Eta-expansion doesn't insert default arguments

This fails in Scalac and Dotty, but seems a bit silly, especially because f _ doesn’t get defaults but f(_) does:

scala> def f(a: Int, b: Boolean = true): Int = a
f: (a: Int, b: Boolean)Int

scala> f: (Int => Int)
<console>:13: error: type mismatch;
 found   : (Int, Boolean) => Int
 required: Int => Int
       f: (Int => Int)
       ^
cala> (f _): (Int => Int)
1 |(f _): (Int => Int)
  |  ^
  |  found:    (Int, Boolean) => Int
  |  required: Int => Int
  |
scala> (f(_)): (Int => Int)
val res0: Int => Int = Lambda$1550/925936450@537c8c7e

This came up in https://github.com/lampepfl/dotty/pull/4276/commits/23464fd64e3a4d1e3c2c2635c289543690d70a5e#diff-b59d25461c7203c18a90d6373be182cdL541

[LPTK]

Following TypeScript, the type of expression f (or f _ for that matter) should be (a: Int, b?: Boolean) => Int where the ? in b? means the parameter is optional; and this would be a subtype of Int => Int.

In TypeScript:

function f(a: number, b: boolean = true): number { return a }
let g: (x: number, y?: boolean) => number = f
let h: (_:number) => number = true ? f : g

IMHO this is yet another example of how full-featured function types – which can properly reflect all method types – would make everything easier and more regular in Scala, the same way they do in TypeScript.

[OlivierBlanvillain]

I will close this as it’s non actionable in it’s current state. It sounds like it would require non trivial changes (the addition of default-parameterized function types?) and would likely be a breaking change.