Fix #7554: Implement TypeTest interface

Using tests from: https://gist.github.com/Blaisorblade/a0eebb6a4f35344e48c4c60dc2a14ce6

Author: nicolasstucki

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

@@ -671,6 +671,10 @@ class Definitions {
   @tu lazy val ClassTagModule: Symbol = ClassTagClass.companionModule
     @tu lazy val ClassTagModule_apply: Symbol = ClassTagModule.requiredMethod(nme.apply)
 
+  @tu lazy val TypeTestClass: ClassSymbol = ctx.requiredClass("scala.reflect.TypeTest")
+  @tu lazy val TypeTestModule: Symbol = TypeTestClass.companionModule
+    @tu lazy val TypeTestModule_identity: Symbol = TypeTestModule.requiredMethod(nme.identity)
+
   @tu lazy val QuotedExprClass: ClassSymbol = ctx.requiredClass("scala.quoted.Expr")
   @tu lazy val QuotedExprModule: Symbol = QuotedExprClass.companionModule
     @tu lazy val QuotedExprModule_nullExpr: Symbol = QuotedExprModule.requiredMethod(nme.nullExpr)

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -48,6 +48,36 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       case _ => EmptyTree
   end synthesizedClassTag
 
+  lazy val synthesizedTypeTest: SpecialHandler =
+    (formal, span) => formal.argInfos match {
+      case arg1 :: arg2 :: Nil if !defn.isBottomClass(arg2.typeSymbol) =>
+        val tp1 = fullyDefinedType(arg1, "TypeTest argument", span)
+        val tp2 = fullyDefinedType(arg2, "TypeTest argument", span)
+        val sym2 = tp2.typeSymbol
+        if tp1 <:< tp2 then
+          ref(defn.TypeTestModule_identity).appliedToType(tp2).withSpan(span)
+        else if sym2 == defn.AnyValClass || sym2 == defn.AnyRefAlias || sym2 == defn.ObjectClass then
+          EmptyTree
+        else
+          // Generate SAM: (s: <tp1>) => if s.isInstanceOf[s.type & <tp2>] then Some(s.asInstanceOf[s.type & <tp2>]) else None
+          def body(args: List[Tree]): Tree = {
+            val arg :: Nil = args
+            val t = arg.tpe & tp2
+            If(
+              arg.select(defn.Any_isInstanceOf).appliedToType(t),
+              ref(defn.SomeClass.companionModule.termRef).select(nme.apply)
+                .appliedToType(t)
+                .appliedTo(arg.select(nme.asInstanceOf_).appliedToType(t)),
+              ref(defn.NoneModule))
+          }
+          val tpe = MethodType(List(nme.s))(_ => List(tp1), mth => defn.OptionClass.typeRef.appliedTo(mth.newParamRef(0) & tp2))
+          val meth = ctx.newSymbol(ctx.owner, nme.ANON_FUN, Synthetic | Method, tpe, coord = span)
+          val typeTestType = defn.TypeTestClass.typeRef.appliedTo(List(tp1, tp2))
+          Closure(meth, tss => body(tss.head).changeOwner(ctx.owner, meth), targetType = typeTestType).withSpan(span)
+      case _ =>
+        EmptyTree
+    }
+
   /** Synthesize the tree for `'[T]` for an implicit `scala.quoted.Type[T]`.
    *  `T` is deeply dealiased to avoid references to local type aliases.
    */
@@ -389,6 +419,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
   val specialHandlers = List(
     defn.ClassTagClass        -> synthesizedClassTag,
+    defn.TypeTestClass        -> synthesizedTypeTest,
     defn.QuotedTypeClass      -> synthesizedTypeTag,
     defn.EqlClass             -> synthesizedEql,
     defn.TupledFunctionClass  -> synthesizedTupleFunction,

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -769,14 +769,16 @@ class Typer extends Namer
    */
   def tryWithClassTag(tree: Typed, pt: Type)(using Context): Tree = tree.tpt.tpe.dealias match {
     case tref: TypeRef if !tref.symbol.isClass && !ctx.isAfterTyper && !(tref =:= pt) =>
-      require(ctx.mode.is(Mode.Pattern))
-      inferImplicit(defn.ClassTagClass.typeRef.appliedTo(tref),
-                    EmptyTree, tree.tpt.span)(using ctx.retractMode(Mode.Pattern)) match {
-        case SearchSuccess(clsTag, _, _) =>
-          typed(untpd.Apply(untpd.TypedSplice(clsTag), untpd.TypedSplice(tree.expr)), pt)
-        case _ =>
-          tree
+      def withTag(tpe: Type): Option[Tree] = {
+        require(ctx.mode.is(Mode.Pattern))
+        inferImplicit(tpe, EmptyTree, tree.tpt.span)(using ctx.retractMode(Mode.Pattern)) match
+          case SearchSuccess(clsTag, _, _) =>
+            Some(typed(untpd.Apply(untpd.TypedSplice(clsTag), untpd.TypedSplice(tree.expr)), pt))
+          case _ =>
+            None
       }
+      withTag(defn.TypeTestClass.typeRef.appliedTo(pt, tref)).orElse(
+        withTag(defn.ClassTagClass.typeRef.appliedTo(tref))).getOrElse(tree)
     case _ => tree
   }
 
