Fix the tparam bounds of exported inherited classes

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

@@ -33,15 +33,11 @@ object TypeApplications {
    */
   object EtaExpansion:
 
-    def apply(tycon: Type)(using Context): Type =
-      assert(tycon.typeParams.nonEmpty, tycon)
-      tycon.etaExpand(tycon.typeParamSymbols)
-
     /** Test that the parameter bounds in a hk type lambda `[X1,...,Xn] => C[X1, ..., Xn]`
      *  contain the bounds of the type parameters of `C`. This is necessary to be able to
      *  contract the hk lambda to `C`.
      */
-    private def weakerBounds(tp: HKTypeLambda, tparams: List[ParamInfo])(using Context): Boolean =
+    private def weakerBounds(tp: HKTypeLambda, fn: Type)(using Context): Boolean =
       val onlyEmptyBounds = tp.typeParams.forall(_.paramInfo == TypeBounds.empty)
       onlyEmptyBounds
         // Note: this pre-test helps efficiency. It is also necessary to workaround  #9965 since in some cases
@@ -50,18 +46,24 @@ object TypeApplications {
         // In this case, we can still return true if we know that the hk lambda bounds
         // are empty anyway.
       || {
+        val tparams = fn.typeParams
         val paramRefs = tparams.map(_.paramRef)
+        val prefix = fn.normalizedPrefix
+        val owner = fn.typeSymbol.maybeOwner
         tp.typeParams.corresponds(tparams) { (param1, param2) =>
-          param2.paramInfo frozen_<:< param1.paramInfo.substParams(tp, paramRefs)
+          // see tests/neg/variances-constr.scala
+          // its B parameter should have info <: Any, using class C as the owner
+          // rather than info <: A, using class Inner2 as the owner
+          param2.paramInfo.asSeenFrom(prefix, owner) frozen_<:< param1.paramInfo.substParams(tp, paramRefs)
         }
       }
 
     def unapply(tp: Type)(using Context): Option[Type] = tp match
-      case tp @ HKTypeLambda(tparams, AppliedType(fn: Type, args))
+      case tp @ HKTypeLambda(tparams, AppliedType(fn, args))
       if fn.typeSymbol.isClass
          && tparams.hasSameLengthAs(args)
          && args.lazyZip(tparams).forall((arg, tparam) => arg == tparam.paramRef)
-         && weakerBounds(tp, fn.typeParams) => Some(fn)
+         && weakerBounds(tp, fn) => Some(fn)
       case _ => None
 
   end EtaExpansion
@@ -244,7 +246,7 @@ class TypeApplications(val self: Type) extends AnyVal {
   def topType(using Context): Type =
     if self.hasSimpleKind then
       defn.AnyType
-    else etaExpand(self.typeParams) match
+    else self.etaExpand match
       case tp: HKTypeLambda =>
         tp.derivedLambdaType(resType = tp.resultType.topType)
       case _ =>
@@ -301,21 +303,44 @@ class TypeApplications(val self: Type) extends AnyVal {
   /** Convert a type constructor `TC` which has type parameters `X1, ..., Xn`
    *  to `[X1, ..., Xn] -> TC[X1, ..., Xn]`.
    */
-  def etaExpand(tparams: List[TypeParamInfo])(using Context): Type =
-    HKTypeLambda.fromParams(tparams, self.appliedTo(tparams.map(_.paramRef)))
-      //.ensuring(res => res.EtaReduce =:= self, s"res = $res, core = ${res.EtaReduce}, self = $self, hc = ${res.hashCode}")
+  def etaExpand(using Context): Type =
+    val tparams = self.typeParams
+    val resType = self.appliedTo(tparams.map(_.paramRef))
+    self.dealias match
+      case self: TypeRef if tparams.nonEmpty && self.symbol.isClass =>
+        val owner = self.symbol.owner
+        // Calling asSeenFrom on the type parameter infos is important
+        // so that class type references within another prefix have
+        // their type parameters' info fixed.
+        // e.g. from pos/i18569:
+        //    trait M1:
+        //      trait A
+        //      trait F[T <: A]
+        //    object M2 extends M1
+        // Type parameter T in M1.F has an upper bound of M1#A
+        // But eta-expanding M2.F should have type parameters with an upper-bound of M2.A.
+        // So we take the prefix M2.type and the F symbol's owner, M1,
+        // to call asSeenFrom on T's info.
+        HKTypeLambda(tparams.map(_.paramName))(
+          tl => tparams.map(p => HKTypeLambda.toPInfo(tl.integrate(tparams, p.paramInfo.asSeenFrom(self.prefix, owner)))),
+          tl => tl.integrate(tparams, resType))
+      case _ =>
+        HKTypeLambda.fromParams(tparams, resType)
 
