Fix #12093: Simplify types after parameter instantiation

Author: OlivierBlanvillain

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

@@ -2834,7 +2834,7 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
           caseLambda match {
             case caseLambda: HKTypeLambda =>
               val instances = paramInstances(new Array(caseLambda.paramNames.length), pat)
-              instantiateParams(instances)(body)
+              instantiateParams(instances)(body).simplified
             case _ =>
               body
           }

tests/pos/12093.scala

@@ -0,0 +1,47 @@
+import scala.compiletime.ops.int.{`*`, +}
+
+// HList
+sealed trait Shape
+final case class #:[H <: Int & Singleton, T <: Shape](head: H, tail: T) extends Shape
+case object Ø extends Shape
+type Ø = Ø.type
+
+// Reduce
+def reduce[T, S <: Shape, A <: Shape](shape: S, axes: A): Reduce[S, A, 0] = ???
+type Reduce[S, Axes <: Shape, I <: Int] <: Shape = S match {
+  case head #: tail => Contains[Axes, I] match {
+    case true => Reduce[tail, Remove[Axes, I], I + 1]
+    case false => head #: Reduce[tail, Axes, I + 1]
+  }
+  case Ø => Axes match {
+    case Ø => Ø
+    // otherwise, do not reduce further
+  }
+}
+type Contains[Haystack <: Shape, Needle <: Int] <: Boolean = Haystack match {
+  case Ø => false
+  case head #: tail => head match {
+    case Needle => true
+    case _ => Contains[tail, Needle]
+  }
+}
+type Remove[From <: Shape, Value <: Int] <: Shape = From match {
+  case Ø => Ø
+  case head #: tail => head match {
+    case Value => Remove[tail, Value]
+    case _ => head #: Remove[tail, Value]
+  }
+}
+
+// Reshape
+def reshape[From <: Shape, To <: Shape](from: From, to: To)
+  (using ev: NumElements[From] =:= NumElements[To]): To = ???
+type NumElements[X <: Shape] <: Int = X match {
+  case Ø => 1
+  case head #: tail => head * NumElements[tail]
+}
+
+// Test cases
+val input = #:(25, #:(256, #:(256, #:(3, Ø))))
+val reduced = reduce(input, #:(3, #:(1, #:(2, Ø))))
+val reshaped: 5 #: 5 #: Ø = reshape(reduced, #:(5, #:(5, Ø)))