Improve the signatures of expand_type and expand_type_by_instance (#14879)

By adding another overload, `CallableType -> CallableType`, we can avoid
the need for several `cast`s across the code base.

Author: AlexWaygood

mypy/checker.py

@@ -1753,8 +1753,7 @@ def expand_typevars(
             result: list[tuple[FuncItem, CallableType]] = []
             for substitutions in itertools.product(*subst):
                 mapping = dict(substitutions)
-                expanded = cast(CallableType, expand_type(typ, mapping))
-                result.append((expand_func(defn, mapping), expanded))
+                result.append((expand_func(defn, mapping), expand_type(typ, mapping)))
             return result
         else:
             return [(defn, typ)]
@@ -7111,7 +7110,6 @@ 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, CallableType)
     return is_callable_compatible(
         exp_signature, other, is_compat=is_more_precise, ignore_return=True
     )

mypy/checkexpr.py

@@ -5518,7 +5518,7 @@ def merge_typevars_in_callables_by_name(
                     variables.append(tv)
                 rename[tv.id] = unique_typevars[name]
 
-            target = cast(CallableType, expand_type(target, rename))
+            target = expand_type(target, rename)
         output.append(target)
 
     return output, variables

mypy/checkmember.py

@@ -1150,7 +1150,7 @@ class B(A[str]): pass
             t = freshen_all_functions_type_vars(t)
             t = bind_self(t, original_type, is_classmethod=True)
             assert isuper is not None
-            t = cast(CallableType, expand_type_by_instance(t, isuper))
+            t = expand_type_by_instance(t, isuper)
             freeze_all_type_vars(t)
         return t.copy_modified(variables=list(tvars) + list(t.variables))
     elif isinstance(t, Overloaded):

mypy/expandtype.py

@@ -47,6 +47,13 @@
 )
 
 
+@overload
+def expand_type(
+    typ: CallableType, env: Mapping[TypeVarId, Type], allow_erased_callables: bool = ...
+) -> CallableType:
+    ...
+
+
 @overload
 def expand_type(
     typ: ProperType, env: Mapping[TypeVarId, Type], allow_erased_callables: bool = ...
@@ -70,6 +77,11 @@ def expand_type(
     return typ.accept(ExpandTypeVisitor(env, allow_erased_callables))
 
 
+@overload
+def expand_type_by_instance(typ: CallableType, instance: Instance) -> CallableType:
+    ...
+
+
 @overload
 def expand_type_by_instance(typ: ProperType, instance: Instance) -> ProperType:
     ...
@@ -133,7 +145,7 @@ def freshen_function_type_vars(callee: F) -> F:
                 tv = ParamSpecType.new_unification_variable(v)
             tvs.append(tv)
             tvmap[v.id] = tv
-        fresh = cast(CallableType, expand_type(callee, tvmap)).copy_modified(variables=tvs)
+        fresh = expand_type(callee, tvmap).copy_modified(variables=tvs)
         return cast(F, fresh)
     else:
         assert isinstance(callee, Overloaded)
@@ -346,7 +358,7 @@ def interpolate_args_for_unpack(
             )
         return (arg_names, arg_kinds, arg_types)
 
-    def visit_callable_type(self, t: CallableType) -> Type:
+    def visit_callable_type(self, t: CallableType) -> CallableType:
         param_spec = t.param_spec()
         if param_spec is not None:
             repl = get_proper_type(self.variables.get(param_spec.id))

mypy/server/astdiff.py

@@ -52,7 +52,7 @@ class level -- these are handled at attribute level (say, 'mod.Cls.method'
 
 from __future__ import annotations
 
-from typing import Sequence, Tuple, Union, cast
+from typing import Sequence, Tuple, Union
 from typing_extensions import TypeAlias as _TypeAlias
 
 from mypy.expandtype import expand_type
@@ -442,7 +442,7 @@ def normalize_callable_variables(self, typ: CallableType) -> CallableType:
                 tv = v.copy_modified(id=tid)
             tvs.append(tv)
             tvmap[v.id] = tv
-        return cast(CallableType, expand_type(typ, tvmap)).copy_modified(variables=tvs)
+        return expand_type(typ, tvmap).copy_modified(variables=tvs)
 
     def visit_tuple_type(self, typ: TupleType) -> SnapshotItem:
         return ("TupleType", snapshot_types(typ.items))