WIP - do not merge - see #1363

Author: Guido van Rossum

mypy/checkexpr.py

@@ -627,6 +627,8 @@ def check_arg(self, caller_type: Type, original_caller_type: Type,
                   callee_type: Type, n: int, m: int, callee: CallableType,
                   context: Context, messages: MessageBuilder) -> None:
         """Check the type of a single argument in a call."""
+        if isinstance(caller_type, Instance) and caller_type.type.fallback_to_any:
+            return
         if isinstance(caller_type, Void):
             messages.does_not_return_value(caller_type, context)
         elif isinstance(caller_type, DeletedType):

mypy/test/data/check-expressions.test

@@ -1382,3 +1382,15 @@ dict(undefined)
 [builtins fixtures/dict.py]
 [out]
 main:1: error: Name 'undefined' is not defined
+
+[case testDictInContext]
+# XXX This doesn't test for the right thing.
+from typing import Any, Dict
+Base = None  # type: Any
+
+class Derived(Base):
+    def to_dict(self) -> Dict[str, Any]:
+        return dict(self)  # fails without the hack
+dict(Derived())  # fails without the hack
+dict(Base())  # Always ok
+[builtins fixtures/dict.py]