Obscure error messages with default args in application

[som-snytt]

Compiler version

Scala compiler version 3.3.0-RC1-bin-SNAPSHOT-git-a1729b0 -- Copyright 2002-2022, LAMP/EPFL

Minimized code

➜  dotty git:(test/current) ./bin/scalac -d /tmp ./tests/untried/neg/t4727.scala
-- [E007] Type Mismatch Error: ./tests/untried/neg/t4727.scala:5:8 -----------------------------------------------------
5 |    new C[Int]
  |        ^^^^^^
  |        Found:    Null
  |        Required: Int
  |        Note that implicit conversions were not tried because the result of an implicit conversion
  |        must be more specific than Int
  |
  | longer explanation available when compiling with `-explain`
1 error found

That is

➜  dotty git:(test/current) cat ./tests/untried/neg/t4727.scala
class C[T](x : T = null)

object Test {
  def main(args: Array[String]): Unit = {
    new C[Int]
  }
}

Output

Scala 2 tweaks the message with a hint that default args are involved.

t4727.scala:5: error: an expression of type Null is ineligible for implicit conversion
Error occurred in an application involving default arguments.
    new C[Int]
    ^
1 error

Expectation

It's a small thing, but maybe when default args are re-engineered or tweaked, a hint could be added along these lines.

[som-snytt]

The longer explanation includes the desired hint in encoded form, as from the tree one can infer it was a default arg at fault.

Experiment with added info in short form, and expanded info in long form.

-- [E007] Type Mismatch Error: tests/neg/t4727.scala:7:8 -----------------------
7 |    new C[Int] // error
  |        ^^^^^^
  |        Found:    Null
  |        Required: Int
  |
  |        Error occurred in an application involving default arguments.
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |
  | Tree: C.$lessinit$greater$default$1[Int]
  | I tried to show that
  |   Null
  | conforms to
  |   Int
  | but none of the attempts shown below succeeded:
  |
  |   ==> Null  <:  Int  = false
  |
  | The tests were made under the empty constraint
  |
  | Expanded application: new C[Int](C.$lessinit$greater$default$1[Int])
   -----------------------------------------------------------------------------