Fix erasing union types

Author: JukkaL

mypy/erasetype.py

@@ -82,7 +82,8 @@ def visit_typeddict_type(self, t: TypedDictType) -> Type:
         return t.fallback.accept(self)
 
     def visit_union_type(self, t: UnionType) -> Type:
-        return AnyType()        # XXX: return underlying type if only one?
+        erased_items = [erase_type(item) for item in t.items]
+        return UnionType.make_simplified_union(erased_items)
 
     def visit_type_type(self, t: TypeType) -> Type:
         return TypeType(t.item.accept(self), line=t.line)