Nothing inferred in polymorphic, inlined method

[biboudis]

minimized code

object Test {
  inline def meth[R <: Int]: R = 3.asInstanceOf[R] // inferred type is Nothing
  def main(args: Array[String]): Unit = {
    val t: Int = meth
  }
}

expectation

It should work as in the normal case.

[biboudis]

There is something weird in inferencing when instantiation is occurring (in Inferencing.scala when Inferencing.isFullyDefined is called from inliner 🤷‍♂) :

committing TS[8, 1, 0] to TS[1, 0], fromConstr = Constraint(
 uninstVars = ;
 constrained types = [R <: Int] => R
 bounds = 
     R := Nothing
 ordering = 
), toConstr = Constraint(
 uninstVars = R;
 constrained types = [R <: Int] => R
 bounds = 
     R <: Int
 ordering = 
)

[odersky]

Inlining instantiates type arguments to the call. You can try to leave out the isFullyDefined call in the Inliner, but I guess bad things would happen. It seems like a scary feature interaction to inline code that references type variables that are not yet instantiated.