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

[Daniel Duan]

> 
> On Jan 31, 2017, at 8:47 AM, Alex Hoppen <alex at ateamer.de> wrote:
> 
> Amendment to the history of the bug after I had a look at the bug reports again: SR-1895 explicitly asked that 
> 
> let s: String? = "hi"
> s.map {print($0)}

This is the anti-pattern we try to discourage. FYI.

> should not produce any warnings while it did so during beta 1. 
> 
> – Alex
> 
>> On 31 Jan 2017, at 09:07, Alex Hoppen via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> This was a deliberate change between Swift 3 beta 1 and beta 2 after a friend of mine pointed the following inconsistency out to me:
>> 
>> struct Foo {
>>  func bar() {}
>> }
>> let foo: Foo? = Foo()
>> foo?.bar() // Does not create a warning
>> true ? foo?.bar() : foo?.bar()  // expression of type '()?' is unused
>> 
>> After some offline discussion at WWDC with the Swift team we decided to move to a consistent model where ()?, ()??, … is always discardable since we didn't want to take the convenience of foo?.bar() away (something that regularly occurs with weak variables, e.g. captures in closures).
>> 
>> So much for the history of this feature.
>> 
>> – Alex
>> 
>> 
>>> On 30 Jan 2017, at 22:58, 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.
>>> 
>>> What do y’all think?
>>> 
>>> Daniel Duan
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>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
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> 
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