[swift-evolution] Proposal: 'T(literal)' should construct T using the appropriate literal protocol if possible

[Xiaodi Wu]

See, I've made this mistake as well, and *not* because I thought it casts
the literal. I had always assumed that something like `UInt16(42)` would
initialize using the integer literal init, since it seems logical that
that's the most specific. The proposed change reflects my currently
erroneous mental model.
On Thu, Jun 2, 2016 at 16:42 Brent Royal-Gordon via swift-evolution <
swift-evolution at swift.org> wrote:

> > In my opinion, using this initializer-call syntax to build an
> explicitly-typed literal is an obvious and natural choice with several
> advantages over the "as" syntax.  However, even if you disagree, it's clear
> that programmers are going to continue to independently try to use it, so
> it's really unfortunate for it to be subtly wrong.
>
> I've seen developers do this; in one memorable case, it resulted in Swift
> taking a ridiculously long time to typecheck an expression, since the
> seemingly pinned-down types of the literals had actually become *more*
> ambiguous, not less.
>
> However, it's not difficult to teach developers to use `as`. Usually
> what's happening is that their mental model of the language is wrong: they
> think of `UInt16(foo)` as a cast to a primitive type, and are surprised to
> learn that it's actually an initializer on a struct and they're
> initializing an instance. Learning this helps them understand how the
> language works, what the difference is between initializers and `as`, and
> how they can write the same things they see in the standard library types.
>
> I think *actually* turning this into magic would be counterproductive. The
> better solution is to make the compiler replace me in that story, by having
> it emit a warning with a fix-it. It keeps initializer calls meaning exactly
> what they say. (And it doesn't require an evolution proposal to do, since
> you can add a warning with a mere bug.)
>
>         UInt16(42)
>         ^~~~~~ ^~
>         Use of initializer with integer literal does not cast '42' to
> 'UInt16'
>         Fix-It: Replace with '42 as UInt16'
>
> --
> Brent Royal-Gordon
> Architechies
>
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