[swift-evolution] Default Generic Arguments

Xiaodi Wu xiaodi.wu at gmail.com
Fri Jan 27 18:32:37 CST 2017

Oh, it's precisely my confidence that a good error message can be devised
which makes me ponder whether "prefer user" is the ideal rule. Having a
stricter rule isn't necessarily bad if the error message makes it easy to

In your example, "prefer user" would object at the line where you make your
Something. I think that makes for a much cleaner error. By contrast, DWIM
necessitates the acrobatics you show above, where the compiler will have to
keep track of a defaulted type for each variable as long as it's in scope
and propose remote fix-its at the declaration site based on how it's later
used. Now what happens if there's an action() that takes only
Something<Int> arguments and an action2() that takes only Something<Int64>
arguments? Will you have an alternating cycle of fix-its that don't fix the

On Fri, Jan 27, 2017 at 18:07 Karl Wagner <razielim at gmail.com> wrote:

> On 27 Jan 2017, at 01:30, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
> Cool, thanks--that makes sense.
> Personally, although DWIM is appealing, I think if we are to go all-out on
> your stance that "adding a default to an existing type parameter should be
> a strict source-breaking change," then "prefer user" is the one rule that
> maximally clarifies the scenario. With that rule, in the evolution
> scenarios that I brought up, either the user-specified default and the
> inferred literal type line up perfectly or it is guaranteed to be
> source-breaking. IMO, that consistency would bring more clarity than DWIM,
> which might prompt a user to be confused why sometimes the compiler "gets
> it" and other times it doesn’t.
> I’m not sure, I think it will be easy enough for users to figure out where
> the problem is because it will create a type-mismatch.
> When type mismatches occur, the only place to look is the variable
> definition, because that is where the type is defined.
> This is such a narrow case that I’m sure we can provide good diagnostics
> for it. The pattern could be:
> - A generic parameter mismatch (i.e. trying to use a value of type
> MyType<X> where type MyType<Y> is expected), and
> - X and Y are both {Whatever}LiteralConvertible, and
> - X is the default type bound to that parameter, and
> - the value was initialised using a {Whatever} literal, where an instance
> of the parameter was expected
> In that case, we could introduce a simple fix-it: replacing one of the
> literal values with "(literal as Y)”
> for example:
> struct Something<T=Int64> { let value: T }
> func action(_: Something<Int>) { … } // Expects a specific kind of
> Something<T>
> let myThing = Something(value: 42) // Fix-it: Did you
> mean ‘Something(value: 42 as Int)’?
> action(myThing)                    // Error: No overload for ‘action’
> which takes a Something<Int64>.
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