Support for variadic type aliases (#15219)

Fixes #15062 

Implementing "happy path" took like couple dozen lines, but there are *a
lot* of edge cases, e.g. where we need to fail gracefully. Also of
course I checked my implementation (mostly) works for recursive variadic
aliases :-) see test.

It looks like several pieces of support for proper variadic types (i.e.
non-aliases, instances etc) are still missing, so I tried to fill in
something where I needed it for type aliases, but not everywhere, some
notable examples:
* Type variable bound checks for instances are still broken, see TODO
item in `semanal_typeargs.py`
* I think type argument count check is still broken for instances (I
think I fixed it for type aliases), there can be fewer than
`len(type_vars) - 1` type arguments, e.g. if one of them is an unpack.
* We should only prohibit multiple *variadic* unpacks in a type list,
multiple fixed length unpacks are fine (I think I fixed this both for
aliases and instances)

Btw I was thinking about an example below, what should we do in such
cases?
```python
from typing import Tuple, TypeVar
from typing_extensions import TypeVarTuple, Unpack

T = TypeVar("T")
S = TypeVar("S")
Ts = TypeVarTuple("Ts")

Alias = Tuple[T, S, Unpack[Ts], S]

def foo(*x: Unpack[Ts]) -> None:
    y: Alias[Unpack[Ts], int, str]
    reveal_type(y)  # <-- what is this type?

# Ts = () => Tuple[int, str, str]
# Ts = (bool) => Tuple[bool, int, str, int]
# Ts = (bool, float) => Tuple[bool, float, int, str, float]
```

Finally, I noticed there is already some code duplication, and I am not
improving it. I am open to suggestions on how to reduce the code
duplication.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

mypy/checkexpr.py

@@ -151,11 +151,13 @@
     UninhabitedType,
     UnionType,
     UnpackType,
+    flatten_nested_tuples,
     flatten_nested_unions,
     get_proper_type,
     get_proper_types,
     has_recursive_types,
     is_named_instance,
+    split_with_prefix_and_suffix,
 )
 from mypy.types_utils import is_generic_instance, is_optional, is_self_type_like, remove_optional
 from mypy.typestate import type_state
@@ -4070,6 +4072,35 @@ class LongName(Generic[T]): ...
             # The _SpecialForm type can be used in some runtime contexts (e.g. it may have __or__).
             return self.named_type("typing._SpecialForm")
 
+    def split_for_callable(
+        self, t: CallableType, args: Sequence[Type], ctx: Context
+    ) -> list[Type]:
+        """Handle directly applying type arguments to a variadic Callable.
+
+        This is needed in situations where e.g. variadic class object appears in
+        runtime context. For example:
+            class C(Generic[T, Unpack[Ts]]): ...
+            x = C[int, str]()
+
+        We simply group the arguments that need to go into Ts variable into a TupleType,
+        similar to how it is done in other places using split_with_prefix_and_suffix().
+        """
+        vars = t.variables
+        if not vars or not any(isinstance(v, TypeVarTupleType) for v in vars):
+            return list(args)
+
+        prefix = next(i for (i, v) in enumerate(vars) if isinstance(v, TypeVarTupleType))
+        suffix = len(vars) - prefix - 1
+        args = flatten_nested_tuples(args)
+        if len(args) < len(vars) - 1:
+            self.msg.incompatible_type_application(len(vars), len(args), ctx)
+            return [AnyType(TypeOfAny.from_error)] * len(vars)
+
+        tvt = vars[prefix]
+        assert isinstance(tvt, TypeVarTupleType)
+        start, middle, end = split_with_prefix_and_suffix(tuple(args), prefix, suffix)
+        return list(start) + [TupleType(list(middle), tvt.tuple_fallback)] + list(end)
+
     def apply_type_arguments_to_callable(
         self, tp: Type, args: Sequence[Type], ctx: Context
     ) -> Type:
@@ -4083,19 +4114,28 @@ def apply_type_arguments_to_callable(
         tp = get_proper_type(tp)
 
