Handle Typeable (#22663)

Fixes #22642 

Turn code comments into actual code.

Author: noti0na1

Diff for: compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -830,6 +830,7 @@ class Definitions {
 
   @tu lazy val ReflectSelectableTypeRef: TypeRef = requiredClassRef("scala.reflect.Selectable")
 
+  @tu lazy val TypeableType: TypeSymbol = requiredPackage("scala.reflect.Typeable$package").moduleClass.requiredType("Typeable")
   @tu lazy val TypeTestClass: ClassSymbol = requiredClass("scala.reflect.TypeTest")
     @tu lazy val TypeTest_unapply: Symbol = TypeTestClass.requiredMethod(nme.unapply)
   @tu lazy val TypeTestModule_identity: Symbol = TypeTestClass.companionModule.requiredMethod(nme.identity)

Diff for: compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -68,8 +68,11 @@ class CheckUnused private (phaseMode: PhaseMode, suffix: String) extends MiniPha
     val name = tree.removeAttachment(OriginalName).getOrElse(nme.NO_NAME)
     if tree.srcPos.isSynthetic && tree.symbol == defn.TypeTest_unapply then
       tree.qualifier.tpe.underlying.finalResultType match
-      case AppliedType(_, args) => // tycon.typeSymbol == defn.TypeTestClass
-        val res = args(1) // T in TypeTest[-S, T]
+      case AppliedType(tycon, args) =>
+        val res =
+          if tycon.typeSymbol == defn.TypeTestClass then args(1) // T in TypeTest[-S, T]
+          else if tycon.typeSymbol == defn.TypeableType then args(0) // T in Typeable[T]
+          else return tree
         val target = res.dealias.typeSymbol
         resolveUsage(target, target.name, res.importPrefix.skipPackageObject) // case _: T =>
       case _ =>

Diff for: tests/warn/i15503d.scala

@@ -1,7 +1,5 @@
 //> using options -Wunused:patvars
 
-import scala.reflect.Typeable
-
 sealed trait Calc
 sealed trait Const extends Calc
 case class Sum(a: Calc, b: Calc) extends Calc
@@ -74,13 +72,6 @@ class C(c0: Option[Int], k0: K):
     for case Some(value) <- List(Option(42))
     yield 27
 
-    /*
-  def tester[A](a: A)(using Typeable[K]) =
-    a match
-    case S(i, j) => i + j
-    case _ => 0
-    */
-
 class Wild:
   def f(x: Any) =
     x match

Diff for: tests/warn/i21525.scala

@@ -1,6 +1,7 @@
 //> using options -Werror -Wunused:imports
 
-import scala.reflect.TypeTest
+import scala.reflect.{Typeable, TypeTest}
+import compiletime.*
 
 trait A {
   type B
@@ -18,3 +19,18 @@ def f(a: A, b: a.B): Boolean = {
       false
   }
 }
+
+trait T:
+  type X
+  given Typeable[X] = deferred
+
+def g(t: T, x: Any) =
+  import t.X
+  x match
+  case _: X => true
+  case _ => false
+
+def typer[T: Typeable](x: Any) =
+  x match
+  case _: T => 1
+  case _ => 0