Fix crash during type inference

Fixes python#674.

mypy/constraints.py

@@ -127,6 +127,7 @@ class ConstraintBuilderVisitor(TypeVisitor[List[Constraint]]):
     """Visitor class for inferring type constraints."""
 
     # The type that is compared against a template
+    # TODO: The value may be None. Is that actually correct?
     actual = Undefined(Type)
 
     def __init__(self, actual: Type, direction: int) -> None:
@@ -154,7 +155,10 @@ def visit_erased_type(self, template: ErasedType) -> List[Constraint]:
     # Non-trivial leaf type
 
     def visit_type_var(self, template: TypeVarType) -> List[Constraint]:
-        return [Constraint(template.id, self.direction, self.actual)]
+        if self.actual:
+            return [Constraint(template.id, self.direction, self.actual)]
+        else:
+            return []
 
     # Non-leaf types
 

mypy/join.py

@@ -54,7 +54,6 @@ def join_types(s: Type, t: Type) -> Type:
 
     If the join does not exist, return an ErrorType instance.
     """
-
     if isinstance(s, AnyType):
         return s
 

mypy/test/data/check-generics.test

@@ -715,3 +715,9 @@ def voidify(n: int) -> None: pass
 def identity(f: Callable[[A], None]) -> Callable[[A], None]:
     return f
 identity(voidify)(3)
+
+[case testTypeVariableUnionAndCallableInTypeInference]
+from typing import Union, Callable, TypeVar
+T = TypeVar('T')
+def f(x: T, y: Union[T, Callable[[T], None]]) -> None: pass
+f('', '')