         if isinstance(tp, CallableType):
-            if len(tp.variables) != len(args):
+            if len(tp.variables) != len(args) and not any(
+                isinstance(v, TypeVarTupleType) for v in tp.variables
+            ):
                 if tp.is_type_obj() and tp.type_object().fullname == "builtins.tuple":
                     # TODO: Specialize the callable for the type arguments
                     return tp
                 self.msg.incompatible_type_application(len(tp.variables), len(args), ctx)
                 return AnyType(TypeOfAny.from_error)
-            return self.apply_generic_arguments(tp, args, ctx)
+            return self.apply_generic_arguments(tp, self.split_for_callable(tp, args, ctx), ctx)
         if isinstance(tp, Overloaded):
             for it in tp.items:
-                if len(it.variables) != len(args):
+                if len(it.variables) != len(args) and not any(
+                    isinstance(v, TypeVarTupleType) for v in it.variables
+                ):
                     self.msg.incompatible_type_application(len(it.variables), len(args), ctx)
                     return AnyType(TypeOfAny.from_error)
-            return Overloaded([self.apply_generic_arguments(it, args, ctx) for it in tp.items])
+            return Overloaded(
+                [
+                    self.apply_generic_arguments(it, self.split_for_callable(it, args, ctx), ctx)
+                    for it in tp.items
+                ]
+            )
         return AnyType(TypeOfAny.special_form)
 
     def visit_list_expr(self, e: ListExpr) -> Type:

mypy/constraints.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Iterable, List, Sequence
+from typing import TYPE_CHECKING, Iterable, List, Sequence, cast
 from typing_extensions import Final
 
 import mypy.subtypes
@@ -46,15 +46,11 @@
     has_recursive_types,
     has_type_vars,
     is_named_instance,
+    split_with_prefix_and_suffix,
 )
 from mypy.types_utils import is_union_with_any
 from mypy.typestate import type_state
-from mypy.typevartuples import (
-    extract_unpack,
-    find_unpack_in_list,
-    split_with_mapped_and_template,
-    split_with_prefix_and_suffix,
-)
+from mypy.typevartuples import extract_unpack, find_unpack_in_list, split_with_mapped_and_template
 
 if TYPE_CHECKING:
     from mypy.infer import ArgumentInferContext
@@ -669,7 +665,7 @@ def visit_instance(self, template: Instance) -> list[Constraint]:
                         instance.type.type_var_tuple_prefix,
                         instance.type.type_var_tuple_suffix,
                     )
-                    tvars = list(tvars_prefix + tvars_suffix)
+                    tvars = cast("list[TypeVarLikeType]", list(tvars_prefix + tvars_suffix))
                 else:
                     mapped_args = mapped.args
                     instance_args = instance.args
@@ -738,7 +734,7 @@ def visit_instance(self, template: Instance) -> list[Constraint]:
                         template.type.type_var_tuple_prefix,
                         template.type.type_var_tuple_suffix,
                     )
-                    tvars = list(tvars_prefix + tvars_suffix)
+                    tvars = cast("list[TypeVarLikeType]", list(tvars_prefix + tvars_suffix))
                 else:
                     mapped_args = mapped.args
                     template_args = template.args

mypy/expandtype.py

@@ -37,14 +37,12 @@
     UninhabitedType,
     UnionType,
     UnpackType,
+    flatten_nested_tuples,
     flatten_nested_unions,
     get_proper_type,
-)
-from mypy.typevartuples import (
-    find_unpack_in_list,
-    split_with_instance,
     split_with_prefix_and_suffix,
 )
+from mypy.typevartuples import find_unpack_in_list, split_with_instance
 
 # WARNING: these functions should never (directly or indirectly) depend on
 # is_subtype(), meet_types(), join_types() etc.
@@ -115,6 +113,7 @@ def expand_type_by_instance(typ: Type, instance: Instance) -> Type:
             instance_args = instance.args
 
         for binder, arg in zip(tvars, instance_args):
+            assert isinstance(binder, TypeVarLikeType)
             variables[binder.id] = arg
 
         return expand_type(typ, variables)
@@ -282,12 +281,14 @@ def visit_type_var_tuple(self, t: TypeVarTupleType) -> Type:
         raise NotImplementedError
 
     def visit_unpack_type(self, t: UnpackType) -> Type:
-        # It is impossible to reasonally implement visit_unpack_type, because
+        # It is impossible to reasonably implement visit_unpack_type, because
         # unpacking inherently expands to something more like a list of types.
         #
         # Relevant sections that can call unpack should call expand_unpack()
         # instead.
-        assert False, "Mypy bug: unpacking must happen at a higher level"
+        # However, if the item is a variadic tuple, we can simply carry it over.
+        # it is hard to assert this without getting proper type.
+        return UnpackType(t.type.accept(self))
 
     def expand_unpack(self, t: UnpackType) -> list[Type] | Instance | AnyType | None:
         return expand_unpack_with_variables(t, self.variables)
@@ -356,7 +357,15 @@ def interpolate_args_for_unpack(
 
