Should error on ignored type parameter in left-hand side of type alias

[goretkin]

  | | |_| | | | (_| |  |  Version 1.6.0-DEV.1653 (2020-12-04)
 _/ |\__'_|_|_|\__'_|  |  Commit 0db72ee7d2* (1 day old master)
|__/                   |

julia> MyType{K, T} = Set{T}
Set

Not mentioned in https://docs.julialang.org/en/v1/manual/types/#UnionAll-Types

Sorry in advance if this is a duplicate issue.

[Seelengrab]

Since they're equivalent, superfluos type parameters are no longer printed on 1.6. Makes types in errors and stacktraces much more compact. I think the relevant PR was #36107.

julia> Set === Set{T} where T
true

[vtjnash]

The LHS is syntax for

julia> MyType = Set{T} where {K, T}
Set

And the unused K gets dropped, so you end up with MyType === Set

[goretkin]

@Seelengrab I think you misunderstood my concern

@vtjnash Thanks for the explanation. I'm not readily finding another conversation about unused type parameters, and that seems to be the reason underlying the non-error on what I want to error. As a side note, I wish you would not close the issues so quickly. Perhaps we want to improve the documentation. Or we want to acknowledge there may be something going on, but still won't fix it, in which case we definitely want to document it. In any case, I know a closed issue doesn't mean there can't be a discussion, so:

Currently, MyType = Set{T} where {K, T} and MyType{K, T} = Set{T} lower slightly differently. Using a simpler example:

julia> Meta.@lower MyType{K} = Int64
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─      const MyType
│        K = Core.TypeVar(:K)
│   %3 = Core.UnionAll(K, Int64)
│        MyType = %3
└──      return %3
))))

julia> Meta.@lower MyType = Int64 where {K}
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─      K = Core.TypeVar(:K)
│   %2 = Core.UnionAll(K, Int64)
│        MyType = %2
└──      return %2
))))

so even if Int64 where {K} does not error, that doesn't seem to force MyType{K} = Int64 to not error as well.

[vtjnash]

Closing issues helps others focus on other work. It's an aspect of triage and email management. We almost never lock them, since continued discussion and PR are welcome.

Yes, more precisely, there's a const decl also, but that didn't seem relevant to your question. To which, what is your question?

[goretkin]

The question is: Can we raise an error on MyType{K, T} = Set{T}?