> On 27 Jan 2017, at 01:30, Xiaodi Wu via swift-evolution
> <swift-evolution@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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