Fix #9000: Change definition of isTrivial

The only difference between checking cyclicity of a match type
in an object O and on the toplevel is that the matchtype itself
has the prefix O.this in the first case, and TermRef(_, pkgObject)
in the second case (this is because the match type is actually looked
up in the package scope, not in the package object scope).
This made a difference in the isTrivial check when computing type parameters.
And this in turn caused spurious cycles to be reported. The fix is
to add the missing case to isTrivial.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeApplications.scala

@@ -10,6 +10,7 @@ import Decorators._
 import util.Stats._
 import Names._
 import NameOps._
+import Flags.Module
 import Variances.variancesConform
 import dotty.tools.dotc.config.Config
 
@@ -145,7 +146,11 @@ class TypeApplications(val self: Type) extends AnyVal {
   final def typeParams(implicit ctx: Context): List[TypeParamInfo] = {
     record("typeParams")
     def isTrivial(prefix: Type, tycon: Symbol) = prefix match {
-      case prefix: ThisType => prefix.cls `eq` tycon.owner
+      case prefix: ThisType =>
+        prefix.cls eq tycon.owner
+      case prefix: TermRef =>
+        val sym = prefix.symbol
+        sym.is(Module) && sym.isStatic && (sym.moduleClass eq tycon.owner)
       case NoPrefix => true
       case _ => false
     }

tests/neg/toplevel-cyclic/defs_1.scala

@@ -1,2 +1 @@
-type A = B  // error: recursion limit exceeded
-
+type A = B

tests/pos/toplevel-match.scala

@@ -0,0 +1,14 @@
+class A
+class B
+
+trait HList
+class HCons[A, As <: HList] extends HList
+class HNil[A] extends HList
+
+type AtoB[Xs <: HList] <: HList = Xs match
+  case HNil[a] => HNil[B]
+  case HCons[a, as] => HCons[B, AtoB[as]]
+
+//type A2B[Xs <: Tuple] <: Tuple = Xs match
+//  case Unit => Unit
+//  case a *: as => B *: A2B[as]