Report error on dynamic metaclass (#2275)

* Avoid crash on dynamic metaclass (force error instead)

* Add a unit test

Author: gvanrossum

mypy/fastparse.py

@@ -371,9 +371,10 @@ def stringify_name(self, n: ast35.AST) -> str:
         if isinstance(n, ast35.Name):
             return n.id
         elif isinstance(n, ast35.Attribute):
-            return "{}.{}".format(self.stringify_name(n.value), n.attr)
-        else:
-            assert False, "can't stringify " + str(type(n))
+            sv = self.stringify_name(n.value)
+            if sv is not None:
+                return "{}.{}".format(sv, n.attr)
+        return None  # Can't do it.
 
     # ClassDef(identifier name,
     #  expr* bases,
@@ -387,6 +388,8 @@ def visit_ClassDef(self, n: ast35.ClassDef) -> ClassDef:
         metaclass = None
         if metaclass_arg:
             metaclass = self.stringify_name(metaclass_arg.value)
+            if metaclass is None:
+                metaclass = '<error>'  # To be reported later
 
         cdef = ClassDef(n.name,
                         self.as_block(n.body, n.lineno),

mypy/semanal.py

@@ -835,6 +835,9 @@ def is_base_class(self, t: TypeInfo, s: TypeInfo) -> bool:
 
     def analyze_metaclass(self, defn: ClassDef) -> None:
         if defn.metaclass:
+            if defn.metaclass == '<error>':
+                self.fail("Dynamic metaclass not supported for '%s'" % defn.name, defn)
+                return
             sym = self.lookup_qualified(defn.metaclass, defn)
             if sym is not None and not isinstance(sym.node, TypeInfo):
                 self.fail("Invalid metaclass '%s'" % defn.metaclass, defn)

test-data/unit/check-classes.test

@@ -2050,3 +2050,8 @@ class A: pass
 class B(object, A): # E: Cannot determine consistent method resolution order (MRO) for "B"
     def readlines(self): pass
     __iter__ = readlines
+
+[case testDynamicMetaclass]
+# flags: --fast-parser
+class C(metaclass=int()):  # E: Dynamic metaclass not supported for 'C'
+    pass