[swift-evolution] [Pitch] Require tuple conversions to be explicit when labels don't match

[Xiaodi Wu]

A sensible solution, IMO. Error with Fix-It when attempting to convert
implicitly between tuples with mismatched labels.

On Mon, May 9, 2016 at 01:47 Jacob Bandes-Storch via swift-evolution <
swift-evolution at swift.org> wrote:

> There was some previous discussion under "[Discussion] Enforce argument
> labels on tuples
> <http://thread.gmane.org/gmane.comp.lang.swift.evolution/14910>".
>
> Halfway through the thread, Haravikk clearly stated the key point:
>
> On Thu, Apr 21, 2016 at 12:14 AM, Haravikk via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> I think the important thing to remember is that the label check is
>> intended to prevent cases like this:
>> let a:(left:Int, right:Int) = (1, 2)
>> var b:(right:Int, left:Int) = a
>> While the two tuples are compatible by type, the meaning of the values
>> may differ due to the different labels; in this case the values are
>> represented in a different order that a developer should have to explicitly
>> reverse to ensure they aren’t making a mistake, or they could represent
>> radically different concepts altogether.
>
>
>
> I agree there's a potential for confusion here, and I suggest we should
> add an error (or warning) imploring the user to make the conversion
> explicit, when the source tuple is labeled:
>
>     func foo() -> (left: Int, right: Int) { return (3, 4) }
>
>     let (left: a, right: b) = foo()  // ok, labels match
>
>     var x = 0, y = 0
>     (left: x, right: y) = (1, 3)  // ok, source is unlabeled
>
>     let (c, d) = foo()
>     // error: conversion between tuple types '(left: Int, right: Int)' and
> '(Int, Int)' requires explicit 'as' operator
>     // suggested fix: "let (c, d) = foo() as (Int, Int)"
>
>     let (right: e, left: f) = foo()
>     // error: conversion between tuple types '(left: Int, right: Int)' and
> '(right: Int, left: Int)' requires explicit 'as' operator
>     // suggested fix: "let (right: e, left: f) = foo() as (right: Int,
> left: Int)"
>
>
> Thoughts?
>
> Jacob
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