Improve user experience when unapplySeq doesn't conform to the expected "structural" type

[lrytz]

In 2.12 this works:

scala> object A { def unapplySeq(x: Int) = Some(collection.mutable.ArrayBuffer(1,2,3)) }
defined object A

scala> 1 match { case A(1, xs @ _*) => xs }
res3: Seq[Int] = ArrayBuffer(2, 3)

2.13 (with scala/scala#7068) gives a cryptic error message:

scala> object A { def unapplySeq(x: Int) = Some(collection.mutable.ArrayBuffer(1,2,3)) }
defined object A

scala> 1 match { case A(1, xs @ _*) => xs }
         ^
       error: error during expansion of this match (this is a scalac bug).
       The underlying error was: type mismatch;
        found   : scala.collection.mutable.ArrayBuffer[Int]
        required: Seq[Int]

The reason here is that drop returns an ArrayBuffer. The interface for name-based pattern matching requires drop to return a scala.Seq.

[lrytz]

Test case added in scala/scala#7120, the neg test should get a better error message.

[szeiger]

The reason here is that drop returns an ArrayBuffer

I was never a fan of expecting different result types with and without drop because it causes unnecessary restrictions as in the example above and creates unnecessary toSeq calls in many other cases. My preference is to add toSeq whenever the type is not statically known to be scala.Seq, with or without drop.

[adriaanm]

Let's improve the error message in 2.13.1. This does not block RC1.