Exponential compile times when pattern matching on type alias #12489
Description
reproduction steps
using Scala 2.13.5,
// Note: Removing `+` here has no effect on the issue:
type ~[+A, +B] = (A, B)
// Dummy type to allow us to map + pattern match
trait A[T] { def map[U](fn: T => U): A[U] = new A[U] {} }
// 25 nested Int's, both the same type
type B = ((((((((((((((((((((((((Int, Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int), Int)
type C = Int ~ Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int ~Int~Int
val ab = new A[B] {}
val ac = new A[C] {}
// Instant compile time:
ab.map { case ((((((((((((((((((((((((_, _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _) => 7 }
// Exponential compile time:
ac.map { case ((((((((((((((((((((((((_, _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _), _) => 7 }problem
Deconstructing a simple alias for a type in a case match causes exponentially increasing compile times, dependent on the depth of the deconstruction.
In the code above, type B and type C are the same, but C is constructed through aliases. Deconstructing type C takes vastly longer than deconstructing type B.
I would expect that ab.map... and ac.map... should take the same length of time to compile.