Nothing inferred when unexpected

[adampauls]

Compiler version

3.2.1-RC2

Minimized code

class Foo {
  def member: String = this.toString
}
def method[T](c: T => Boolean): T = ???
val y: Foo => Boolean = _ => true
val x : String = "5"
method(y).member // error

Output

Found:    Nothing
Required: ?{ member: ? }
Note that implicit conversions were not tried because the result of an implicit conversion
must be more specific than ?{ member: <?> }

Expectation

Should compile. I understand that Nothing is inferrable here and that I should not expect .member to be able to constrain the type of T. Since Nothing <: Person I suppose it makes sense to infer the tightest type, but does it? Scala widens types in many places and I'm not sure there's any reason to ever want to infer Nothing here. This case is simple enough that I suspect it is intended behavior, but I'm still curious.

[dwijnand]

I'm not sure. Is this a bug? Should T be instantiating to Foo?

[smarter]

We do have logic to avoid either uninstantiated type variables or type variables instantiated to Nothing in similar situations: https://github.com/lampepfl/dotty/pull/10205/files, so I think it's worth digging into why this doesn't trigger here.

[smarter]

Maybe T is instantiated early in interpolateTypeVars because it only appears in covariant positions (in which case we choose the most specific type), so we never reach couldInstantiateTypeVar. Not sure how to reconcile these two heuristics.

[odersky]

Yes, that's it. We interpolate before we have a chance to approximate by the upper bound in couldInstantiateTypeVar.

I think we can fix that by a adding a targeted special case in interpolateTypeVars.