Warn about unnecessary unsafeCoerce

unsafeCoerce has been around a lot longer than coerce, so older code tends to use it even if coerce could do the job. It would be nice if the type checker could produce a warning when it detected such a situation.

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Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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changed weight to 5

added Tfeature request Trac import typechecker labels

mentioned in issue #9757 (closed)

I'm intrigued by this idea. But, it would surely have a performance impact, if extra analysis must be done on every use of unsafeCoerce. Would this be opt-in?

I would tend to think it should be opt-in anyways, at least for a couple compiler versions, since it would just annoy people who want to support those older compiler versions without coerce.

It's be a bit awkward to implement: it amounts to emitting a constraint, and then reporting a warning if the constraint can be solved, but nothing if it ''can't''. But I'm sure it's possible.

Trac metadata

Trac field	Value
TypeOfFailure	OtherFailure → IncorrectWarning

changed weight to 3

Trac metadata

Trac field	Value
Priority	normal → low

marked this issue as related to #15429 (closed)

added Plow label