AssertionError on anonymous class with refinement type and missing type parameters

[propensive]

The following combination causes a compiler crash in typer with Scala 2.12.1:

trait Foo[T]
object Foo {
  val foo: Foo[Any] { type Bar = Any } = new Foo { def baz(): Any = () }
}

I think the correct behavior should be to just output the error regarding the missing parameters on new Foo, and give up. Full output is here: error.txt.

The crash also appears with 2.12.0, but not 2.11.8.

[SethTisue]

someone want to check if this 1) should remain open and 2) isn't a duplicate?

[joroKr21]

It's still a problem. There are 5 other issues about foundType is Erroneous:
#10803, #10015, #10003, #9960, #1472

[joroKr21]

tree.tpe = <error>{}
tree.tpe.isErroneous = false

🤔

[SethTisue]

Dotty:

  |                                   found:    Object with Foo[Nothing]{...}
  |                                   required: Foo[Any]{Bar = Any}

[joroKr21]

But dotty doesn't issue an error about trait Foo takes type parameters because it has type inference on extends (I think there's an issue about that but can't find it). The problem is that:

new Foo { def baz(): Any = () }

desugars to something like:

{
  class anon$1 extends Foo { // here dotty infers Nothing but scalac doesn't try to
    def baz(): Any = ()
  }
  new anon$1
}

Instead anon$1 ends up having an error type as a parent as in class anon$1 extends <error>. But TypeMap and by extension isErroneous don't look in the parents.

However AdaptTypeError exposes the parents:
https://github.com/scala/scala/blob/284a9c03c650c76e2b5295fee5c03565bcdf5246/src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala#L212-L215

[joroKr21]

I believe we should not try so hard to construct a meaningful type mismatch error for an anonymous class with erroneous definition.

[joroKr21]

PR here scala/scala#7635