Allow simple type args of parent types to back reference

Can't do so for higher-kinded type arguments, as eta-expanding them
requires completing them.  Nor for applied type arguments, as typing the
applied tree type requires completing the type constructor.

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

@@ -695,6 +695,9 @@ object SymDenotations {
       containsOpaques ||
       is(Module, butNot = Package) && owner.seesOpaques
 
+    def isTouched(using Context): Boolean =
+      flagsUNSAFE.is(Touched) // do not force the info to check the flag
+
     def isProvisional(using Context): Boolean =
       flagsUNSAFE.is(Provisional) // do not force the info to check the flag
 

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

@@ -279,16 +279,16 @@ class TypeApplications(val self: Type) extends AnyVal {
       case tp: TypeRef => tp.symbol == defn.AnyKindClass
       case _ => false
     }
-    val selfResult = self.hkResult
-    val otherResult = other.hkResult
-    isAnyKind(selfResult) || isAnyKind(otherResult) ||
-    { if (selfResult.exists)
-        otherResult.exists &&
-        selfResult.hasSameKindAs(otherResult) &&
-        self.typeParams.corresponds(other.typeParams)((sparam, oparam) =>
-          sparam.paramInfo.hasSameKindAs(oparam.paramInfo))
-      else !otherResult.exists
-    }
+    val sparams = self.typeParams
+    val oparams = other.typeParams
+    if sparams.isEmpty && oparams.isEmpty then true // defer calling hkResult to avoid completing types
+    else
+      val selfResult  = self.hkResult
+      val otherResult = other.hkResult
+      isAnyKind(selfResult) || isAnyKind(otherResult)
+      || !selfResult.exists && !otherResult.exists
+      || selfResult.hasSameKindAs(otherResult)
+        && sparams.corresponds(oparams)((sp, op) => sp.paramInfo.hasSameKindAs(op.paramInfo))
   }
 
   /** Dealias type if it can be done without forcing the TypeRef's info */

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -170,7 +170,7 @@ object Checking {
    *  A NoType paramBounds is used as a sign that checking should be suppressed.
    */
   def preCheckKind(arg: Tree, paramBounds: Type)(using Context): Tree =
-    if (arg.tpe.widen.isRef(defn.NothingClass) ||
+    if (arg.tpe.isExactlyNothing ||
         !paramBounds.exists ||
         arg.tpe.hasSameKindAs(paramBounds.bounds.hi)) arg
     else errorTree(arg, em"Type argument ${arg.tpe} does not have the same kind as its bound $paramBounds")
@@ -336,7 +336,29 @@ object Checking {
             if (locked.contains(tp) || tp.symbol.infoOrCompleter.isInstanceOf[NoCompleter])
               throw CyclicReference(tp.symbol)
             locked += tp
-            try if (!tp.symbol.isClass) checkInfo(tp.info)
+            try
+              if !tp.symbol.isClass
+                && (!cycleOK || tp.symbol.isTouched)
+                // from pos/i8900-cycle.scala
+                //     [T <: Cov[U], U <: T]
+                //
+                // if we force U while checking T
+                // then U <: T will encounter T with a NoCompleter
+                // and throw while cycleOK=false (bad)
+                //
+                // so instead we delay U to later
+                // then U <: T will see T <: Cov[U] and then encounter U with a NoCompleter
+                // and throw but then cycleOK=true because it's a type argument
+                //
+                // for any case of true illegal cycles, e.g. T1 in neg/cycles.scala
+                //     type X = (U, U)   // error: cycle (old)
+                //     type U = X & Int  // error: cycle (new)
+                // skipping untouched U will move the error to when U is completed
+                //
+                // previously running preCheckKind while typing Cov[U] and (U, U) forced the `U`s
+                // now completion is avoided in hasSameKindAs by looking at type parameters, leaving the `U`s untouched
+              then
+                checkInfo(tp.info)
             finally locked -= tp
             tp.withPrefix(pre1)
           }

tests/neg/cycles.scala

@@ -30,12 +30,12 @@ class E {
 }
 
 class T1 {
-  type X = (U, U) // error: cycle
-  type U = X & Int
+  type X = (U, U)
+  type U = X & Int // error: cycle
 }
 class T2 {
-  type X = (U, U) // error: cycle
-  type U = X | Int
+  type X = (U, U)
+  type U = X | Int // error: cycle
 }
 object T12 {
   val _ : (T1 {})#U = ??? // old-error: conflicting bounds

tests/neg/i15177.app.scala

@@ -0,0 +1,9 @@
+// like tests/pos/i15177.scala
+// but with an applied type B[D]
+class X[T] {
+  type Id
+}
+object A extends X[B[D]]
+class B[C](id: A
+  .Id) // error
+class D

tests/neg/i15177.hk.scala

@@ -0,0 +1,8 @@
+// like tests/pos/i15177.scala
+// but with B being higher kinded
+class X[T[_]] {
+  type Id
+}
+object A extends X[B]
+class B[C](id: A
+  .Id) // error: type Id is not a member of object A

tests/neg/i15177.ub.scala

@@ -0,0 +1,13 @@
+// like tests/pos/i15177.scala
+// but with T having an upper bound
+// that B doesn't conform to
+// just to be sure that not forcing B
+// doesn't backdoor an illegal X[B]
+class X[T <: C] {
+  type Id
+}
+object A
+  extends X[ // error
+    B] // error
+class B(id: A.Id)
+class C

tests/neg/i4368.scala

@@ -98,8 +98,8 @@ object Test6 {
 
 object Test7 {
   class Fix[F[_]] {
-    class Foo { type R >: F[T] <: F[T] } // error: cyclic
-    type T = F[Foo#R]
+    class Foo { type R >: F[T] <: F[T] }
+    type T = F[Foo#R] // error: cyclic
   }
 
   object App {

tests/neg/i4369b.scala

@@ -1,2 +1,2 @@
-trait X[R <: Z, Z >: X[R, R] <: X[R, R]] // error // error
+trait X[R <: Z, Z >: X[R, R] <: X[R, R]] // error
 class Z extends X[Z, Z]

tests/pos/i15177.FakeEnum.scala

@@ -0,0 +1,20 @@
+trait FakeEnum[A, @specialized(Byte, Short, Int, Long) B]
+{
+  trait Value {
+    self: A =>
+      def name: String
+      def id: B
+  }
+}
+
+object FakeEnumType
+  extends FakeEnum[FakeEnumType, Short]
+{
+  val MEMBER1 = new FakeEnumType((0: Short), "MEMBER1") {}
+  val MEMBER2 = new FakeEnumType((1: Short), "MEMBER2") {}
+}
+
+sealed abstract
+class FakeEnumType(val id: Short, val name: String)
+  extends FakeEnumType.Value
+{}

tests/pos/i15177.scala

@@ -0,0 +1,5 @@
+class X[T] {
+  type Id
+}
+object A extends X[B]
+class B(id: A.Id)