Fix #3443: adapt child compatibility check

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

@@ -606,20 +606,19 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
       }
     }
 
-    val tvars = tp1.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds) }
-    val protoTp1 = thisTypeMap(tp1.appliedTo(tvars))
-
-    // replace type parameter references with fresh type vars or bounds
+    // replace type parameter references with bounds
     val typeParamMap = new TypeMap {
       def apply(t: Type): Type = t match {
 
         case tp: TypeRef if tp.symbol.is(TypeParam) && tp.underlying.isInstanceOf[TypeBounds] =>
           // See tests/patmat/gadt.scala  tests/patmat/exhausting.scala
-          val bound =
-            if (variance == 0) tp.underlying.bounds      // non-variant case is not well-founded
-            else if (variance == 1) TypeBounds.upper(tp)
-            else TypeBounds.lower(tp)
-          newTypeVar(bound)
+          val exposed =
+            if (variance == 0) newTypeVar(tp.underlying.bounds)
+            else if (variance == 1) expose(tp, true)
+            else expose(tp, false)
+
+          debug.println(s"$tp exposed to =====> " + exposed)
+          exposed
         case tp: RefinedType if tp.refinedInfo.isInstanceOf[TypeBounds] =>
           // Ideally, we would expect type inference to do the job
           // Check tests/patmat/t9657.scala
@@ -629,6 +628,9 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
       }
     }
 
+    val tvars = tp1.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds) }
+    val protoTp1 = thisTypeMap(tp1.appliedTo(tvars))
+
     if (protoTp1 <:< tp2 && isFullyDefined(protoTp1, ForceDegree.noBottom)) protoTp1
     else {
       val protoTp2 = typeParamMap(tp2)
@@ -778,41 +780,41 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
    *
    *  A <: X :> Y  B <: U :> V   M { type T <: A :> B }  ~~>  M { type T <: X :> V }
    */
-  def expose(tp: Type, refineCtx: Boolean = false, up: Boolean = true): Type = tp match {
+  def expose(tp: Type, up: Boolean = true): Type = tp match {
     case tp: AppliedType =>
-      tp.derivedAppliedType(expose(tp.tycon, refineCtx, up), tp.args.map(expose(_, refineCtx, up)))
+      tp.derivedAppliedType(expose(tp.tycon, up), tp.args.map(expose(_, up)))
 
     case tp: TypeAlias =>
-      val hi = expose(tp.alias, refineCtx, up)
-      val lo = expose(tp.alias, refineCtx, up)
+      val hi = expose(tp.alias, up)
+      val lo = expose(tp.alias, up)
 
       if (hi =:= lo)
         tp.derivedTypeAlias(hi)
       else
         tp.derivedTypeBounds(lo, hi)
 
     case tp @ TypeBounds(lo, hi) =>
-      tp.derivedTypeBounds(expose(lo, refineCtx, false), expose(hi, refineCtx, true))
+      tp.derivedTypeBounds(expose(lo, false), expose(hi, true))
 
     case tp: RefinedType =>
       tp.derivedRefinedType(
         expose(tp.parent),
         tp.refinedName,
-        expose(tp.refinedInfo, true, up)
+        expose(tp.refinedInfo, up)
       )
-    case tp: TypeProxy if refineCtx =>
+    case tp: TypeProxy =>
       tp.underlying match {
         case TypeBounds(lo, hi) =>
-          expose(if (up) hi else lo, refineCtx, up)
+          expose(if (up) hi else lo, up)
         case _ =>
           tp
       }
 
     case OrType(tp1, tp2) =>
-      OrType(expose(tp1, refineCtx, up), expose(tp2, refineCtx, up))
+      OrType(expose(tp1, up), expose(tp2, up))
 
     case AndType(tp1, tp2) =>
-      AndType(expose(tp1, refineCtx, up), expose(tp2, refineCtx, up))
+      AndType(expose(tp1, up), expose(tp2, up))
 
     case _ => tp
   }

tests/patmat/i3443.scala

@@ -0,0 +1,19 @@
+object Test {
+  // shapeless.Coproduct
+  sealed trait Coproduct extends Product with Serializable
+  sealed trait :+:[+H, +T <: Coproduct] extends Coproduct
+  final case class Inl[+H, +T <: Coproduct](head : H) extends :+:[H, T]
+  final case class Inr[+H, +T <: Coproduct](tail : T) extends :+:[H, T]
+  sealed trait CNil extends Coproduct
+
+  // Note that this only appears when T is a type parameter. Replaying T with
+  // a concrete type (CNil or another :+:) leads to accurate warnnings
+  def f[T <: Coproduct](fa: Int :+: T) =
+    fa match {
+      case Inl(x) => 1
+      case Inr(x) => 2 // Dotty thinks this unreachable, but it is!
+    }
+
+  def main(args: Array[String]): Unit =
+    assert(f(Inr(Inl(-1))) == 2) // Example of reachability
+}