@@ -1705,8 +1707,8 @@ class Typer extends Namer
     val body1 = typed(tree.body, pt)
     body1 match {
       case UnApply(fn, Nil, arg :: Nil)
-      if fn.symbol.exists && fn.symbol.owner == defn.ClassTagClass && !body1.tpe.isError =>
-        // A typed pattern `x @ (e: T)` with an implicit `ctag: ClassTag[T]`
+      if fn.symbol.exists && (fn.symbol.owner == defn.ClassTagClass || fn.symbol.owner.derivesFrom(defn.TypeTestClass)) && !body1.tpe.isError =>
+        // A typed pattern `x @ (e: T)` with an implicit `ctag: ClassTag[T]` or `ctag: TypeTest[T]`
         // was rewritten to `x @ ctag(e)` by `tryWithClassTag`.
         // Rewrite further to `ctag(x @ e)`
         tpd.cpy.UnApply(body1)(fn, Nil,

docs/docs/reference/other-new-features/type-test.md

@@ -0,0 +1,70 @@
+---
+layout: doc-page
+title: "TypeTest"
+---
+
+TypeTest
+--------
+
+`TypeTest` provides a replacement for `ClassTag.unapply` where the type of the argument is generalized.
+`TypeTest.unapply` will return `Some(x.asInstanceOf[Y])` if `x` conforms to `Y`, otherwise it returns `None`.
+
+```scala
+trait TypeTest[-S, T] extends Serializable {
+  def unapply(s: S): Option[s.type & T]
+}
+```
+
+Just like `ClassTag` used to do, it can be used to perform type checks in patterns.
+
+```scala
+type X
+type Y <: X
+given TypeTest[X, Y] = ...
+(x: X) match {
+  case y: Y => ... // safe checked downcast
+  case _ => ...
+}
+```
+
+
+Examples
+--------
+
+Given the following abstract definition of `Peano` numbers that provides `TypeTest[Nat, Zero]` and `TypeTest[Nat, Succ]`
+
+```scala
+trait Peano {
+  type Nat
+  type Zero <: Nat
+  type Succ <: Nat
+  def safeDiv(m: Nat, n: Succ): (Nat, Nat)
+  val Zero: Zero
+  val Succ: SuccExtractor
+  trait SuccExtractor {
+    def apply(nat: Nat): Succ
+    def unapply(nat: Succ): Option[Nat]
+  }
+  given TypeTest[Nat, Zero] = typeTestOfZero
+  protected def typeTestOfZero: TypeTest[Nat, Zero]
+  given TypeTest[Nat, Succ]
+  protected def typeTestOfSucc: TypeTest[Nat, Succ]
+```
+
+it will be possible to write the following program
+
+```scala
+val peano: Peano = ...
+import peano.{_, given}
+def divOpt(m: Nat, n: Nat): Option[(Nat, Nat)] = {
+  n match {
+    case Zero => None
+    case s @ Succ(_) => Some(safeDiv(m, s))
+  }
+}
+val two = Succ(Succ(Zero))
+val five = Succ(Succ(Succ(two)))
+println(divOpt(five, two))
+```
+
+Note that without the `TypeTest[Nat, Succ]` the pattern `Succ.unapply(nat: Succ)` would be unchecked.

library/src/scala/reflect/TypeTest.scala

@@ -0,0 +1,31 @@
+package scala.reflect
+
+/** A `TypeTest[S, T] contains the logic needed to know at runtime if a value of
+ *  type `S` can be downcasted to `T`.
+ *
+ *  If a pattern match is performed on a term of type `s: S` that is uncheckable with `s.isInstanceOf[T]` and
+ *  the pattern are of the form:
+ *    - `t: T`
+ *    - `t @ X()` where the `X.unapply` has takes an argument of type `T`
+ *  then a given instance of `TypeTest[S, T]` is summoned and used to perform the test.
+ */
+@scala.annotation.implicitNotFound(msg = "No TypeTest available for [${S}, ${T}]")
+trait TypeTest[-S, T] extends Serializable {
+
+  /** A TypeTest[S, T] can serve as an extractor that matches only S of type T.
+   *
+   * The compiler tries to turn unchecked type tests in pattern matches into checked ones
+   * by wrapping a `(_: T)` type pattern as `tt(_: T)`, where `tt` is the `TypeTest[S, T]` instance.
+   * Type tests necessary before calling other extractors are treated similarly.
+   * `SomeExtractor(...)` is turned into `tt(SomeExtractor(...))` if `T` in `SomeExtractor.unapply(x: T)`
+   * is uncheckable, but we have an instance of `TypeTest[S, T]`.
+   */
+  def unapply(x: S): Option[x.type & T]
+
+}
+
+object TypeTest {
+
+  def identity[T]: TypeTest[T, T] = Some(_)
+
+}