   /** If self is not lambda-bound, eta expand it. */
   def ensureLambdaSub(using Context): Type =
-    if (isLambdaSub) self else EtaExpansion(self)
+    if isLambdaSub then self
+    else
+      assert(self.typeParams.nonEmpty, self)
+      self.etaExpand
 
   /** Eta expand if `self` is a (non-lambda) class reference and `bound` is a higher-kinded type */
   def etaExpandIfHK(bound: Type)(using Context): Type = {
     val hkParams = bound.hkTypeParams
     if (hkParams.isEmpty) self
     else self match {
-      case self: TypeRef if self.symbol.isClass && self.typeParams.length == hkParams.length =>
-        EtaExpansion(self)
+      case self: TypeRef if self.symbol.isClass && self.typeParams.hasSameLengthAs(hkParams) =>
+        etaExpand
       case _ => self
     }
   }

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

@@ -594,7 +594,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             if (base.typeSymbol == cls2) return true
           }
           else if tp1.typeParams.nonEmpty && !tp1.isAnyKind then
-            return recur(tp1, EtaExpansion(tp2))
+            return recur(tp1, tp2.etaExpand)
         fourthTry
     }
 
@@ -734,7 +734,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           case _ =>
             val tparams1 = tp1.typeParams
             if (tparams1.nonEmpty)
-              return recur(tp1.etaExpand(tparams1), tp2) || fourthTry
+              return recur(tp1.etaExpand, tp2) || fourthTry
             tp2 match {
               case EtaExpansion(tycon2: TypeRef) if tycon2.symbol.isClass && tycon2.symbol.is(JavaDefined) =>
                 recur(tp1, tycon2) || fourthTry
@@ -2820,7 +2820,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         tp.symbol match
           case cls: ClassSymbol =>
             if cls == defn.SingletonClass then defn.AnyType
-            else if cls.typeParams.nonEmpty then EtaExpansion(tp)
+            else if cls.typeParams.nonEmpty then tp.etaExpand
             else tp
           case sym =>
             if !ctx.erasedTypes && sym == defn.FromJavaObjectSymbol then defn.AnyType

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -850,7 +850,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         }
         else if args.nonEmpty then
           tycon.safeAppliedTo(EtaExpandIfHK(sym.typeParams, args.map(translateTempPoly)))
-        else if (sym.typeParams.nonEmpty) tycon.etaExpand(sym.typeParams)
+        else if (sym.typeParams.nonEmpty) tycon.etaExpand
         else tycon
       case TYPEBOUNDStpe =>
         val lo = readTypeRef()

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

@@ -165,7 +165,7 @@ trait Deriving {
           // case (a) ... see description above
           val derivedParams = clsParams.dropRight(instanceArity)
           val instanceType =
-            if (instanceArity == clsArity) clsType.etaExpand(clsParams)
+            if (instanceArity == clsArity) clsType.etaExpand
             else {
               val derivedParamTypes = derivedParams.map(_.typeRef)
 

