Fix cyclic check, regardless of definition order (#22342)

Author: dwijnand

Diff for: compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -287,11 +287,16 @@ object Checking {
      */
     def checkInfo(tp: Type): Type = tp match {
       case tp @ TypeAlias(alias) =>
-        tp.derivedAlias(checkPart(alias, "alias"))
+        val lo1 = atVariance(-1)(checkPart(alias, "alias"))
+        val hi1 = checkUpper(alias, "alias")
+        if lo1 eq hi1 then
+          tp.derivedAlias(lo1)
+        else
+          tp.derivedTypeBounds(lo1, hi1)
       case tp @ MatchAlias(alias) =>
-        tp.derivedAlias(checkUpper(alias, "match"))
+        tp.derivedAlias(atVariance(0)(checkUpper(alias, "match")))
       case tp @ TypeBounds(lo, hi) =>
-        tp.derivedTypeBounds(checkPart(lo, "lower bound"), checkUpper(hi, "upper bound"))
+        tp.derivedTypeBounds(atVariance(-1)(checkPart(lo, "lower bound")), checkUpper(hi, "upper bound"))
       case _ =>
         tp
     }
@@ -312,12 +317,12 @@ object Checking {
       case tp: TermRef =>
         this(tp.info)
         mapOver(tp)
-      case tp @ AppliedType(tycon, args) =>
-        tp.derivedAppliedType(this(tycon), args.mapConserve(this(_, nestedCycleOK, nestedCycleOK)))
       case tp @ RefinedType(parent, name, rinfo) =>
         tp.derivedRefinedType(this(parent), name, this(rinfo, nestedCycleOK, nestedCycleOK))
       case tp: RecType =>
         tp.rebind(this(tp.parent))
+      case tp: LazyRef =>
+        tp
       case tp @ TypeRef(pre, _) =>
         try {
           // A prefix is interesting if it might contain (transitively) a reference
@@ -350,14 +355,17 @@ object Checking {
 
           if isInteresting(pre) then
             CyclicReference.trace(i"explore ${tp.symbol} for cyclic references"):
-              val pre1 = this(pre, false, false)
+              val pre1 = atVariance(variance max 0)(this(pre, false, false))
               if locked.contains(tp)
                   || tp.symbol.infoOrCompleter.isInstanceOf[NoCompleter]
+                  && tp.symbol == sym
               then
                 throw CyclicReference(tp.symbol)
               locked += tp
               try
-                if tp.symbol.isOpaqueAlias then
+                if tp.symbol.infoOrCompleter.isInstanceOf[NoCompleter] then
+                  ; // skip checking info (and avoid forcing the symbol with .isOpaqueAlias/etc)
+                else if tp.symbol.isOpaqueAlias then
                   checkInfo(TypeAlias(tp.translucentSuperType))
                 else if !tp.symbol.isClass then
                   checkInfo(tp.info)
@@ -375,6 +383,16 @@ object Checking {
         }
       case _ => mapOver(tp)
     }
+
+    override def mapArg(arg: Type, tparam: ParamInfo): Type =
+      val varianceDiff = variance != tparam.paramVarianceSign
+      atVariance(variance * tparam.paramVarianceSign):
+        // Using tests/pos/i22257.scala as an example,
+        // if we consider FP's lower-bound of Fixed[Node]
+        // than `Node` is a type argument in contravariant
+        // position, while the type parameter is covariant.
+        val nestedCycleOK1 = nestedCycleOK || variance != 0 && varianceDiff
+        this(arg, nestedCycleOK, nestedCycleOK1)
   }
 
   /** Under -Yrequire-targetName, if `sym` has an operator name, check that it has a

Diff for: tests/neg/i4368.scala

@@ -98,7 +98,7 @@ object Test6 {
 
 object Test7 {
   class Fix[F[_]] {
-    class Foo { type R >: F[T] <: F[T] } // error: cyclic
+    class Foo { type R >: F[T] <: F[T] } // error
     type T = F[Foo#R]
   }
 
@@ -149,9 +149,9 @@ object Test9 {
 object i4369 {
   trait X { self =>
     type R <: Z
-    type Z >: X { type R = self.R; type Z = self.R } // error: cyclic // error: cyclic // error: cyclic
+    type Z >: X { type R = self.R; type Z = self.R } // error: cyclic
   }
-  class Foo extends X { type R = Foo; type Z = Foo }
+  class Foo extends X { type R = Foo; type Z = Foo } // error
 }
 object i4370 {
   class Foo { type R = A }

