Detail why not baseType

Author: dwijnand

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -625,6 +625,14 @@ object SpaceEngine {
         // but end up with A1[<?>] and A2[<?>].
         // So we widen (like AppliedType superType does) away
         // non-class type constructors.
+        //
+        // Can't use `tpOriginal.baseType(cls)` because it causes
+        // i15893 to return exhaustivity warnings, because instead of:
+        //    <== refineUsingParent(N, class Succ, []) = Succ[<? <: NatT>]
+        //    <== isSub(Succ[<? <: NatT>] <:< Succ[Succ[<?>]]) = true
+        // we get
+        //    <== refineUsingParent(NatT, class Succ, []) = Succ[NatT]
+        //    <== isSub(Succ[NatT] <:< Succ[Succ[<?>]]) = false
         def getAppliedClass(tp: Type): Type = tp match
           case tp @ AppliedType(_: HKTypeLambda, _)                        => tp
           case tp @ AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass => tp

tests/warn/i15893.min.scala

@@ -0,0 +1,13 @@
+sealed trait NatT
+case class Zero()                 extends NatT
+case class Succ[+N <: NatT](n: N) extends NatT
+
+type Mod2[N <: NatT] <: NatT = N match
+  case Zero                  => Zero
+  case Succ[Zero]            => Succ[Zero]
+  case Succ[Succ[predPredN]] => Mod2[predPredN]
+
+def dependentlyTypedMod2[N <: NatT](n: N): Mod2[N] = n match
+  case Zero(): Zero                         => Zero() // warn
+  case Succ(Zero()): Succ[Zero]             => Succ(Zero()) // warn
+  case Succ(Succ(predPredN)): Succ[Succ[?]] => dependentlyTypedMod2(predPredN) // warn