self.fail("Cannot determine type of '%s' in base class '%s'" % (name, base), context)
+
     def invalid_method_type(self, sig: CallableType, context: Context) -> None:
         self.fail('Invalid method type', context)
 

mypy/test/data/check-multiple-inheritance.test

@@ -231,3 +231,18 @@ class G(Generic[T]):
 class A(G[int]):
     def f(self, s: int) -> 'A': pass
 class B(A, int): pass
+
+[case testCannotDetermineTypeInMultipleInheritance]
+class A(B, C):
+    def f(self): pass
+class B:
+    @dec
+    def f(self): pass
+class C:
+    @dec
+    def f(self): pass
+def dec(f): return f
+[out]
+main: note: In class "A":
+main:1: error: Cannot determine type of 'f' in base class 'B'
+main:1: error: Cannot determine type of 'f' in base class 'C'