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

@@ -1200,7 +1200,7 @@ class Namer { typer: Typer =>
               val forwarderName = checkNoConflict(alias.toTypeName, isPrivate = false, span)
               var target = pathType.select(sym)
               if target.typeParams.nonEmpty then
-                target = target.etaExpand(target.typeParams)
+                target = target.etaExpand
               newSymbol(
                 cls, forwarderName,
                 MandatoryExportTypeFlags | (sym.flags & RetainedExportTypeFlags),

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

@@ -377,7 +377,7 @@ object RefChecks {
      */
     def checkOverride(checkSubType: (Type, Type) => Context ?=> Boolean, member: Symbol, other: Symbol): Unit =
       def memberTp(self: Type) =
-        if (member.isClass) TypeAlias(member.typeRef.etaExpand(member.typeParams))
+        if (member.isClass) TypeAlias(member.typeRef.etaExpand)
         else self.memberInfo(member)
       def otherTp(self: Type) =
         self.memberInfo(other)

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

@@ -4283,7 +4283,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             tree1.withType(tp1)
           else
             // Eta-expand higher-kinded type
-            val tp1 = tree.tpe.etaExpand(tp.typeParamSymbols)
+            val tp1 = tree.tpe.etaExpand
             tree.withType(tp1)
         }
       if (ctx.mode.is(Mode.Pattern) || ctx.mode.isQuotedPattern || tree1.tpe <:< pt) tree1

tests/neg/i7820.check

@@ -0,0 +1,34 @@
+-- [E046] Cyclic Error: tests/neg/i7820.scala:1:23 ---------------------------------------------------------------------
+1 |trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
+  |                       ^
+  |                       Cyclic reference involving type F
+  |
+  |                        Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E046] Cyclic Error: tests/neg/i7820.scala:2:23 ---------------------------------------------------------------------
+2 |trait A2 { type F[X <: F, Y] }        // error: cyclic reference involving type F
+  |                       ^
+  |                       Cyclic reference involving type F
+  |
+  |                        Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E046] Cyclic Error: tests/neg/i7820.scala:3:23 ---------------------------------------------------------------------
+3 |trait A3 { type F[X >: F, Y] }        // error: cyclic reference involving type F
+  |                       ^
+  |                       Cyclic reference involving type F
+  |
+  |                        Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`
+-- Warning: tests/neg/i7820.scala:1:25 ---------------------------------------------------------------------------------
+1 |trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
+  |                         ^
+  |                         `_` is deprecated for wildcard arguments of types: use `?` instead
+  |                         This construct can be rewritten automatically under -rewrite -source 3.4-migration.
+-- Warning: tests/neg/i7820.scala:1:28 ---------------------------------------------------------------------------------
+1 |trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
+  |                            ^
+  |                            `_` is deprecated for wildcard arguments of types: use `?` instead
+  |                            This construct can be rewritten automatically under -rewrite -source 3.4-migration.

tests/neg/i7820.scala

@@ -1,3 +1,3 @@
 trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
-trait A2 { type F[X <: F, Y] }        // error: cyclic reference involving type F // error
-trait A3 { type F[X >: F, Y] }        // error: cyclic reference involving type F // error
+trait A2 { type F[X <: F, Y] }        // error: cyclic reference involving type F
+trait A3 { type F[X >: F, Y] }        // error: cyclic reference involving type F

tests/pos/i18569.reg1.scala

@@ -0,0 +1,11 @@
+// Minimisation of the CI failure
+// in scala-parallel-collections
+// to do with how EtaExpansion is used
+// by typeOfNew when typing a New tree
+
+trait Foo[+A]:
+  class Bar[B >: A]
+
+class Test:
+  def t1[X](foo: Foo[X]): Unit =
+    val bar = new foo.Bar()

tests/pos/i18569.scala

@@ -0,0 +1,11 @@
+trait M1:
+  trait A
+  trait F[T <: A]
+  type G[T <: A] = F[T]
+
+object M2 extends M1
+
+trait Test:
+  export M2.*
+  def y: F[A]
+  def z: G[A]