[Sema] Improve error for unqualified use of static variable in method

[aeu]

Resolves

#48759

Summary

This PR improves the diagnostic emitted when a static member is referenced through an instance.
The previous diagnostic was misleading because it stated that the static member “cannot be used on an instance,” without clarifying the correct usage.
This change updates the diagnostic to explicitly state that static members must be referenced on the type.

Changes made

1. Added a new diagnostic to include/swift/AST/DiagnosticsSema.def
2. Updated lib/Sema/CSDiagnostics.cpp to extract the relevant tokens and emit the updated message
3. Added test coverage in test/Sema/static_member_on_instance.swift

Original Example (from Issue)

struct HasStatic {
    func foo() {
        print(cvar)
    }
    static let cvar = 123
}

Previous Output

$ swiftc 48759.swift 
48759.swift:3:15: error: static member 'cvar' cannot be used on instance of type 'HasStatic'
1 | struct HasStatic {
2 |     func foo() {
3 |         print(cvar)
  |               `- error: static member 'cvar' cannot be used on instance of type 'HasStatic'
4 |     }
5 |     static let cvar = 123

Updated Output

$ swiftc 48759.swift 
48759.swift:3:15: error: static member 'cvar' can only be used on the type 'HasStatic', not on the instance self
1 | struct HasStatic {
2 |     func foo() {
3 |         print(cvar)
  |               `- error: static member 'cvar' can only be used on the type 'HasStatic', not on the instance self
4 |     }
5 |     static let cvar = 123

[xedin]

Thank you for the contribution! I left a comment inline regarding the code itself and I think the new wording might not actually make things clearer.

[xedin]

I am not sure whether the diagnostic is any better but regardless of that this is not the way to get the instance reference, it should be recorded as part of recording a fix.

[xedin]

If I remember correctly we had some previous attempts at re-phrasing this diagnostic too, you might want to look for the discussion older PRs.

[aeu]

If I remember correctly we had some previous attempts at re-phrasing this diagnostic too, you might want to look for the discussion older PRs.

Hi @xedin - I was looking at #48759 which referenced SR-6207 and that was where I got the diagnostic suggestion from user @huonw (below) which is what I implemented.

static member 'cvar' can only be used on the type 'HasStatic', not the instance 'X'

I found another discussion in #12644 in which @jrose-apple was in agreement with Huon's recommendation.

I'm open to suggestions though, I was unable to think of anything better myself so I just went with what was in the original defect.

[aeu]

... but regardless of that this is not the way to get the instance reference, it should be recorded as part of recording a fix.

Hi @xedin - If I am reading your suggestion correctly, I should create a fix and record it with the base during constraint solving and then have the emitter read it rather than pulling it from the source text?