Offer additional note for why method with Self reference cannot be used on any SomeType

[ktoso]

Motivation

The following snippet;

protocol Base {
    associatedtype SomeID: Hashable
}

protocol Alpha: Base {
    func take(id: Self.SomeID)
}

struct Use {
    let alpha: any Alpha
    
    func test() {
        alpha.take(id: 12) // error: Member 'take' cannot be used on value of type 'any Alpha'; consider using a generic constraint instead
    }
}

is correct in rejecting the line, since we need Self.SomeID to be well typed, but we dont know what its type is via the any.

It'd be helpful if we can point out that "why" the method can't be used via a note, in a method with many parameters one can sometimes miss the Self reference.

Proposed solution

Adding a note on the offending parameter would be good

Alternatives considered

No response

Additional information

No response

[amritpan]

@GunjanRawat26 This could be a good intro to diagnostics

[ktoso]

Linking with radar rdar://135445764

Contributions welcome here, this could be an simple first issue maybe?

[flash1729]

hey @ktoso , i would like to work on this as my first issue

from what i have understood is we need to give a more refined compiler message for this error and for it

1. identify where in the compiler this error message is generated
2. change the message there to be more specific

it would be great if you could guide me further on anything that i am missing

[flash1729]

Current progress :
I found the code related to error message in DiagnosticsSema.def

ERROR(could_not_use_member_on_existential,none,
      "member %1 cannot be used on value of type %0; consider using a"
      " generic constraint instead",
      (Type, DeclNameRef))

// error: member 'take' cannot be used on value of type 'any Alpha'; consider using a generic constraint instead

This is where changes will be made to show different error message

Questions

1. Are there any related places where this code is rewritten or referenced so changes will be made there as well ?
2. this is my first contribution so i dont know how to set up environment to test our changes so for that i am reading GettingStarted.md. I am a bit unsure about exactly what needs to be done rn.
3. I am understanding what error is exactly. also are there any specific guidelines for wording error message ?