Keep NoneType in Union TypeVar values

Author: cdce8p

mypy/expandtype.py

@@ -52,22 +52,30 @@
 
 
 @overload
-def expand_type(typ: CallableType, env: Mapping[TypeVarId, Type]) -> CallableType: ...
+def expand_type(
+    typ: CallableType, env: Mapping[TypeVarId, Type], *, keep_none_type: bool = ...
+) -> CallableType: ...
 
 
 @overload
-def expand_type(typ: ProperType, env: Mapping[TypeVarId, Type]) -> ProperType: ...
+def expand_type(
+    typ: ProperType, env: Mapping[TypeVarId, Type], *, strict_optional: bool = ...
+) -> ProperType: ...
 
 
 @overload
-def expand_type(typ: Type, env: Mapping[TypeVarId, Type]) -> Type: ...
+def expand_type(
+    typ: Type, env: Mapping[TypeVarId, Type], *, strict_optional: bool = ...
+) -> Type: ...
 
 
-def expand_type(typ: Type, env: Mapping[TypeVarId, Type]) -> Type:
+def expand_type(
+    typ: Type, env: Mapping[TypeVarId, Type], *, strict_optional: bool = False
+) -> Type:
     """Substitute any type variable references in a type given by a type
     environment.
     """
-    return typ.accept(ExpandTypeVisitor(env))
+    return typ.accept(ExpandTypeVisitor(env, strict_optional))
 
 
 @overload
@@ -184,8 +192,9 @@ class ExpandTypeVisitor(TrivialSyntheticTypeTranslator):
 
     variables: Mapping[TypeVarId, Type]  # TypeVar id -> TypeVar value
 
-    def __init__(self, variables: Mapping[TypeVarId, Type]) -> None:
+    def __init__(self, variables: Mapping[TypeVarId, Type], strict_optional: bool = False) -> None:
         self.variables = variables
+        self.strict_optional = strict_optional
         self.recursive_guard: set[Type | tuple[int, Type]] = set()
 
     def visit_unbound_type(self, t: UnboundType) -> Type:
@@ -460,7 +469,7 @@ def visit_union_type(self, t: UnionType) -> Type:
         # might be subtypes of others, however calling  make_simplified_union()
         # can cause recursion, so we just remove strict duplicates.
         simplified = UnionType.make_union(
-            remove_trivial(flatten_nested_unions(expanded)), t.line, t.column
+            remove_trivial(flatten_nested_unions(expanded), self.strict_optional), t.line, t.column
         )
         # This call to get_proper_type() is unfortunate but is required to preserve
         # the invariant that ProperType will stay ProperType after applying expand_type(),
@@ -508,7 +517,7 @@ def expand_self_type(var: Var, typ: Type, replacement: Type) -> Type:
     return typ
 
 
-def remove_trivial(types: Iterable[Type]) -> list[Type]:
+def remove_trivial(types: Iterable[Type], strict_optional: bool = False) -> list[Type]:
     """Make trivial simplifications on a list of types without calling is_subtype().
 
     This makes following simplifications:
@@ -523,7 +532,7 @@ def remove_trivial(types: Iterable[Type]) -> list[Type]:
         p_t = get_proper_type(t)
         if isinstance(p_t, UninhabitedType):
             continue
-        if isinstance(p_t, NoneType) and not state.strict_optional:
+        if isinstance(p_t, NoneType) and not state.strict_optional and not strict_optional:
             removed_none = True
             continue
         if isinstance(p_t, Instance) and p_t.type.fullname == "builtins.object":

mypy/typeanal.py

@@ -1898,7 +1898,7 @@ def fix_instance(
     t.args = tuple(args)
     fix_type_var_tuple_argument(t)
     if not t.type.has_type_var_tuple_type:
-        fixed = expand_type(t, env)
+        fixed = expand_type(t, env, strict_optional=True)
         assert isinstance(fixed, Instance)
         t.args = fixed.args