Support ParamSpec variables in type aliases (#14159)

Fixes #11855 
Fixes #7084
Fixes #10445
Should fix #4987

After thinking about this for some time, it looks like the best way to
implement this is by switching type aliases from unbound to bound type
variables. Then I can essentially simply share (or copy in one small
place, to avoid cyclic imports) all the logic that currently exists for
`ParamSpec` and `Concatenate` in `expand_type()` etc.

This will also address a big piece of tech debt, and will get some
benefits (almost) for free, such as checking bounds/values for alias
type variables, and much tighter handling of unbound type variables.

Note that in this PR I change logic for emitting some errors, I try to
avoid showing multiple errors for the same location/reason. But this is
not an essential part of this PR (it is just some test cases would
otherwise fail with even more error messages), I can reconsider if there
are objections.

docs/source/generics.rst

@@ -916,9 +916,5 @@ defeating the purpose of using aliases.  Example:
 
     OIntVec = Optional[Vec[int]]
 
-.. note::
-
-    A type alias does not define a new type. For generic type aliases
-    this means that variance of type variables used for alias definition does not
-    apply to aliases. A parameterized generic alias is treated simply as an original
-    type with the corresponding type variables substituted.
+Using type variable bounds or values in generic aliases, has the same effect
+as in generic classes/functions.

mypy/checker.py

@@ -7115,7 +7115,7 @@ def visit_uninhabited_type(self, t: UninhabitedType) -> Type:
         return t
 
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:
-        # Target of the alias cannot by an ambiguous <nothing>, so we just
+        # Target of the alias cannot be an ambiguous <nothing>, so we just
         # replace the arguments.
         return t.copy_modified(args=[a.accept(self) for a in t.args])
 

mypy/checkexpr.py

@@ -3854,10 +3854,8 @@ def visit_type_application(self, tapp: TypeApplication) -> Type:
 
         There are two different options here, depending on whether expr refers
         to a type alias or directly to a generic class. In the first case we need
-        to use a dedicated function typeanal.expand_type_aliases. This
-        is due to the fact that currently type aliases machinery uses
-        unbound type variables, while normal generics use bound ones;
-        see TypeAlias docstring for more details.
+        to use a dedicated function typeanal.expand_type_alias(). This
+        is due to some differences in how type arguments are applied and checked.
         """
         if isinstance(tapp.expr, RefExpr) and isinstance(tapp.expr.node, TypeAlias):
             # Subscription of a (generic) alias in runtime context, expand the alias.

mypy/erasetype.py

@@ -176,8 +176,8 @@ def visit_param_spec(self, t: ParamSpecType) -> Type:
         return t
 
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:
-        # Type alias target can't contain bound type variables, so
-        # it is safe to just erase the arguments.
+        # Type alias target can't contain bound type variables (not bound by the type
+        # alias itself), so it is safe to just erase the arguments.
         return t.copy_modified(args=[a.accept(self) for a in t.args])
 
 

mypy/expandtype.py

@@ -15,7 +15,6 @@
     NoneType,
     Overloaded,
     Parameters,
-    ParamSpecFlavor,
     ParamSpecType,
     PartialType,
     ProperType,
@@ -34,6 +33,7 @@
     UninhabitedType,
     UnionType,
     UnpackType,
+    expand_param_spec,
     get_proper_type,
 )
 from mypy.typevartuples import (
@@ -212,32 +212,8 @@ def visit_param_spec(self, t: ParamSpecType) -> Type:
             # TODO: what does prefix mean in this case?
             # TODO: why does this case even happen? Instances aren't plural.
             return repl
-        elif isinstance(repl, ParamSpecType):
-            return repl.copy_modified(
-                flavor=t.flavor,
-                prefix=t.prefix.copy_modified(
-                    arg_types=t.prefix.arg_types + repl.prefix.arg_types,
-                    arg_kinds=t.prefix.arg_kinds + repl.prefix.arg_kinds,
-                    arg_names=t.prefix.arg_names + repl.prefix.arg_names,
-                ),
-            )
-        elif isinstance(repl, Parameters) or isinstance(repl, CallableType):
-            # if the paramspec is *P.args or **P.kwargs:
-            if t.flavor != ParamSpecFlavor.BARE:
-                assert isinstance(repl, CallableType), "Should not be able to get here."
-                # Is this always the right thing to do?
-                param_spec = repl.param_spec()
-                if param_spec:
-                    return param_spec.with_flavor(t.flavor)
-                else:
-                    return repl
-            else:
-                return Parameters(
-                    t.prefix.arg_types + repl.arg_types,
-                    t.prefix.arg_kinds + repl.arg_kinds,
-                    t.prefix.arg_names + repl.arg_names,
-                    variables=[*t.prefix.variables, *repl.variables],
-                )
+        elif isinstance(repl, (ParamSpecType, Parameters, CallableType)):
+            return expand_param_spec(t, repl)
         else:
             # TODO: should this branch be removed? better not to fail silently
             return repl
@@ -446,8 +422,8 @@ def visit_type_type(self, t: TypeType) -> Type:
         return TypeType.make_normalized(item)
 
     def visit_type_alias_type(self, t: TypeAliasType) -> Type:
-        # Target of the type alias cannot contain type variables,
-        # so we just expand the arguments.
+        # 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))
 
     def expand_types(self, types: Iterable[Type]) -> list[Type]:

mypy/fixup.py

@@ -180,6 +180,8 @@ def visit_var(self, v: Var) -> None:
 
     def visit_type_alias(self, a: TypeAlias) -> None:
         a.target.accept(self.type_fixer)
+        for v in a.alias_tvars:
+            v.accept(self.type_fixer)
 
 
 class TypeFixer(TypeVisitor[None]):

mypy/mixedtraverser.py

@@ -25,6 +25,9 @@
 class MixedTraverserVisitor(TraverserVisitor, TypeTraverserVisitor):
     """Recursive traversal of both Node and Type objects."""
 
+    def __init__(self) -> None:
+        self.in_type_alias_expr = False
+
     # Symbol nodes
 
     def visit_var(self, var: Var) -> None:
@@ -45,7 +48,9 @@ def visit_class_def(self, o: ClassDef) -> None:
 
     def visit_type_alias_expr(self, o: TypeAliasExpr) -> None:
         super().visit_type_alias_expr(o)
+        self.in_type_alias_expr = True
         o.type.accept(self)
+        self.in_type_alias_expr = False
 
     def visit_type_var_expr(self, o: TypeVarExpr) -> None:
         super().visit_type_var_expr(o)

mypy/nodes.py

@@ -12,6 +12,7 @@
     Callable,
     Dict,
     Iterator,
+    List,
     Optional,
     Sequence,
     Tuple,
@@ -2546,7 +2547,7 @@ class TypeAliasExpr(Expression):
 
     # The target type.
     type: mypy.types.Type
-    # Names of unbound type variables used to define the alias
+    # Names of type variables used to define the alias
     tvars: list[str]
     # Whether this alias was defined in bare form. Used to distinguish
     # between
@@ -2559,7 +2560,7 @@ class TypeAliasExpr(Expression):
     def __init__(self, node: TypeAlias) -> None:
         super().__init__()
         self.type = node.target
-        self.tvars = node.alias_tvars
+        self.tvars = [v.name for v in node.alias_tvars]
         self.no_args = node.no_args
         self.node = node
 
@@ -3309,10 +3310,9 @@ class TypeAlias(SymbolNode):
     class-valued attributes. See SemanticAnalyzerPass2.check_and_set_up_type_alias
     for details.
 
-    Aliases can be generic. Currently, mypy uses unbound type variables for
-    generic aliases and identifies them by name. Essentially, type aliases
-    work as macros that expand textually. The definition and expansion rules are
-    following:
+    Aliases can be generic. We use bound type variables for generic aliases, similar
+    to classes. Essentially, type aliases work as macros that expand textually.
+    The definition and expansion rules are following:
 
         1. An alias targeting a generic class without explicit variables act as
         the given class (this doesn't apply to TypedDict, Tuple and Callable, which
@@ -3363,11 +3363,11 @@ def f(x: B[T]) -> T: ...  # without T, Any would be used here
 
     Meaning of other fields:
 
-    target: The target type. For generic aliases contains unbound type variables
-        as nested types.
+    target: The target type. For generic aliases contains bound type variables
+        as nested types (currently TypeVar and ParamSpec are supported).
     _fullname: Qualified name of this type alias. This is used in particular
         to track fine grained dependencies from aliases.
-    alias_tvars: Names of unbound type variables used to define this alias.
+    alias_tvars: Type variables used to define this alias.
     normalized: Used to distinguish between `A = List`, and `A = list`. Both
         are internally stored using `builtins.list` (because `typing.List` is
         itself an alias), while the second cannot be subscripted because of
@@ -3396,7 +3396,7 @@ def __init__(
         line: int,
         column: int,
         *,
-        alias_tvars: list[str] | None = None,
+        alias_tvars: list[mypy.types.TypeVarLikeType] | None = None,
         no_args: bool = False,
         normalized: bool = False,
         eager: bool = False,
@@ -3446,12 +3446,16 @@ def name(self) -> str:
     def fullname(self) -> str:
         return self._fullname
 
+    @property
+    def has_param_spec_type(self) -> bool:
+        return any(isinstance(v, mypy.types.ParamSpecType) for v in self.alias_tvars)
+
     def serialize(self) -> JsonDict:
         data: JsonDict = {
             ".class": "TypeAlias",
             "fullname": self._fullname,
             "target": self.target.serialize(),
-            "alias_tvars": self.alias_tvars,
+            "alias_tvars": [v.serialize() for v in self.alias_tvars],
             "no_args": self.no_args,
             "normalized": self.normalized,
             "line": self.line,
@@ -3466,7 +3470,8 @@ def accept(self, visitor: NodeVisitor[T]) -> T:
     def deserialize(cls, data: JsonDict) -> TypeAlias:
         assert data[".class"] == "TypeAlias"
         fullname = data["fullname"]
-        alias_tvars = data["alias_tvars"]
+        alias_tvars = [mypy.types.deserialize_type(v) for v in data["alias_tvars"]]
+        assert all(isinstance(t, mypy.types.TypeVarLikeType) for t in alias_tvars)
         target = mypy.types.deserialize_type(data["target"])
         no_args = data["no_args"]
         normalized = data["normalized"]
@@ -3477,7 +3482,7 @@ def deserialize(cls, data: JsonDict) -> TypeAlias:
             fullname,
             line,
             column,
-            alias_tvars=alias_tvars,
+            alias_tvars=cast(List[mypy.types.TypeVarLikeType], alias_tvars),
             no_args=no_args,
             normalized=normalized,
         )