support recursive flattening

Author: TH3CHARLie

mypy/checkexpr.py

@@ -19,7 +19,7 @@
     PartialType, DeletedType, UninhabitedType, TypeType, TypeOfAny, LiteralType, LiteralValue,
     is_named_instance, FunctionLike,
     StarType, is_optional, remove_optional, is_generic_instance, get_proper_type, ProperType,
-    get_proper_types, flatten_nested_unions, TypeAliasType
+    get_proper_types, flatten_nested_unions
 )
 from mypy.nodes import (
     NameExpr, RefExpr, Var, FuncDef, OverloadedFuncDef, TypeInfo, CallExpr,
@@ -2521,10 +2521,9 @@ def check_op(self, method: str, base_type: Type,
             left_variants = [base_type]
             base_type = get_proper_type(base_type)
             if isinstance(base_type, UnionType):
-                items = [get_proper_type(item) if isinstance(item, TypeAliasType)
-                         else item for item in base_type.relevant_items()]
                 left_variants = [item for item in
-                                 flatten_nested_unions(items)]
+                                 flatten_nested_unions(base_type.relevant_items(),
+                                                       handle_type_alias_type=True)]
             right_type = self.accept(arg)
 
             # Step 1: We first try leaving the right arguments alone and destructure
@@ -2566,11 +2565,9 @@ def check_op(self, method: str, base_type: Type,
             right_variants = [(right_type, arg)]
             right_type = get_proper_type(right_type)
             if isinstance(right_type, UnionType):
-                items = [get_proper_type(item) if isinstance(item, TypeAliasType)
-                         else item for item in right_type.relevant_items()]
                 right_variants = [(item, TempNode(item, context=context))
-                                  for item in flatten_nested_unions(items)]
-
+                                  for item in flatten_nested_unions(right_type.relevant_items(),
+                                                                    handle_type_alias_type=True)]
             msg = self.msg.clean_copy()
             msg.disable_count = 0
             all_results = []

mypy/types.py

@@ -2231,15 +2231,20 @@ def has_type_vars(typ: Type) -> bool:
     return typ.accept(HasTypeVars())
 
 
-def flatten_nested_unions(types: Iterable[Type]) -> List[Type]:
+def flatten_nested_unions(types: Iterable[Type],
+                          handle_type_alias_type: bool = False) -> List[Type]:
     """Flatten nested unions in a type list."""
     # This and similar functions on unions can cause infinite recursion
     # if passed a "pathological" alias like A = Union[int, A] or similar.
     # TODO: ban such aliases in semantic analyzer.
     flat_items = []  # type: List[Type]
+    if handle_type_alias_type:
+        types = [get_proper_type(item) if isinstance(item, TypeAliasType)
+                 else item for item in types]
     for tp in types:
         if isinstance(tp, ProperType) and isinstance(tp, UnionType):
-            flat_items.extend(flatten_nested_unions(tp.items))
+            flat_items.extend(flatten_nested_unions(tp.items,
+                                        handle_type_alias_type=handle_type_alias_type))
         else:
             flat_items.append(tp)
     return flat_items