More regression tests

Closes scala#9287
Closes scala#12640
Closes scala#12896

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 (used to crash)
+         case None => summon[CpsMonad[F]].pure(Empty[F]())
+      }

tests/neg/i9287.scala

@@ -0,0 +1,7 @@
+object Foo {
+  Nil {
+    class i0 { a: i2 =>  // error
+      class i1
+    }
+  }  // error
+}

tests/pos/i12896.scala

@@ -0,0 +1,12 @@
+trait IO[E] {
+  def map[B](f: Any => B): IO[E] = ???
+  def flatMap[C](f: Any => IO[C]): IO[E | C] = ???
+}
+
+class Test {
+  def test: Unit = {
+    val a: IO[Nothing] = ???
+
+    val d = a.flatMap(y => a.flatMap(z => a.map(_ => z)))
+  }
+}