Integrate prune logic in addOneBound

New cyclic bounds can also appear as a consequence of unification.
The latter goes through addOneBound, but not through addConstraint.
So the prune logic that avoids cycles should go to addOneBound.

ExprType checking could logically stay with addConstraint but it
was simpler to move it as well.

Author: odersky

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

@@ -87,55 +87,85 @@ trait ConstraintHandling[AbstractContext] {
     nonParamBounds(param).derivedTypeBounds(fullLowerBound(param), fullUpperBound(param))
 
   protected def addOneBound(param: TypeParamRef, rawBound: Type, isUpper: Boolean)(implicit actx: AbstractContext): Boolean =
-    !constraint.contains(param) || {
-      def avoidCycle(tp: Type): Type = tp match
-        case tp: AndOrType =>
-          tp.derivedAndOrType(avoidCycle(tp.tp1), avoidCycle(tp.tp2))
-        case tp: TypeParamRef =>
-          if tp eq param then
-            //println(i"stripping $tp from $rawBound, upper = $isUpper in $constraint")
-            if isUpper then defn.AnyType else defn.NothingType
-          else constraint.entry(tp) match
-            case TypeBounds(lo, hi) => if lo eq hi then avoidCycle(lo) else tp
-            case inst => avoidCycle(inst)
-        case tp: TypeVar =>
-          val underlying1 = avoidCycle(tp.underlying)
-          if underlying1 eq tp.underlying then tp else underlying1
-        case _ =>
-          tp
+
+    /** Normalize the bound `tp` in the following ways:
+     *
+     *   1. Any toplevel occurrences of the compared parameter `param` are
+     *      replaced by `Nothing` if bound is from below or `Any` otherwise.
+     *   2. Toplevel occurrences of TypeVars over TypeRefs in the current
+     *      constraint are dereferenced.
+     *   3. Toplevel occurrences of TypeRefs that are instantiated in the current
+     *      constraint are also dereferenced.
+     *   4. Toplevel occurrences of ExprTypes lead to a `NoType` return, which
+     *      causes the addOneBound operation to fail.
+     *
+     *   An occurrence is toplevel if it is the bound itself,
+     *   or the bound is a union or intersection, and the ocurrence is
+     *   toplevel in one of the operands of the `&` or `|`.
+     */
+    def avoidCycle(tp: Type): Type = tp match
+      case tp: AndOrType =>
+        val p1 = avoidCycle(tp.tp1)
+        val p2 = avoidCycle(tp.tp2)
+        if p1.exists && p2.exists then tp.derivedAndOrType(p1, p2) else NoType
+      case tp: TypeParamRef =>
+        if tp eq param then
+          //println(i"stripping $tp from $rawBound, upper = $isUpper in $constraint")
+          if isUpper then defn.AnyType else defn.NothingType
+        else constraint.entry(tp) match
+          case TypeBounds(lo, hi) => if lo eq hi then avoidCycle(lo) else tp
+          case inst => avoidCycle(inst)
+      case tp: TypeVar =>
+        val underlying1 = avoidCycle(tp.underlying)
+        if underlying1 eq tp.underlying then tp else underlying1
+      case tp: ExprType =>
+        // ExprTypes are not value types, so type parameters should not
+        // be instantiated to ExprTypes. A scenario where such an attempted
+        // instantiation can happen is if we unify (=> T) => () with A => ()
+        // where A is a TypeParamRef. See the comment on EtaExpansion.etaExpand
+        // why types such as (=> T) => () can be constructed and i7969.scala
+        // as a test where this happens.
+        // Note that scalac by contrast allows such instantiations. But letting
+        // type variables be ExprTypes has its own problems (e.g. you can't write
+        // the resulting types down) and is largely unknown terrain.
+        NoType
+      case _ =>
+        tp
+
+    if !constraint.contains(param) then false
+    else
       val bound = avoidCycle(rawBound)
 
