Annotations on polymorphic method parameter/return types get lost

[j2rodrig]

Annotations on parameter/return types disappear when the type of a polymorphic method is built. This only happens where the annotations have an underlying PolyParam.

trait Annot extends annotation.StaticAnnotation with annotation.TypeConstraint

trait PolyBug {

    // This works
    type T
    def m(param1: T @Annot): T @Annot

    // These do not: annotations are dropped
    def m1[P](param1: P @Annot): P @Annot
    def m2[P](param1: List[P @Annot] @Annot): List[P @Annot] @Annot

    // This doesn't work either, but the annotations on T remain intact
    def m3[P](param1: Map[P @Annot,T @Annot] @Annot): Map[P @Annot,T @Annot] @Annot
}

In all cases above, the annotations exist on the typed tree. Annotations also exist on the typed tree's type, except for the type of the DefDef tree.

The computed types of the methods above are:

m: (param1: PolyBug.this.T @Annot)PolyBug.this.T @Annot
m1: [P](param1: P)P
m2: [P](param1: scala.collection.immutable.List[P])scala.collection.immutable.List[P]
m3: [P](param1: scala.collection.immutable.Map[P, PolyBug.this.T @Annot])
        scala.collection.immutable.Map[P, PolyBug.this.T @Annot]

Annotations still exist on the ValDef trees' types, but they are lost when computing the DefDef trees' types.

[j2rodrig]

Here's another example that looks related. Here, the annotation @annot is incorrectly dropped when computing the type of r1.constrained:

trait PolyBug2 {
    var unconstrained: AnyRef

    trait R1 {
        type T <: AnyRef
        val constrained: T @Annot
        unconstrained = this.constrained  // causes an error (constraint is retained)
    }

    val r1: R1
    unconstrained = r1.constrained        // CONSTRAINT ANNOTATION IS DROPPED HERE
}