Merge pull request #12649 from dotty-staging/fix-12640

Show root of stackoverflow in IsFullyDefinedAccumulator

Author: odersky

tests/neg/i12640.scala

@@ -0,0 +1,17 @@
+package x
+
+trait CpsMonad[F[_]]:
+  def pure[A](x:A): F[A]
+  def flatMap[A,B](fa:F[A])(f: A=>F[B]): F[B]
+
+abstract sealed class CpsStream[-F[_],+T]
+
+case class Cons[F[_],T](head:T, tailFun: ()=>F[CpsStream[F,T]]) extends CpsStream[F,T]
+
+case class Empty[F[_]]() extends CpsStream[F,Nothing]
+
+def unfold[S,F[_]:CpsMonad,T](s0:S)(f:S => F[Option[(S,T)]]):F[CpsStream[F,T]] =
+      summon[CpsMonad[F]].flatMap(f(s0)){
+         case Some(s1,a) => Cons(a, () => unfold(s1,f))  // error // error
+         case None => summon[CpsMonad[F]].pure(Empty[F]())
+      }