Fix #6127: Drop followAlias

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

@@ -79,8 +79,10 @@ object Inferencing {
    *      constrained upper bound != given upper bound and
    *      constrained lower bound == given lower bound).
    *  If (1) and (2) do not apply:
-   *   3. T is maximized if it appears only contravariantly in the given type.
-   *   4. T is minimized in all other cases.
+   *   3. T is minimized if forceDegree is minimizeAll.
+   *   4. Otherwise, T is maximized if it appears only contravariantly in the given type,
+   *      or if forceDegree is `noBottom` and T's minimized value is a bottom type.
+   *   5. Otherwise, T is minimized.
    *
    *  The instantiation is done in two phases:
    *  1st Phase: Try to instantiate minimizable type variables to
@@ -89,32 +91,29 @@ object Inferencing {
    *  to their upper bound.
    */
   private class IsFullyDefinedAccumulator(force: ForceDegree.Value)(implicit ctx: Context) extends TypeAccumulator[Boolean] {
+
     private def instantiate(tvar: TypeVar, fromBelow: Boolean): Type = {
       val inst = tvar.instantiate(fromBelow)
       typr.println(i"forced instantiation of ${tvar.origin} = $inst")
       inst
     }
+
     private[this] var toMaximize: Boolean = false
+
     def apply(x: Boolean, tp: Type): Boolean = tp.dealias match {
       case _: WildcardType | _: ProtoType =>
         false
       case tvar: TypeVar
       if !tvar.isInstantiated && ctx.typerState.constraint.contains(tvar) =>
         force.appliesTo(tvar) && {
           val direction = instDirection(tvar.origin)
-          if (direction != 0) {
-            //if (direction > 0) println(s"inst $tvar dir = up")
-            instantiate(tvar, direction < 0)
-          }
-          else {
-            val minimize =
-              force.minimizeAll ||
-              variance >= 0 && !(
-                !force.allowBottom &&
-                defn.isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true)))
-            if (minimize) instantiate(tvar, fromBelow = true)
-            else toMaximize = true
-          }
+          def avoidBottom =
+            !force.allowBottom &&
+            defn.isBottomType(ctx.typeComparer.approximation(tvar.origin, fromBelow = true))
+          def preferMin = force.minimizeAll || variance >= 0 && !avoidBottom
+          if (direction != 0) instantiate(tvar, direction < 0)
+          else if (preferMin) instantiate(tvar, fromBelow = true)
+          else toMaximize = true
           foldOver(x, tvar)
         }
       case tp =>

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

@@ -2535,32 +2535,6 @@ class Typer extends Namer
       }
     }
 
-    /** If `tp` is a TypeVar which is fully constrained (i.e. its upper bound `hi` conforms
-     *  to its lower bound `lo`), replace `tp` by `hi`. This is necessary to
-     *  keep the right constraints for some implicit search problems. The paradigmatic case
-     *  is `implicitNums.scala`. Without the healing done in `followAlias`, we cannot infer
-     *  implicitly[_3], where _2 is the typelevel number 3. The problem here is that if a
-     *  prototype is, say, Succ[Succ[Zero]], we can infer that it's argument type is Succ[Zero].
-     *  But if the prototype is N? >: Succ[Succ[Zero]] <: Succ[Succ[Zero]], the same
-     *  decomposition does not work - we'd get a N?#M where M is the element type name of Succ
-     *  instead.
-     */
-    def followAlias(tp: Type)(implicit ctx: Context): Type = {
-      val constraint = ctx.typerState.constraint
-      def inst(tp: Type): Type = tp match {
-        case TypeBounds(lo, hi)
-        if (lo eq hi) || ctx.test(implicit ctx => hi <:< lo) =>
-          inst(lo)
-        case tp: TypeParamRef =>
-          constraint.typeVarOfParam(tp).orElse(tp)
-        case _ => tp
-      }
-      tp match {
-        case tp: TypeVar if constraint.contains(tp) => inst(constraint.entry(tp.origin))
-        case _ => tp
-      }
-    }
-
     /** Reveal ignored parts of prototype when synthesizing the receiver
      *  of an extension method. This is necessary for pos/i5773a.scala
      */
@@ -2809,7 +2783,7 @@ class Typer extends Namer
     }
 
     def resultMatches(wtp: Type, pt: Type) =
-      constrainResult(tree.symbol, wtp, revealProtoOfExtMethod(followAlias(pt)))
+      constrainResult(tree.symbol, wtp, revealProtoOfExtMethod(pt))
 
     def adaptNoArgs(wtp: Type): Tree = {
       val ptNorm = underlyingApplied(pt)

tests/pos/i6127.scala

@@ -0,0 +1,18 @@
+import reflect.ClassTag
+class Co[+S]
+object Co {
+  def empty[X: ClassTag]: Co[X] = ???
+}
+class Contra[-S]
+object Contra {
+  def empty[X: ClassTag]: Contra[X] = ???
+}
+class Foo[+FT](x: FT) {
+  def fooArray: Foo[Array[String]] = new Foo(Array.empty)
+  val y1: Array[String] = Array.empty
+  def fooCo: Foo[Co[String]] = new Foo(Co.empty)
+  val y2: Co[String] = Co.empty
+  def fooContra: Foo[Contra[String]] = new Foo(Contra.empty)
+  val y3: Contra[String] = Contra.empty
+}
+