Model tuple type aliases better (#20967)

Fixes #17133 (surprisingly has four upvotes, but maybe people are
mistriaging, e.g. the other comment in #17133 is something else that I
fixed in #20872)

Author: hauntsaninja

mypy/checkexpr.py

@@ -4975,7 +4975,7 @@ class C(Generic[T, Unpack[Ts]]): ...
         # This code can be only called either from checking a type application, or from
         # checking a type alias (after the caller handles no_args aliases), so we know it
         # was initially an IndexExpr, and we allow empty tuple type arguments.
-        if not validate_instance(fake, self.chk.fail, empty_tuple_index=True):
+        if not validate_instance(fake, self.chk.fail, indexed=True):
             fix_instance(
                 fake, self.chk.fail, self.chk.note, disallow_any=False, options=self.chk.options
             )

mypy/semanal.py

@@ -4037,8 +4037,8 @@ def analyze_alias(
                 variadic = True
             new_tvar_defs.append(td)
 
-        empty_tuple_index = typ.empty_tuple_index if isinstance(typ, UnboundType) else False
-        return analyzed, new_tvar_defs, depends_on, empty_tuple_index
+        indexed = bool(isinstance(typ, UnboundType) and (typ.args or typ.empty_tuple_index))
+        return analyzed, new_tvar_defs, depends_on, indexed
 
     def is_pep_613(self, s: AssignmentStmt) -> bool:
         if s.unanalyzed_type is not None and isinstance(s.unanalyzed_type, UnboundType):
@@ -4137,10 +4137,10 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
             res = NoneType()
             alias_tvars: list[TypeVarLikeType] = []
             depends_on: set[str] = set()
-            empty_tuple_index = False
+            indexed = False
         else:
             tag = self.track_incomplete_refs()
-            res, alias_tvars, depends_on, empty_tuple_index = self.analyze_alias(
+            res, alias_tvars, depends_on, indexed = self.analyze_alias(
                 lvalue.name,
                 rvalue,
                 allow_placeholder=True,
@@ -4180,12 +4180,11 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
         no_args = (
             isinstance(res, ProperType)
             and isinstance(res, Instance)
-            and not res.args
-            and not empty_tuple_index
             and not pep_695
+            and not indexed
         )
         if isinstance(res, ProperType) and isinstance(res, Instance):
-            if not validate_instance(res, self.fail, empty_tuple_index):
+            if not validate_instance(res, self.fail, indexed):
                 fix_instance(res, self.fail, self.note, disallow_any=False, options=self.options)
         # Aliases defined within functions can't be accessed outside
         # the function, since the symbol table will no longer
@@ -5689,7 +5688,7 @@ def visit_type_alias_stmt(self, s: TypeAliasStmt) -> None:
                 return
 
             tag = self.track_incomplete_refs()
-            res, alias_tvars, depends_on, empty_tuple_index = self.analyze_alias(
+            res, alias_tvars, depends_on, indexed = self.analyze_alias(
                 s.name.name,
                 s.value.expr(),
                 allow_placeholder=True,

mypy/typeanal.py

@@ -2160,6 +2160,15 @@ def instantiate_type_alias(
         # Note that we keep the kind of Any for consistency.
         return set_any_tvars(node, [], ctx.line, ctx.column, options, special_form=True)
 
+    if (
+        no_args
+        and isinstance(node.target, ProperType)
+        and isinstance(node.target, Instance)
+        and node.target.type.fullname == "builtins.tuple"
+        and len(args)
+    ):
+        no_args = False
+
     max_tv_count = len(node.alias_tvars)
     act_len = len(args)
     if (
@@ -2480,13 +2489,14 @@ def make_optional_type(t: Type) -> Type:
         return UnionType([t, NoneType()], t.line, t.column)
 
 
-def validate_instance(t: Instance, fail: MsgCallback, empty_tuple_index: bool) -> bool:
+def validate_instance(t: Instance, fail: MsgCallback, indexed: bool) -> bool:
     """Check if this is a well-formed instance with respect to argument count/positions."""
     # TODO: combine logic with instantiate_type_alias().
     if any(unknown_unpack(a) for a in t.args):
         # This type is not ready to be validated, because of unknown total count.
         # TODO: is it OK to fill with TypeOfAny.from_error instead of special form?
         return False
+    empty_tuple_index = indexed and not t.args
     if t.type.has_type_var_tuple_type:
         min_tv_count = sum(
             not tv.has_default() and not isinstance(tv, TypeVarTupleType)

mypy/types.py

@@ -1043,6 +1043,7 @@ def __init__(
         self,
         name: str,
         args: Sequence[Type] | None = None,
+        *,
         line: int = -1,
         column: int = -1,
         optional: bool = False,

test-data/unit/check-generics.test

@@ -1002,7 +1002,7 @@ y: U
 reveal_type(x)  # N: Revealed type is "builtins.int | None"
 reveal_type(y)  # N: Revealed type is "builtins.int"
 
-U[int]  # E: Type application targets a non-generic function or class
+U[int]  # E: Bad number of arguments for type alias, expected 0, given 1
 O[int]  # E: Bad number of arguments for type alias, expected 0, given 1 \
         # E: Type application is only supported for generic classes
 

test-data/unit/check-python310.test

@@ -1402,6 +1402,42 @@ def print_test(m: object, typ: type[T]) -> T:
             reveal_type(m)  # N: Revealed type is "builtins.object"
     raise
 
+[case testMatchClassPatternTupleAlias]
+# flags: --strict-equality --warn-unreachable
+from typing import Tuple
+
+tuple_alias = tuple
+Tuple_alias = Tuple
+
+def main(m: object):
+    match m:
+        case tuple():
+            reveal_type(m)  # N: Revealed type is "builtins.tuple[Any, ...]"
+        case _:
+            reveal_type(m)  # N: Revealed type is "builtins.object"
+
+    match m:
+        case tuple_alias():
+            reveal_type(m)  # N: Revealed type is "builtins.tuple[Any, ...]"
+        case _:
+            reveal_type(m)  # N: Revealed type is "builtins.object"
+
+    match m:
+        # With real typeshed you'll get an error like this instead:
+        # Expected type in class pattern; found "typing._SpecialForm"
+        case Tuple():  # E: Expected type in class pattern; found "builtins.int"
+            reveal_type(m)  # E: Statement is unreachable
+        case _:
+            reveal_type(m)  # N: Revealed type is "builtins.object"
+
+    match m:
+        # This is a false negative, it will raise at runtime
+        case Tuple_alias():
+            reveal_type(m)  # N: Revealed type is "builtins.tuple[Any, ...]"
+        case _:
+            reveal_type(m)  # N: Revealed type is "builtins.object"
+[builtins fixtures/tuple.pyi]
+
 [case testMatchNonFinalMatchArgs]
 class A:
     __match_args__ = ("a", "b")