less ambitious

Author: hauntsaninja

mypy/typeops.py

@@ -529,6 +529,10 @@ def callable_corresponding_argument(
             not (by_name.required or by_pos.required)
             and by_pos.name is None
             and by_name.pos is None
+            # This is not principled, but prevents a crash. It's weird to have a FormalArgument
+            # that has an UnpackType.
+            and not isinstance(by_name.typ, UnpackType)
+            and not isinstance(by_pos.typ, UnpackType)
         ):
             return FormalArgument(
                 by_name.name, by_pos.pos, meet_types(by_name.typ, by_pos.typ), False

mypy/types.py

@@ -1923,27 +1923,6 @@ def __hash__(self) -> int:
         return hash((self.name, self.pos, self.typ, self.required))
 
 
-def _synthesize_arg_from_vararg(
-    vararg: FormalArgument | None, position: int | None
-) -> FormalArgument | None:
-    if vararg is None:
-        return None
-    typ = vararg.typ
-    if isinstance(typ, UnpackType):
-        # Similar to logic in ExpressionChecker.visit_tuple_index_helper
-        unpacked = get_proper_type(typ.type)
-        if isinstance(unpacked, TypeVarTupleType):
-            bound = get_proper_type(unpacked.upper_bound)
-            assert isinstance(bound, Instance)
-            assert bound.type.fullname == "builtins.tuple"
-            typ = bound.args[0]
-        else:
-            assert isinstance(unpacked, Instance)
-            assert unpacked.type.fullname == "builtins.tuple"
-            typ = unpacked.args[0]
-    return FormalArgument(None, position, typ, False)
-
-
 class Parameters(ProperType):
     """Type that represents the parameters to a function.
 
@@ -2090,7 +2069,11 @@ def try_synthesizing_arg_from_kwarg(self, name: str | None) -> FormalArgument |
             return None
 
     def try_synthesizing_arg_from_vararg(self, position: int | None) -> FormalArgument | None:
-        return _synthesize_arg_from_vararg(self.var_arg(), position)
+        var_arg = self.var_arg()
+        if var_arg is not None:
+            return FormalArgument(None, position, var_arg.typ, False)
+        else:
+            return None
 
     def accept(self, visitor: TypeVisitor[T]) -> T:
         return visitor.visit_parameters(self)
@@ -2435,7 +2418,11 @@ def try_synthesizing_arg_from_kwarg(self, name: str | None) -> FormalArgument |
             return None
 
     def try_synthesizing_arg_from_vararg(self, position: int | None) -> FormalArgument | None:
-        return _synthesize_arg_from_vararg(self.var_arg(), position)
+        var_arg = self.var_arg()
+        if var_arg is not None:
+            return FormalArgument(None, position, var_arg.typ, False)
+        else:
+            return None
 
     @property
     def items(self) -> list[CallableType]: