[swift-evolution] Warn about unused Optional.some(())

[Daniel Duan]

Not sure if this requires a proposal. The code change is pretty trivial: https://github.com/apple/swift/pull/7154 <https://github.com/apple/swift/pull/7154> .

> On Jan 30, 2017, at 3:25 PM, Slava Pestov <spestov at apple.com> wrote:
> 
>> 
>> On Jan 30, 2017, at 2:58 PM, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Hi all,
>> 
>> Right now, expressions that evaluates to Optional<()>, Optional<Optional<()>>… gets special treatment when it’s unused. For example:
>> 
>> func f(s: String) {}
>> let s: String = “”
>> s.map(f) // no warning here, even tho the resulting type is `Optional<()>` and unused.
>> 
>> func g() throws {}
>> try? g() // no warnings here neither.
>> 
>> This is convenient, but encourages composing map/filter/reduce, etc with side-effect-ful functions, which we have found a few cases of in our production code recently. Granted, these cases could’ve been caught with more careful code reviews. But we wouldn’t have missed them if this “feature” didn’t exist.
>> 
>> I think we should remove the special treatment so that code in the example above would generate a warning about `()?` being unused. Users can silence it manually by assigning the result to `_`. 
>> 
>> OTOH, this would undermine the convenience of `try?` when the throwing function don’t return anything.
> 
> IMHO, using ‘try?’ to ignore an error result, instead of just turning it into an optional, is an anti-pattern, and forcing users to write ‘_ = try? foo()’ might not be so bad…
> 
>> 
>> What do y’all think?
>> 
>> Daniel Duan
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