I’m not entirely sure what an “expr-collection” is. Does your proposal mean that in this code: func foo() -> Int {...} var w = 0 var x = T(foo()) var y = T(w) var z = T(0) different initializers would be used for `x`,`y`, and `z`? If so, that seems a potential source of much subtler problems.
I don’t disagree that you’ve identified a potential source of issues, but it’s conceivable that there might be circumstances where the "semantically very different results” are desired. I can’t think of any off the top of my head, but I’m not convinced that means they don’t exist. So… I’m tentatively -1 - Dave Sweeris > On Jun 2, 2016, at 11:08 AM, John McCall via swift-evolution > <swift-evolution@swift.org> wrote: > > The official way to build a literal of a specific type is to write the > literal in an explicitly-typed context, like so: > let x: UInt16 = 7 > or > let x = 7 as UInt16 > > Nonetheless, programmers often try the following: > UInt16(7) > > Unfortunately, this does not attempt to construct the value using the > appropriate literal protocol; it instead performs overload resolution using > the standard rules, i.e. considering only single-argument unlabelled > initializers of a type which conforms to IntegerLiteralConvertible. Often > this leads to static ambiguities or, worse, causes the literal to be built > using a default type (such as Int); this may have semantically very different > results which are only caught at runtime. > > 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. > > Therefore, I propose that we adopt the following typing rule: > > Given a function call expression of the form A(B) (that is, an expr-call > with a single, unlabelled argument) where B is an expr-literal or > expr-collection, if A has type T.Type for some type T and there is a declared > conformance of T to an appropriate literal protocol for B, then the > expression is always resolves as a literal construction of type T (as if the > expression were written "B as A") rather than as a general initializer call. > > Formally, this would be a special form of the argument conversion constraint, > since the type of the expression A may not be immediately known. > > Note that, as specified, it is possible to suppress this typing rule by > wrapping the literal in parentheses. This might seem distasteful; it would > be easy enough to allow the form of B to include extra parentheses. It's > potentially useful to have a way to suppress this rule and get a normal > construction, but there are several other ways of getting that effect, such > as explicitly typing the literal argument (e.g. writing "A(Int(B))"). > > A conditional conformance counts as a declared conformance even if the > generic arguments are known to not satisfy the conditional conformance. This > permits the applicability of the rule to be decided without having to first > decide the type arguments, which greatly simplifies the type-checking problem > (and may be necessary for soundness; I didn't explore this in depth, but it > certainly feels like a very nasty sort of dependence). We could potentially > weaken this for cases where A is a direct type reference with bound > parameters, e.g. Foo<Int>([]) or the same with a typealias, but I think > there's some benefit from having a simpler specification, both for the > implementation and for the explicability of the model. > > John. > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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