Instantiate for invariance in paramInstances

Author: dwijnand

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

@@ -3095,7 +3095,7 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
     def paramInstances(canApprox: Boolean) = new TypeAccumulator[Array[Type]]:
       def apply(insts: Array[Type], t: Type) = t match
         case param @ TypeParamRef(b, n) if b eq caseLambda =>
-          def range1(tp: Type) = Range(tp, tp)
+          def range1(tp: Type) = if variance == 0 then tp else Range(tp, tp)
           insts(n) =
             if canApprox then
               approximation(param, fromBelow = variance >= 0, Int.MaxValue).simplified

tests/neg/i17149.scala

@@ -0,0 +1,18 @@
+type Ext1[S] = S match {
+  case Seq[t] => t
+}
+type Ext2[S] = S match {
+  case Seq[_] => Int
+}
+type Ext3[S] = S match {
+  case Array[t] => t
+}
+type Ext4[S] = S match {
+  case Array[_] => Int
+}
+def foo[T <: Seq[Any], A <: Array[B], B] =
+  summon[Ext1[T] =:= T]    // error
+  summon[Ext2[T] =:= Int]  // ok
+  summon[Ext3[A] =:= B]    // ok
+  summon[Ext4[A] =:= Int]  // ok
+