## Summary
Came across this while debugging some ecosystem changes in
https://github.com/astral-sh/ruff/pull/18347. I think the meta-type of a
typevar-annotated variable should be equal to `type`, not `<class
'object'>`.
## Test Plan
New Markdown tests.
## Summary
Allow a typevar to be callable if it is bound to a callable type, or
constrained to callable types.
I spent some time digging into why this support didn't fall out
naturally, and ultimately the reason is that we look up `__call__` on
the meta type (since its a dunder), and our implementation of
`Type::to_meta_type` for `Type::Callable` does not return a type with
`__call__`.
A more general solution here would be to have `Type::to_meta_type` for
`Type::Callable` synthesize a protocol with `__call__` and return an
intersection with that protocol (since for a type to be callable, we
know its meta-type must have `__call__`). That solution could in
principle also replace the special-case handling of `Type::Callable`
itself, here in `Type::bindings`. But that more general approach would
also be slower, and our protocol support isn't quite ready for that yet,
and handling this directly in `Type::bindings` is really not bad.
Fixes https://github.com/astral-sh/ty/issues/480
## Test Plan
Added mdtests.
It's possible for a typevar to list another typevar as its default
value:
```py
class C[T, U = T]: ...
```
When specializing this class, if a type isn't provided for `U`, we would
previously use the default as-is, leaving an unspecialized `T` typevar
in the specialization. Instead, we want to use what `T` is mapped to as
the type of `U`.
```py
reveal_type(C()) # revealed: C[Unknown, Unknown]
reveal_type(C[int]()) # revealed: C[int, int]
reveal_type(C[int, str]()) # revealed: C[int, str]
```
This is especially important for the `slice` built-in type.
