Merge pull request #9085 from lampepfl/revert-9084-fix-#9061

Revert "Fix #9061: Treat `type F <: [X] => G` like `type F[X] <: G`."

Author: smarter

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

@@ -156,6 +156,4 @@ object Variances {
     if (v > 0) "+"
     else if (v < 0) "-"
     else ""
-
-  val alwaysInvariant: Any => Invariant.type = Function.const(Invariant)
 }

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

@@ -15,7 +15,6 @@ import util.Spans._
 import util.Property
 import collection.mutable
 import tpd.ListOfTreeDecorator
-import Variances.alwaysInvariant
 import config.{Config, Feature}
 import config.Printers.typr
 import Annotations._
@@ -969,10 +968,7 @@ class Namer { typer: Typer =>
     override final def typeSig(sym: Symbol): Type =
       val tparamSyms = completerTypeParams(sym)(ictx)
       given ctx as Context = nestedCtx
-
-      def abstracted(tp: TypeBounds): TypeBounds =
-        HKTypeLambda.boundsFromParams(tparamSyms, tp)
-
+      def abstracted(tp: TypeBounds): TypeBounds = HKTypeLambda.boundsFromParams(tparamSyms, tp)
       val dummyInfo1 = abstracted(TypeBounds.empty)
       sym.info = dummyInfo1
       sym.setFlag(Provisional)
@@ -1002,22 +998,8 @@ class Namer { typer: Typer =>
       }
       sym.info = dummyInfo2
 
-      // Treat the parameters of an upper type lambda bound on the RHS as non-variant.
-      // E.g.   type F <: [X] =>> G   and   type F[X] <: G
-      // are treated alike.
-      def addVariances(tp: Type): Type = tp match
-        case tp: TypeBounds =>
-          def recur(tp: Type): Type = tp match
-            case tp: HKTypeLambda if !tp.isDeclaredVarianceLambda =>
-              tp.withVariances(tp.paramNames.map(alwaysInvariant))
-               .derivedLambdaType(resType = recur(tp.resType))
-            case tp => tp
-          tp.derivedTypeBounds(tp.lo, recur(tp.hi))
-        case _ =>
-          tp
-
       val rhs1 = typedAheadType(rhs)
-      val rhsBodyType: TypeBounds = addVariances(rhs1.tpe).toBounds
+      val rhsBodyType: TypeBounds = rhs1.tpe.toBounds
       val unsafeInfo = if (isDerived) rhsBodyType else abstracted(rhsBodyType)
 
       def opaqueToBounds(info: Type): Type =

compiler/src/dotty/tools/dotc/util/common.scala

@@ -6,8 +6,8 @@ import core.Types.WildcardType
 /** Common values hoisted out for performance */
 object common {
 
-  val alwaysTrue: Any => Boolean = Function.const(true)
-  val alwaysFalse: Any => Boolean = Function.const(false)
-  val alwaysZero: Any => Int = Function.const(0)
-  val alwaysWildcardType: Any => WildcardType.type = Function.const(WildcardType)
+  val alwaysTrue: Any => Boolean = Function.const(true) _
+  val alwaysFalse: Any => Boolean = Function.const(false) _
+  val alwaysZero: Any => Int = Function.const(0) _
+  val alwaysWildcardType: Any => WildcardType.type = Function.const(WildcardType) _
 }