Refactor instantiate

This if for better understandability only since it avoids the deeply nested
return.

Also, add a new test showing how to deal with tracked parameters in typeclass
arguments.

Author: odersky

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

@@ -4946,8 +4946,7 @@ object Types extends TypeUtils {
      *  is also a singleton type.
      */
     def instantiate(fromBelow: Boolean)(using Context): Type =
-      val tp = typeToInstantiateWith(fromBelow)
-      instantiateWith(tp)
+      instantiateWith(typeToInstantiateWith(fromBelow))
 
     /** Widen unions when instantiating this variable in the current context? */
     def widenUnions(using Context): Boolean = !ctx.typerState.constraint.isHard(this)

tests/pos/ord-over-tracked.scala

@@ -0,0 +1,15 @@
+import language.experimental.modularity
+
+trait Ord[T]:
+  def lt(x: T, y: T): Boolean
+
+given Ord[Int] = ???
+
+case class D(tracked val x: Int)
+given [T <: D]: Ord[T] = (a, b) => a.x < b.x
+
+def mySort[T: Ord](x: Array[T]): Array[T] = ???
+
+def test =
+  val arr = Array(D(1))
+  val arr1 = mySort(arr) // error: no given instance of  type Ord[D{val x: (1 : Int)}]