[SR-6118] mechanism to hand through #file/#line in subscripts

[weissi]

Previous ID	SR-6118
Radar	None
Original Reporter	@weissi
Type	New Feature
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	New Feature
Assignee	@slavapestov
Priority	Medium

md5: b802d9f73456058327d450079e18b6e4

Issue Description:

For my hypothetical MyArray implementation, in normal functions I can write

function get(index: Int, file: StaticString = #file, line: UInt = #line) -> Value {
    if !checkIndex(index) {
        fatalError("index \(index) out of bound", file: file, line: line)
    }
    ...
}

and I'll be able to create good error messages for the user.

In subscripts however, default arguments are disallowed so

subscript(index: Int) -> Value {
    if !checkIndex(index) {
        fatalError("index \(index) out of bound")
    }
    ....
}

will always show its literal line and file rather than the callers.

[belkadan]

That should look like this:

struct A {
  subscript(a: Int, file: String = #file) -> Int {
    print(file)
    return a
  }
}

print(A()[1])

but we don't allow default arguments in subscripts at the moment (because it's unclear how they would work with inout subscripts).

[weissi]

@belkadan, sorry, could you expand on the inout subscripts? I don't know what they are 🙂

[slavapestov]

#23734

[swift-ci]

Comment by Joseph Twomey (JIRA)

@swift-ci create