Collapse into 1 etaExpand (to rule them all)

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

@@ -34,10 +34,6 @@ 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`.
@@ -245,7 +241,7 @@ 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 _ =>
@@ -302,45 +298,49 @@ 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 match
+      case self: TypeRef if tparams.nonEmpty && self.symbol.isClass =>
+        val prefix = self.prefix
+        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(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
     }
   }
 
-  // Like `target.etaExpand(target.typeParams)`
-  // except call `asSeenFrom` to fix class type parameter bounds
-  // e.g. from pos/i18569:
-  //    trait M1:
-  //      trait A
-  //      trait F[T <: A]
-  //    object M2 extends M1
-  // Type parameter T in M2.F has an upper bound of M1#A instead of M2.A
-  // So we take the prefix M2.type and the F symbol's owner, M1,
-  // to call asSeenFrom on T's info.
-  def etaExpandWithAsf(using Context): Type = self match
-    case self: TypeRef if self.symbol.isClass =>
-      val tparams = self.symbol.typeParams
-      val prefix = self.prefix
-      val owner = self.symbol.owner
-      HKTypeLambda(tparams.map(_.paramName))(
-        tl => tparams.map(p => HKTypeLambda.toPInfo(tl.integrate(tparams, p.info.asSeenFrom(prefix, owner)))),
-        tl => tl.integrate(tparams, self.appliedTo(tparams.map(_.paramRef))))
-    case _ => etaExpand(typeParams)
-
   /** Maps [Ts] => C[Ts] to C */
   def etaCollapse(using Context): Type = self match
     case EtaExpansion(classType) => classType

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

@@ -606,7 +606,7 @@ 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
     }
 
@@ -746,7 +746,7 @@ 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

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

@@ -832,7 +832,7 @@ 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

@@ -1197,7 +1197,7 @@ class Namer { typer: Typer =>
               val forwarderName = checkNoConflict(alias.toTypeName, isPrivate = false, span)
               var target = pathType.select(sym)
               if target.typeParams.nonEmpty then
-                target = target.etaExpandWithAsf
+                target = target.etaExpand
               newSymbol(
                 cls, forwarderName,
                 Exported | Final,

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

@@ -372,7 +372,7 @@ 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

@@ -4306,7 +4306,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               AppliedType(tree.tpe, tp.typeParams.map(Function.const(TypeBounds.empty)))
             else
               // Eta-expand higher-kinded type
-              tree.tpe.etaExpand(tp.typeParamSymbols)
+              tree.tpe.etaExpand
           tree.withType(tp1)
         }
       if (ctx.mode.is(Mode.Pattern) || ctx.mode.isQuotedPattern || tree1.tpe <:< pt) tree1