Compiler hang with covariant type constructor in GADT #14287
Description
Compiler version
3.1.1-RC2
Minimized code
// using scala 3.1.1-RC2 enum Free[+F[_], A]: case Return(a: A) case Suspend(s: F[A]) case FlatMap[F[_], A, B]( s: Free[F, A], f: A => Free[F, B]) extends Free[F, B] def flatMap[F2[x] >: F[x], B](f: A => Free[F2,B]): Free[F2,B] = FlatMap(this, f) @annotation.tailrec final def step: Free[F, A] = this match case FlatMap(FlatMap(fx, f), g) => fx.flatMap(x => f(x).flatMap(y => g(y))).step case FlatMap(Return(x), f) => f(x).step case _ => thisOutput
Compilation hangs when using scala-cli. In a larger code base, I've seen crashes with stacks like the following, though I haven't reproduced the crash in a standalone program. The hang & crash are both related to making F covariant in the definition of Free.
In the example above, changing the left-associated flatMap case to case FlatMap(FlatMap(fx, f), g) => ??? fixes the issue.
[error] dotty.tools.dotc.core.TypeOps$.dotty$tools$dotc$core$TypeOps$$anon$1$$_$op$proxy5$1(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$.dotty$tools$dotc$core$TypeOps$$anon$1$$_$op$proxy5$1(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$.dotty$tools$dotc$core$TypeOps$$anon$1$$_$op$proxy5$1(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:451) [error] dotty.tools.dotc.core.TypeOps$.dotty$tools$dotc$core$TypeOps$$anon$1$$_$op$proxy5$1(TypeOps.scala:451)Another workaround is explicitly providing types to the flatMap calls in the problematic expression:
case FlatMap(FlatMap(fx, f), g) => fx.flatMap[F, A](x => f(x).flatMap[F, A](g)).step