Fix #9000: Avoid spurious cyclic errors for toplevel matches

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
     }

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

@@ -154,8 +154,8 @@ class CyclicReference private (val denot: SymDenotation) extends TypeError {
 object CyclicReference {
   def apply(denot: SymDenotation)(implicit ctx: Context): CyclicReference = {
     val ex = new CyclicReference(denot)
-    if (!(ctx.mode is Mode.CheckCyclic)) {
-      cyclicErrors.println(s"Cyclic reference involving $denot")
+    if (!(ctx.mode is Mode.CheckCyclic) || ctx.settings.Ydebug.value) {
+      cyclicErrors.println(s"Cyclic reference involving! $denot")
       for (elem <- ex.getStackTrace take 200)
         cyclicErrors.println(elem.toString)
     }

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2238,7 +2238,11 @@ object messages {
 
   class IllegalCyclicTypeReference(sym: Symbol, where: String, lastChecked: Type)(using ctx: Context)
     extends CyclicMsg(IllegalCyclicTypeReferenceID) {
-    def msg = i"illegal cyclic type reference: ${where} ${hl(lastChecked.show)} of $sym refers back to the type itself"
+    def msg =
+      val lastCheckedStr =
+        try lastChecked.show
+        catch case ex: CyclicReference => "..."
+      i"illegal cyclic type reference: ${where} ${hl(lastCheckedStr)} of $sym refers back to the type itself"
     def explain = ""
   }
 

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]