Wrong error message when instantiating a trait from scala 2.13 stdlib

[prolativ]

Compiler version

3.2.1 and before

Minimized code

new concurrent.ExecutionContext

Output

-- [E042] Type Error: ----------------------------------------------------------
1 |new concurrent.ExecutionContext
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |    Unit is abstract; it cannot be instantiated
  |
  | longer explanation available when compiling with `-explain`

With -explain enabled:

-- [E042] Type Error: ----------------------------------------------------------
1 |new concurrent.ExecutionContext
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |    Unit is abstract; it cannot be instantiated
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | Abstract classes and traits need to be extended by a concrete class or object
  | to make their functionality accessible.
  |
  | You may want to create an anonymous class extending Unit with
  |   class Unit { }
  |
  | or add a companion object with
  |   object Unit extends Unit
  |
  | You need to implement any abstract members in both cases.
   -----------------------------------------------------------------------------

Expectation

The actual name of the instantiated trait (e.g. ExecutionContext in the example above) should be displayed instead of Unit.

[yzia2000]

Hi I would like to tackle this issue. It seems that scala 2 type symbol passed into checkInstantiable is Unit while scala 3 has the right symbol.

Since this is my first dotty issue, @prolativ may I know why this issue was marked as good-first-issue? Have we dealt with such cases in the past?

[nizhikov]

Hello. I prepared PR to fix this - #16635

[nizhikov]

@prolativ Can you, please, help me. What is the next steps to get this fix (#16635) merged?

[yzia2000]

Hey @nizhikov, why is it that the symbol in Apply type (tree.tpe) has the right name for Scala 3 but not for Scala 2? While the symbol in New (nu.tpe) does have the right symbol in it for both? Should the difference be there in the first place?