[Feature Request] Support explicit syntax for partial parameter binding of Self

[soraros]

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

As title. Instead of this:

struct Pointer[
  T: AnyType,
  origin: Origin,
  address_space: AddressSpace,
]:
  alias _with_origin = Pointer[T, _, address_space]

  fn __init__(
    other: Self._with_origin[_],
    out self: Self._with_origin[_immutable_cast[other.origin].result],
  ):
    ...

I want to write:

struct Pointer[
  T: AnyType,
  origin: Origin,
  address_space: AddressSpace,
]:
  fn __init__(
    other: Self[origin=_],  # unbinding
    out self: Self[origin=_immutable_cast[other.origin].result],  # rebinding
  ):
    ...

What is your motivation for this change?

• It's useful.
• This behaviour is consistent with other type aliases.
• It avoids the need to create ad-hoc names that don't make much sense.

Any other details?

As an added benefit, we get Float64[width=4] etc, which is also vastly useful.

Related: #1904.

[owenhilyard]

I like this train of thought, but I think we need a way to also do parameter kwargs.

[soraros]

As an example, #3083 would look much cleaner with this feature.