             # Extract the typevartuple so we can get a tuple fallback from it.
             expanded_unpacked_tvt = expanded_unpack.type
-            assert isinstance(expanded_unpacked_tvt, TypeVarTupleType)
+            if isinstance(expanded_unpacked_tvt, TypeVarTupleType):
+                fallback = expanded_unpacked_tvt.tuple_fallback
+            else:
+                # This can happen when tuple[Any, ...] is used to "patch" a variadic
+                # generic type without type arguments provided.
+                assert isinstance(expanded_unpacked_tvt, ProperType)
+                assert isinstance(expanded_unpacked_tvt, Instance)
+                assert expanded_unpacked_tvt.type.fullname == "builtins.tuple"
+                fallback = expanded_unpacked_tvt
 
             prefix_len = expanded_unpack_index
             arg_names = t.arg_names[:star_index] + [None] * prefix_len + t.arg_names[star_index:]
@@ -368,11 +377,7 @@ def interpolate_args_for_unpack(
                 + expanded_items[:prefix_len]
                 # Constructing the Unpack containing the tuple without the prefix.
                 + [
-                    UnpackType(
-                        TupleType(
-                            expanded_items[prefix_len:], expanded_unpacked_tvt.tuple_fallback
-                        )
-                    )
+                    UnpackType(TupleType(expanded_items[prefix_len:], fallback))
                     if len(expanded_items) - prefix_len > 1
                     else expanded_items[0]
                 ]
@@ -456,9 +461,12 @@ def expand_types_with_unpack(
         indicates use of Any or some error occurred earlier. In this case callers should
         simply propagate the resulting type.
         """
+        # TODO: this will cause a crash on aliases like A = Tuple[int, Unpack[A]].
+        # Although it is unlikely anyone will write this, we should fail gracefully.
+        typs = flatten_nested_tuples(typs)
         items: list[Type] = []
         for item in typs:
-            if isinstance(item, UnpackType):
+            if isinstance(item, UnpackType) and isinstance(item.type, TypeVarTupleType):
                 unpacked_items = self.expand_unpack(item)
                 if unpacked_items is None:
                     # TODO: better error, something like tuple of unknown?
@@ -523,7 +531,11 @@ def visit_type_type(self, t: TypeType) -> Type:
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:
         # Target of the type alias cannot contain type variables (not bound by the type
         # alias itself), so we just expand the arguments.
-        return t.copy_modified(args=self.expand_types(t.args))
+        args = self.expand_types_with_unpack(t.args)
+        if isinstance(args, list):
+            return t.copy_modified(args=args)
+        else:
+            return args
 
     def expand_types(self, types: Iterable[Type]) -> list[Type]:
         a: list[Type] = []

mypy/nodes.py

@@ -3471,6 +3471,7 @@ def f(x: B[T]) -> T: ...  # without T, Any would be used here
         "normalized",
         "_is_recursive",
         "eager",
+        "tvar_tuple_index",
     )
 
     __match_args__ = ("name", "target", "alias_tvars", "no_args")
@@ -3498,6 +3499,10 @@ def __init__(
         # it is the cached value.
         self._is_recursive: bool | None = None
         self.eager = eager
+        self.tvar_tuple_index = None
+        for i, t in enumerate(alias_tvars):
+            if isinstance(t, mypy.types.TypeVarTupleType):
+                self.tvar_tuple_index = i
         super().__init__(line, column)
 
     @classmethod

mypy/semanal.py

@@ -270,6 +270,7 @@
     TypeOfAny,
     TypeType,
     TypeVarLikeType,
+    TypeVarTupleType,
     TypeVarType,
     UnboundType,
     UnpackType,
@@ -3424,8 +3425,18 @@ def analyze_alias(
                 allowed_alias_tvars=tvar_defs,
             )
 
+        # There can be only one variadic variable at most, the error is reported elsewhere.
+        new_tvar_defs = []
+        variadic = False
+        for td in tvar_defs:
+            if isinstance(td, TypeVarTupleType):
+                if variadic:
+                    continue
+                variadic = True
+            new_tvar_defs.append(td)
+
         qualified_tvars = [node.fullname for _name, node in found_type_vars]
-        return analyzed, tvar_defs, depends_on, qualified_tvars
+        return analyzed, new_tvar_defs, depends_on, qualified_tvars
 
     def is_pep_613(self, s: AssignmentStmt) -> bool:
         if s.unanalyzed_type is not None and isinstance(s.unanalyzed_type, UnboundType):

mypy/semanal_typeargs.py

@@ -18,6 +18,7 @@
 from mypy.options import Options
 from mypy.scope import Scope
 from mypy.subtypes import is_same_type, is_subtype
+from mypy.typeanal import set_any_tvars
 from mypy.types import (
     AnyType,
     Instance,
@@ -32,8 +33,10 @@
     TypeVarType,
     UnboundType,
     UnpackType,
+    flatten_nested_tuples,
     get_proper_type,
     get_proper_types,
+    split_with_prefix_and_suffix,
 )
 
