refactor: Use @overload for expand_type (#13641)

Removing a bunch of `TODO` items.

mypy/checker.py

@@ -6762,7 +6762,6 @@ def overload_can_never_match(signature: CallableType, other: CallableType) -> bo
     exp_signature = expand_type(
         signature, {tvar.id: erase_def_to_union_or_bound(tvar) for tvar in signature.variables}
     )
-    assert isinstance(exp_signature, ProperType)
     assert isinstance(exp_signature, CallableType)
     return is_callable_compatible(
         exp_signature, other, is_compat=is_more_precise, ignore_return=True

mypy/checkexpr.py

@@ -1506,7 +1506,7 @@ def analyze_type_type_callee(self, item: ProperType, context: Context) -> Type:
             res = type_object_type(item.type, self.named_type)
             if isinstance(res, CallableType):
                 res = res.copy_modified(from_type_type=True)
-            expanded = get_proper_type(expand_type_by_instance(res, item))
+            expanded = expand_type_by_instance(res, item)
             if isinstance(expanded, CallableType):
                 # Callee of the form Type[...] should never be generic, only
                 # proper class objects can be.

mypy/checkmember.py

@@ -731,7 +731,7 @@ def analyze_var(
                     signature, dispatched_type, var.is_classmethod, mx.context, name, mx.msg
                 )
                 signature = bind_self(signature, mx.self_type, var.is_classmethod)
-                expanded_signature = get_proper_type(expand_type_by_instance(signature, itype))
+                expanded_signature = expand_type_by_instance(signature, itype)
                 freeze_type_vars(expanded_signature)
                 if var.is_property:
                     # A property cannot have an overloaded type => the cast is fine.
@@ -1111,7 +1111,7 @@ class B(A[str]): pass
             ]
         )
     if isuper is not None:
-        t = cast(ProperType, expand_type_by_instance(t, isuper))
+        t = expand_type_by_instance(t, isuper)
     return t
 
 

mypy/expandtype.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import Iterable, Mapping, Sequence, TypeVar, cast
+from typing import Iterable, Mapping, Sequence, TypeVar, cast, overload
 
 from mypy.types import (
     AnyType,
@@ -37,18 +37,36 @@
 from mypy.typevartuples import split_with_instance, split_with_prefix_and_suffix
 
 
+@overload
+def expand_type(typ: ProperType, env: Mapping[TypeVarId, Type]) -> ProperType:
+    ...
+
+
+@overload
+def expand_type(typ: Type, env: Mapping[TypeVarId, Type]) -> Type:
+    ...
+
+
 def expand_type(typ: Type, env: Mapping[TypeVarId, Type]) -> Type:
     """Substitute any type variable references in a type given by a type
     environment.
     """
-    # TODO: use an overloaded signature? (ProperType stays proper after expansion.)
     return typ.accept(ExpandTypeVisitor(env))
 
 
+@overload
+def expand_type_by_instance(typ: ProperType, instance: Instance) -> ProperType:
+    ...
+
+
+@overload
+def expand_type_by_instance(typ: Type, instance: Instance) -> Type:
+    ...
+
+
 def expand_type_by_instance(typ: Type, instance: Instance) -> Type:
     """Substitute type variables in type using values from an Instance.
     Type variables are considered to be bound by the class declaration."""
-    # TODO: use an overloaded signature? (ProperType stays proper after expansion.)
     if not instance.args:
         return typ
     else:
@@ -87,7 +105,6 @@ def freshen_function_type_vars(callee: F) -> F:
         tvs = []
         tvmap: dict[TypeVarId, Type] = {}
         for v in callee.variables:
-            # TODO(PEP612): fix for ParamSpecType
             if isinstance(v, TypeVarType):
                 tv: TypeVarLikeType = TypeVarType.new_unification_variable(v)
             elif isinstance(v, TypeVarTupleType):

mypy/maptype.py

@@ -3,17 +3,7 @@
 import mypy.typeops
 from mypy.expandtype import expand_type
 from mypy.nodes import TypeInfo
-from mypy.types import (
-    AnyType,
-    Instance,
-    ProperType,
-    TupleType,
-    Type,
-    TypeOfAny,
-    TypeVarId,
-    get_proper_type,
-    has_type_vars,
-)
+from mypy.types import AnyType, Instance, TupleType, Type, TypeOfAny, TypeVarId, has_type_vars
 
 
 def map_instance_to_supertype(instance: Instance, superclass: TypeInfo) -> Instance:
@@ -37,7 +27,7 @@ def map_instance_to_supertype(instance: Instance, superclass: TypeInfo) -> Insta
                 # Unfortunately we can't support this for generic recursive tuples.
                 # If we skip this special casing we will fall back to tuple[Any, ...].
                 env = instance_to_type_environment(instance)
-                tuple_type = get_proper_type(expand_type(instance.type.tuple_type, env))
+                tuple_type = expand_type(instance.type.tuple_type, env)
                 if isinstance(tuple_type, TupleType):
                     return mypy.typeops.tuple_fallback(tuple_type)
 
@@ -101,7 +91,6 @@ def map_instance_to_direct_supertypes(instance: Instance, supertype: TypeInfo) -
         if b.type == supertype:
             env = instance_to_type_environment(instance)
             t = expand_type(b, env)
-            assert isinstance(t, ProperType)
             assert isinstance(t, Instance)
             result.append(t)