[SR-12093] Checked Casts With Literals Are Deeply Unintuitive

[CodaFi]

Previous ID	SR-12093
Radar	rdar://problem/58999135
Original Reporter	@CodaFi
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, StarterProposal
Assignee	inatonix (JIRA)
Priority	Medium

md5: f23b80191801a31166f6b799915c7a23

cloned from:

• SR-11421 Checked Cast Diagnostics Should Be More Specific When Literals Are Involved

Issue Description:

Checked casts on literals can have surprising effects at face value

let character = "A" as Character
let bad = "A" as? Character

The former is a character literal and is statically coerced. The latter is a String literal that is being dynamically cast to a Character. This cast will always fail, and we note as much - this also works with as! force casts.

warning: cast from 'String' to unrelated type 'Character' always fails

This behavior is deeply unintuitive and should be revisited by a proposal to unify casting semantics for literals.

[beccadax]

@swift-ci create

[LucianoPAlmeida]

Just for reference, link to a PR related to the cloned bug #29493

[swift-ci]

Comment by shuya inada (JIRA)

Is it okay to tackle on this task? This is my first commit for Swift.

[CodaFi]

inatonix (JIRA User) You should know that this might involve a lot of work up front for one's first bug. You'll need to take a look at all the places where we aren't consistent between the static and dynamic coercions, figure out what we're missing, and draw up a proposal that provides a unified and consistent semantics for the two. Then you'll need to implement your proposal and drive it through evolution.

If this still sounds like a path you'd like to pursue, then by all means let me know how I can help. But if you're interest in a separate starter bug recommendation, I can give that too.

[swift-ci]

Comment by shuya inada (JIRA)

@CodaFi Thank you for your comment! I got it, this problem sounds pretty difficult as my first task on Swift.. Do you have any recommendation task like separated starter bugs as the first contribution to Swift?