 
@@ -79,10 +82,34 @@ def visit_type_alias_type(self, t: TypeAliasType) -> None:
         self.seen_aliases.add(t)
         # Some recursive aliases may produce spurious args. In principle this is not very
         # important, as we would simply ignore them when expanding, but it is better to keep
-        # correct aliases.
-        if t.alias and len(t.args) != len(t.alias.alias_tvars):
-            t.args = [AnyType(TypeOfAny.from_error) for _ in t.alias.alias_tvars]
+        # correct aliases. Also, variadic aliases are better to check when fully analyzed,
+        # so we do this here.
         assert t.alias is not None, f"Unfixed type alias {t.type_ref}"
+        args = flatten_nested_tuples(t.args)
+        if t.alias.tvar_tuple_index is not None:
+            correct = len(args) >= len(t.alias.alias_tvars) - 1
+            if any(
+                isinstance(a, UnpackType) and isinstance(get_proper_type(a.type), Instance)
+                for a in args
+            ):
+                correct = True
+        else:
+            correct = len(args) == len(t.alias.alias_tvars)
+        if not correct:
+            if t.alias.tvar_tuple_index is not None:
+                exp_len = f"at least {len(t.alias.alias_tvars) - 1}"
+            else:
+                exp_len = f"{len(t.alias.alias_tvars)}"
+            self.fail(
+                f"Bad number of arguments for type alias, expected: {exp_len}, given: {len(args)}",
+                t,
+                code=codes.TYPE_ARG,
+            )
+            t.args = set_any_tvars(
+                t.alias, t.line, t.column, self.options, from_error=True, fail=self.fail
+            ).args
+        else:
+            t.args = args
         is_error = self.validate_args(t.alias.name, t.args, t.alias.alias_tvars, t)
         if not is_error:
             # If there was already an error for the alias itself, there is no point in checking
@@ -101,6 +128,17 @@ def visit_instance(self, t: Instance) -> None:
     def validate_args(
         self, name: str, args: Sequence[Type], type_vars: list[TypeVarLikeType], ctx: Context
     ) -> bool:
+        # TODO: we need to do flatten_nested_tuples and validate arg count for instances
+        # similar to how do we do this for type aliases above, but this may have perf penalty.
+        if any(isinstance(v, TypeVarTupleType) for v in type_vars):
+            prefix = next(i for (i, v) in enumerate(type_vars) if isinstance(v, TypeVarTupleType))
+            tvt = type_vars[prefix]
+            assert isinstance(tvt, TypeVarTupleType)
+            start, middle, end = split_with_prefix_and_suffix(
+                tuple(args), prefix, len(type_vars) - prefix - 1
+            )
+            args = list(start) + [TupleType(list(middle), tvt.tuple_fallback)] + list(end)
+
         is_error = False
         for (i, arg), tvar in zip(enumerate(args), type_vars):
             if isinstance(tvar, TypeVarType):
@@ -167,7 +205,11 @@ def visit_unpack_type(self, typ: UnpackType) -> None:
             return
         if isinstance(proper_type, Instance) and proper_type.type.fullname == "builtins.tuple":
             return
-        if isinstance(proper_type, AnyType) and proper_type.type_of_any == TypeOfAny.from_error:
+        if (
+            isinstance(proper_type, UnboundType)
+            or isinstance(proper_type, AnyType)
+            and proper_type.type_of_any == TypeOfAny.from_error
+        ):
             return
 
         # TODO: Infer something when it can't be unpacked to allow rest of

mypy/subtypes.py

@@ -661,6 +661,8 @@ def visit_type_var_tuple(self, left: TypeVarTupleType) -> bool:
     def visit_unpack_type(self, left: UnpackType) -> bool:
         if isinstance(self.right, UnpackType):
             return self._is_subtype(left.type, self.right.type)
+        if isinstance(self.right, Instance) and self.right.type.fullname == "builtins.object":
+            return True
         return False
 
     def visit_parameters(self, left: Parameters) -> bool: