Make Union[str, None] equivalent to str

Closes #572.

Author: JukkaL

mypy/test/data/check-unions.test

@@ -97,3 +97,9 @@ s = ''
 s = f(int)
 s = f(1)    # E: Incompatible types in assignment (expression has type "int", variable has type "str")
 x = f(int)  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+
+[case testUnionWithNoneItem]
+from typing import Union
+def f() -> Union[int, None]: pass
+x = 1
+x = f()

mypy/test/data/semanal-types.test

@@ -1466,3 +1466,45 @@ MypyFile:1(
     BaseType(
       __main__.B)
     PassStmt:7()))
+
+[case testUnionType]
+from typing import Union
+def f(x: Union[int, str]) -> None: pass
+[out]
+MypyFile:1(
+  ImportFrom:1(typing, [Union : Union])
+  FuncDef:2(
+    f
+    Args(
+      Var(x))
+    def (x: Union[builtins.int, builtins.str])
+    Block:2(
+      PassStmt:2())))
+
+[case testUnionTypeWithNoneItem]
+from typing import Union
+def f(x: Union[int, None]) -> None: pass
+[out]
+MypyFile:1(
+  ImportFrom:1(typing, [Union : Union])
+  FuncDef:2(
+    f
+    Args(
+      Var(x))
+    def (x: builtins.int)
+    Block:2(
+      PassStmt:2())))
+
+[case testUnionTypeWithNoneItemAndTwoItems]
+from typing import Union
+def f(x: Union[int, None, str]) -> None: pass
+[out]
+MypyFile:1(
+  ImportFrom:1(typing, [Union : Union])
+  FuncDef:2(
+    f
+    Args(
+      Var(x))
+    def (x: Union[builtins.int, builtins.str])
+    Block:2(
+      PassStmt:2())))

mypy/typeanal.py

@@ -89,7 +89,9 @@ def visit_unbound_type(self, t: UnboundType) -> Type:
                 return TupleType(self.anal_array(t.args),
                                  self.builtin_type('builtins.tuple'))
             elif sym.node.fullname() == 'typing.Union':
-                return UnionType(self.anal_array(t.args))
+                items = self.anal_array(t.args)
+                items = [item for item in items if not isinstance(item, Void)]
+                return UnionType.make_union(items)
             elif sym.node.fullname() == 'typing.Callable':
                 return self.analyze_function_type(t)
             elif sym.kind == TYPE_ALIAS: