[ty] Add support for generic PEP695 type aliases (#20219)

## Summary

Adds support for generic PEP695 type aliases, e.g.,
```python
type A[T] = T
reveal_type(A[int]) # A[int]
```

Resolves https://github.com/astral-sh/ty/issues/677.

crates/ty_python_semantic/resources/mdtest/generics/pep695/aliases.md

@@ -0,0 +1,172 @@
+# Generic type aliases: PEP 695 syntax
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+## Defining a generic alias
+
+At its simplest, to define a type alias using PEP 695 syntax, you add a list of `TypeVar`s,
+`ParamSpec`s or `TypeVarTuple`s after the alias name.
+
+```py
+from ty_extensions import generic_context
+
+type SingleTypevar[T] = ...
+type MultipleTypevars[T, S] = ...
+type SingleParamSpec[**P] = ...
+type TypeVarAndParamSpec[T, **P] = ...
+type SingleTypeVarTuple[*Ts] = ...
+type TypeVarAndTypeVarTuple[T, *Ts] = ...
+
+# revealed: tuple[T@SingleTypevar]
+reveal_type(generic_context(SingleTypevar))
+# revealed: tuple[T@MultipleTypevars, S@MultipleTypevars]
+reveal_type(generic_context(MultipleTypevars))
+
+# TODO: support `ParamSpec`/`TypeVarTuple` properly
+# (these should include the `ParamSpec`s and `TypeVarTuple`s in their generic contexts)
+reveal_type(generic_context(SingleParamSpec))  # revealed: tuple[()]
+reveal_type(generic_context(TypeVarAndParamSpec))  # revealed: tuple[T@TypeVarAndParamSpec]
+reveal_type(generic_context(SingleTypeVarTuple))  # revealed: tuple[()]
+reveal_type(generic_context(TypeVarAndTypeVarTuple))  # revealed: tuple[T@TypeVarAndTypeVarTuple]
+```
+
+You cannot use the same typevar more than once.
+
+```py
+# error: [invalid-syntax] "duplicate type parameter"
+type RepeatedTypevar[T, T] = ...
+```
+
+## Specializing type aliases explicitly
+
+The type parameter can be specified explicitly:
+
+```py
+from typing import Literal
+
+type C[T] = T
+
+def _(a: C[int], b: C[Literal[5]]):
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: Literal[5]
+```
+
+The specialization must match the generic types:
+
+```py
+# error: [too-many-positional-arguments] "Too many positional arguments: expected 1, got 2"
+reveal_type(C[int, int])  # revealed: Unknown
+```
+
+And non-generic types cannot be specialized:
+
+```py
+type B = ...
+
+# error: [non-subscriptable] "Cannot subscript non-generic type alias"
+reveal_type(B[int])  # revealed: Unknown
+
+# error: [non-subscriptable] "Cannot subscript non-generic type alias"
+def _(b: B[int]): ...
+```
+
+If the type variable has an upper bound, the specialized type must satisfy that bound:
+
+```py
+type Bounded[T: int] = ...
+type BoundedByUnion[T: int | str] = ...
+
+class IntSubclass(int): ...
+
+reveal_type(Bounded[int])  # revealed: Bounded[int]
+reveal_type(Bounded[IntSubclass])  # revealed: Bounded[IntSubclass]
+
+# TODO: update this diagnostic to talk about type parameters and specializations
+# error: [invalid-argument-type] "Argument is incorrect: Expected `int`, found `str`"
+reveal_type(Bounded[str])  # revealed: Unknown
+
+# TODO: update this diagnostic to talk about type parameters and specializations
+# error: [invalid-argument-type] "Argument is incorrect: Expected `int`, found `int | str`"
+reveal_type(Bounded[int | str])  # revealed: Unknown
+
+reveal_type(BoundedByUnion[int])  # revealed: BoundedByUnion[int]
+reveal_type(BoundedByUnion[IntSubclass])  # revealed: BoundedByUnion[IntSubclass]
+reveal_type(BoundedByUnion[str])  # revealed: BoundedByUnion[str]
+reveal_type(BoundedByUnion[int | str])  # revealed: BoundedByUnion[int | str]
+```
+
+If the type variable is constrained, the specialized type must satisfy those constraints:
+
+```py
+type Constrained[T: (int, str)] = ...
+
+reveal_type(Constrained[int])  # revealed: Constrained[int]
+
+# TODO: error: [invalid-argument-type]
+# TODO: revealed: Constrained[Unknown]
+reveal_type(Constrained[IntSubclass])  # revealed: Constrained[IntSubclass]
+
+reveal_type(Constrained[str])  # revealed: Constrained[str]
+
+# TODO: error: [invalid-argument-type]
+# TODO: revealed: Unknown
+reveal_type(Constrained[int | str])  # revealed: Constrained[int | str]
+
+# TODO: update this diagnostic to talk about type parameters and specializations
+# error: [invalid-argument-type] "Argument is incorrect: Expected `int | str`, found `object`"
+reveal_type(Constrained[object])  # revealed: Unknown
+```
+
+If the type variable has a default, it can be omitted:
+
+```py
+type WithDefault[T, U = int] = ...
+
+reveal_type(WithDefault[str, str])  # revealed: WithDefault[str, str]
+reveal_type(WithDefault[str])  # revealed: WithDefault[str, int]
+```
+
+If the type alias is not specialized explicitly, it is implicitly specialized to `Unknown`:
+
+```py
+type G[T] = list[T]
+
+def _(g: G):
+    reveal_type(g)  # revealed: list[Unknown]
+```
+
+Unless a type default was provided:
+
+```py
+type G[T = int] = list[T]
+
+def _(g: G):
+    reveal_type(g)  # revealed: list[int]
+```
+
+## Aliases are not callable
+
+```py
+type A = int
+type B[T] = T
+
+# error: [call-non-callable] "Object of type `TypeAliasType` is not callable"
+reveal_type(A())  # revealed: Unknown
+
+# error: [call-non-callable] "Object of type `GenericAlias` is not callable"
+reveal_type(B[int]())  # revealed: Unknown
+```
+
+## Recursive Truthiness
+
+Make sure we handle cycles correctly when computing the truthiness of a generic type alias:
+
+```py
+type X[T: X] = T
+
+def _(x: X):
+    assert x
+```

crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md

@@ -40,13 +40,6 @@ You cannot use the same typevar more than once.
 class RepeatedTypevar[T, T]: ...
 ```
 
-You can only use typevars (TODO: or param specs or typevar tuples) in the class's generic context.
-
-```py
-# TODO: error
-class GenericOfType[int]: ...
-```
-
 You can also define a generic class by inheriting from some _other_ generic class, and specializing
 it with typevars. With PEP 695 syntax, you must explicitly list all of the typevars that you use in
 your base classes.

crates/ty_python_semantic/resources/mdtest/pep695_type_aliases.md

@@ -188,7 +188,7 @@ T = TypeVar("T")
 IntAnd = TypeAliasType("IntAndT", tuple[int, T], type_params=(T,))
 
 def f(x: IntAnd[str]) -> None:
-    reveal_type(x)  # revealed: @Todo(Generic PEP-695 type alias)
+    reveal_type(x)  # revealed: @Todo(Generic manual PEP-695 type alias)
 ```
 
 ### Error cases