[swift-evolution] Default Generic Arguments

[Karl Wagner]

> 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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