       val oldBounds @ TypeBounds(lo, hi) = constraint.nonParamBounds(param)
       val equalBounds = isUpper && (lo eq bound) || !isUpper && (bound eq hi)
-      if (equalBounds &&
-         !bound.existsPart(bp => bp.isInstanceOf[WildcardType] || (bp eq param))) {
+      if !bound.exists then false
+      else if equalBounds
+        && !bound.existsPart(bp => bp.isInstanceOf[WildcardType] || (bp eq param))
+      then
         // The narrowed bounds are equal and do not contain wildcards,
         // so we can remove `param` from the constraint.
         // (Handling wildcards requires choosing a bound, but we don't know which
         // bound to choose here, this is handled in `ConstraintHandling#approximation`)
         constraint = constraint.replace(param, bound)
         true
-      }
-      else {
+      else
         // Narrow one of the bounds of type parameter `param`
         // If `isUpper` is true, ensure that `param <: `bound`, otherwise ensure
         // that `param >: bound`.
-        val narrowedBounds = {
+        val narrowedBounds =
           val saved = homogenizeArgs
           homogenizeArgs = Config.alignArgsInAnd
           try
             if (isUpper) oldBounds.derivedTypeBounds(lo, hi & bound)
             else oldBounds.derivedTypeBounds(lo | bound, hi)
           finally homogenizeArgs = saved
-        }
         val c1 = constraint.updateEntry(param, narrowedBounds)
         (c1 eq constraint) || {
           constraint = c1
           val TypeBounds(lo, hi) = constraint.entry(param)
           isSubType(lo, hi)
         }
-      }
-    }
+  end addOneBound
 
   private def location(implicit ctx: Context) = "" // i"in ${ctx.typerState.stateChainStr}" // use for debugging
 
@@ -201,7 +231,6 @@ trait ConstraintHandling[AbstractContext] {
     up.forall(addOneBound(_, lo, isUpper = false))
   }
 
-
   protected def isSubType(tp1: Type, tp2: Type, whenFrozen: Boolean)(implicit actx: AbstractContext): Boolean =
     if (whenFrozen)
       isSubTypeWhenFrozen(tp1, tp2)
@@ -461,49 +490,6 @@ trait ConstraintHandling[AbstractContext] {
           if (fromBelow) isSubType(bound, tp) else isSubType(tp, bound)
       }
 
-    /** Normalize the bound `bnd` in the following ways:
-     *
-     *   1. Any toplevel occurrences of the compared parameter `param` are
-     *      replaced by `Nothing` if bound is from below or `Any` otherwise.
-     *   2. Toplevel occurrences of TypeVars over TypeRefs in the current
-     *      constraint are dereferenced.
-     *   3. Toplevel occurrences of TypeRefs that are instantiated in the current
-     *      constraint are also referenced.
-     *   4. Toplevel occurrences of ExprTypes lead to a `NoType` return, which
-     *      causes the addConstraint operation to fail.
-     *
-     *   An occurrence is toplevel if it is the bound itself,
-     *   or the bound is a union or intersection, and the ocurrence is
-     *   toplevel in one of the operands of the `&` or `|`.
-     */
-    def prune(bnd: Type): Type = bnd match
-      case bnd: AndOrType =>
-        val p1 = prune(bnd.tp1)
-        val p2 = prune(bnd.tp2)
-        if (p1.exists && p2.exists) bnd.derivedAndOrType(p1, p2)
-        else NoType
-      case bnd: TypeVar if constraint contains bnd.origin => // (2)
-        prune(bnd.underlying)
-      case bnd: TypeParamRef =>
-        if bnd eq param then // (1)
-          if fromBelow then defn.NothingType else defn.AnyType
-        else constraint.entry(bnd) match
-          case TypeBounds(lo, hi) => if lo eq hi then prune(lo) else bnd
-          case inst => prune(inst) // (3)
-      case bnd: ExprType => // (4)
-        // ExprTypes are not value types, so type parameters should not
-        // be instantiated to ExprTypes. A scenario where such an attempted
-        // instantiation can happen is if we unify (=> T) => () with A => ()
-        // where A is a TypeParamRef. See the comment on EtaExpansion.etaExpand
-        // why types such as (=> T) => () can be constructed and i7969.scala
-        // as a test where this happens.
-        // Note that scalac by contrast allows such instantiations. But letting
-        // type variables be ExprTypes has its own problems (e.g. you can't write
-        // the resulting types down) and is largely unknown terrain.
-        NoType
-      case _ =>
-        bnd
-
     def kindCompatible(tp1: Type, tp2: Type): Boolean =
       val tparams1 = tp1.typeParams
       val tparams2 = tp2.typeParams
@@ -521,9 +507,8 @@ trait ConstraintHandling[AbstractContext] {
         case bound: TypeParamRef if constraint contains bound =>
           addParamBound(bound)
         case _ =>
-          val pbound = prune(pruneLambdaParams(bound))
-          pbound.exists
-          && kindCompatible(param, pbound)
+          val pbound = pruneLambdaParams(bound)
+          kindCompatible(param, pbound)
           && (if fromBelow then addLowerBound(param, pbound) else addUpperBound(param, pbound))
       finally addConstraintInvocations -= 1
     }