Contextual vararg parameters

[prolativ]

Compiler version

3.3.2-RC1-bin-20230627-588a0b1-NIGHTLY and before

Minimized code

def foo(using xs: Int*) = xs
val f = foo

Output

[error] UsingVarargs.scala:2:12
[error] no implicit argument of type Int* was found for parameter xs of method foo
[error] val f = foo
[error]            ^

Expectation

There seems to be no way to define an instance of a vararg type, e.g. given Int* = 0 won't work.
Even if one defines a given instance of a corresponding non-vararg type or a type wrapped into a Seq (given Int = 0 or given Seq[Int] = Seq(0)) it won't be picked by the implicits resolution. The only way to pass such parameter seems to be foo(using 0).
Taking that into account it seems reasonable to raise a warning when a user defines a using parameter with a vararg type.

[nicolasstucki]

This should be an error. The signature is non-sensical as we could find no implicit or the same one and provide it an arbitrary number of times.

[nicolasstucki]

This should probably be checked while typing the def.

[Kordyjan]

What's funny is that this function is still usable:

foo(using 1, 2, 3)

works just fine.

I agree that declarations like this are nonsensical and should be either a warning on an error.

[scala-center-bot]

This issue was picked for the Issue Spree No. 33 of 11 July 2023 which takes place in 7 days. @smarter, @eugenefle, @iusildra will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.