Sharpen range approximation also for non-class type constructors

Author: odersky

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

@@ -5948,8 +5948,6 @@ object Types {
             if (distributeArgs(args, tp.tyconTypeParams))
               range(tp.derivedAppliedType(tycon, loBuf.toList),
                     tp.derivedAppliedType(tycon, hiBuf.toList))
-            else if tycon.isLambdaSub || args.exists(isRangeOfNonTermTypes) then
-              range(defn.NothingType, defn.AnyType)
             else if ctx.phase == Phases.checkCapturesPhase && args.forall(differsOnlyInCaptureSet) then
               // This is a stopgap to avoid approximating the core type to Nothing. It can probably be
               // dropped once we capture set ranges that we can keep as inference results.
@@ -5968,6 +5966,8 @@ object Types {
               // We don't have that abstraction yet, so for now we approximate by the bounds, which
               // avoids the failures, even though its soundness status is currently unclear.
               tp.derivedAppliedType(tycon, args.map(rangeToBounds))
+            else if tycon.isLambdaSub || args.exists(isRangeOfNonTermTypes) then
+              range(defn.NothingType, defn.AnyType)
             else
               // See lampepfl/dotty#14152
               range(defn.NothingType, tp.derivedAppliedType(tycon, args.map(rangeToBounds)))

tests/pos-custom-args/captures/i16226a.scala

@@ -0,0 +1,13 @@
+class Name
+class TermName extends Name
+class TypeName extends Name
+
+trait ParamInfo:
+  type ThisName <: Name
+  def variance: Long
+object ParamInfo:
+  type Of[N <: Name] = ParamInfo { type ThisName = N }
+
+def test(tparams1: List[ParamInfo{ type ThisName = TypeName }], tparams2: List[ParamInfo.Of[TypeName]]) =
+  tparams1.lazyZip(tparams2).map((p1, p2) => p1.variance + p2.variance)
+