Diff for: tests/neg/i4369c.scala

@@ -1,5 +1,18 @@
 trait X { self =>
   type R <: Z
-  type Z >: X { type R = self.R; type Z = self.R } // error // error // error
+  type Z >:
+    X { // error
+      type R = // was-error
+        self.R
+      type Z = // was-error
+        self.R
+    }
+}
+
+class Foo // error
+    extends X {
+  type R =
+    Foo
+  type Z =
+    Foo
 }
-class Foo extends X { type R = Foo; type Z = Foo }

Diff for: tests/neg/mt-deskolemize-2.scala

@@ -19,8 +19,8 @@ trait Description:
   type Elem <: Tuple
 
 class PrimBroken extends Expr:
-  type Value = Alias
-  type Alias = Value // error
+  type Value = Alias // error
+  type Alias = Value
 
 class Prim extends Expr:
   type Value = BigInt

Diff for: tests/neg/toplevel-cyclic/defs_1.scala

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

Diff for: tests/neg/toplevel-cyclic/moredefs_1.scala

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

Diff for: tests/pos/i22257.fixed.scala

@@ -0,0 +1,52 @@
+
+object Scaffold {
+
+  trait Arrow
+  object Arrow {
+    trait Outbound extends Arrow
+  }
+
+  trait NodeKOrGraphK {}
+
+  trait NodeK extends NodeKOrGraphK {
+
+    type FBound <: Induction
+
+    protected def getInduction: Seq[FBound]
+  }
+
+  trait Induction {
+    def arrow: Arrow
+    def node: NodeK
+  }
+
+  object Induction {
+
+    trait FP[+N <: NodeK] extends Induction { // short for "fixed point"
+      def node: N
+    }
+  }
+
+  trait GraphK extends NodeKOrGraphK {
+
+    type Batch[+T] <: Iterable[T]
+
+    type _Node <: NodeK
+
+    def entries: Batch[_Node]
+  }
+
+  trait Topology {
+
+    type Node = NodeK { type FBound <: Topology.this.FBound }
+    trait Node_ extends NodeK {
+      type FBound = Topology.this.FBound
+    }
+
+    type FP = Induction.FP[Node]
+    type FBound <: FP
+
+    type Graph = GraphK { type _Node <: Node }
+  }
+
+}

Diff for: tests/pos/i22257.orig.scala

@@ -0,0 +1,52 @@
+
+object Scaffold {
+
+  trait Arrow
+  object Arrow {
+    trait Outbound extends Arrow
+  }
+
+  trait NodeKOrGraphK {}
+
+  trait NodeK extends NodeKOrGraphK {
+
+    type FBound <: Induction
+
+    protected def getInduction: Seq[FBound]
+  }
+
+  trait Induction {
+    def arrow: Arrow
+    def node: NodeK
+  }
+
+  object Induction {
+
+    trait FP[+N <: NodeK] extends Induction { // short for "fixed point"
+      def node: N
+    }
+  }
+
+  trait GraphK extends NodeKOrGraphK {
+
+    type Batch[+T] <: Iterable[T]
+
+    type _Node <: NodeK
+
+    def entries: Batch[_Node]
+  }
+
+  trait Topology {
+
+    type FP = Induction.FP[Node]
+    type FBound <: FP
+
+    type Node = NodeK { type FBound <: Topology.this.FBound }
+    trait Node_ extends NodeK {
+      type FBound = Topology.this.FBound
+    }
+
+    type Graph = GraphK { type _Node <: Node }
+  }
+
+}

Diff for: tests/pos/i22257.scala

@@ -0,0 +1,26 @@
+trait NodeK { type FBound }
+trait Fixed[+N <: NodeK]
+
+type Bound1 <: FP1
+type FP1     = Fixed[Node1]
+type Node1   = NodeK { type FBound <: Bound1 } // was-error
+
+type FP2     = Fixed[Node2]                    // was-error
+type Bound2 <: FP2
+type Node2   = NodeK { type FBound <: Bound2 }
+
+type Node3   = NodeK { type FBound <: Bound3 }
+type FP3     = Fixed[Node3]
+type Bound3 <: FP3
+
+type Bound4 <: FP4
+type Node4   = NodeK { type FBound <: Bound4 } // was-error
+type FP4     = Fixed[Node4]
+
+type FP5     = Fixed[Node5]                    // was-error
+type Node5   = NodeK { type FBound <: Bound5 }
+type Bound5 <: FP5
+
+type Node6   = NodeK { type FBound <: Bound6 }
+type Bound6 <: FP6
+type FP6     = Fixed[Node6]