tests/neg-custom-args/fatal-warnings/IsInstanceOfClassTag.scala

@@ -0,0 +1,24 @@
+import scala.reflect.ClassTag
+
+object IsInstanceOfClassTag {
+  def safeCast[T: ClassTag](x: Any): Option[T] = {
+    x match {
+      case x: T => Some(x) // TODO error: deprecation waring
+      case _ => None
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    safeCast[List[String]](List[Int](1)) match {
+      case None =>
+      case Some(xs) =>
+        xs.head.substring(0)
+    }
+
+    safeCast[List[_]](List[Int](1)) match {
+      case None =>
+      case Some(xs) =>
+        xs.head.substring(0) // error
+    }
+  }
+}

tests/neg-custom-args/fatal-warnings/IsInstanceOfClassTag2.scala

@@ -0,0 +1,22 @@
+import scala.reflect.TypeTest
+
+object IsInstanceOfClassTag {
+  def safeCast[T](x: Any)(using TypeTest[Any, T]): Option[T] = {
+    x match {
+      case x: T => Some(x)
+      case _ => None
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    safeCast[List[String]](List[Int](1)) match { // error
+      case None =>
+      case Some(xs) =>
+    }
+
+    safeCast[List[_]](List[Int](1)) match {
+      case None =>
+      case Some(xs) =>
+    }
+  }
+}

tests/neg-custom-args/fatal-warnings/type-test-paths-2.scala

@@ -0,0 +1,41 @@
+import scala.reflect.TypeTest
+
+trait R {
+  type Nat
+  type Succ <: Nat
+  type Idx
+  given TypeTest[Nat, Succ] = typeTestOfSucc
+  protected def typeTestOfSucc: TypeTest[Nat, Succ]
+  def n: Nat
+  def one: Succ
+}
+
+object RI extends R {
+  type Nat = Int
+  type Succ = Int
+  type Idx = Int
+  protected def typeTestOfSucc: TypeTest[Nat, Succ] = new {
+    def unapply(x: Int): Option[x.type & Succ] =
+      if x > 0 then Some(x) else None
+  }
+  def n: Nat = 4
+  def one: Succ = 1
+}
+
+object Test {
+  val r1: R = RI
+  val r2: R = RI
+
+  r1.n match {
+    case n: r2.Nat => // error: the type test for Test.r2.Nat cannot be checked at runtime
+    case n: r1.Idx => // error: the type test for Test.r1.Idx cannot be checked at runtime
+    case n: r1.Succ => // Ok
+    case n: r1.Nat => // Ok
+  }
+
+  r1.one match {
+    case n: r2.Nat => // error: the type test for Test.r2.Nat cannot be checked at runtime
+    case n: r1.Idx => // error: the type test for Test.r1.Idx cannot be checked at runtime
+    case n: r1.Nat => // Ok
+  }
+}

tests/neg-custom-args/fatal-warnings/type-test-paths.scala

@@ -0,0 +1,37 @@
+import scala.reflect.TypeTest
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val p1: T = T1
+    val p2: T = T1
+
+    (p1.y: p1.X) match {
+      case x: p2.Y => // error: unchecked
+      case x: p1.Y =>
+      case _ =>
+    }
+  }
+
+}
+
+trait T {
+  type X
+  type Y <: X
+  def x: X
+  def y: Y
+  given TypeTest[X, Y] = typeTestOfY
+  protected def typeTestOfY: TypeTest[X, Y]
+}
+
+object T1 extends T {
+  type X = Boolean
+  type Y = true
+  def x: X = false
+  def y: Y = true
+  protected def typeTestOfY: TypeTest[X, Y] = new {
+    def unapply(x: X): Option[x.type & Y] = x match
+      case x: (true & x.type) => Some(x)
+      case _ => None
+  }
+
+}

tests/neg-custom-args/fatal-warnings/type-test-syntesize.scala

@@ -0,0 +1,29 @@
+import scala.reflect.TypeTest
+
+object Test {
+  def test[S, T](using TypeTest[S, T]): Unit = ()
+  val a: A = ???
+
+  test[Any, Any]
+  test[Int, Int]
+
+  test[Int, Any]
+  test[String, Any]
+  test[String, AnyRef]
+
+  test[Any, Int]
+  test[Any, String]
+  test[Any, Some[_]]
+  test[Any, Array[Int]]
+  test[Seq[Int], List[Int]]
+
+  test[Any, Some[Int]] // error
+  test[Any, a.X] // error
+  test[a.X, a.Y] // error
+
+}
+
+class A {
+  type X
+  type Y <: X
+}