Fix scala#7554: Implement TypeTest interface

`TypeTest` is a replacemnt for the same functionallity performed by the `ClassTag.unaplly`.
Using `ClassTag` instances happend to be unsound.
`TypeTest` fixes that unsoundess and adds extra flexibility with the `S` type.
`ClassTag` type tests will still be supported but a warining will be emitted after 3.0.

Author: nicolasstucki

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

@@ -790,6 +790,8 @@ class Definitions {
   @tu lazy val ClassTagModule: Symbol = ClassTagClass.companionModule
     @tu lazy val ClassTagModule_apply: Symbol = ClassTagModule.requiredMethod(nme.apply)
 
+  @tu lazy val TypeTestClass: ClassSymbol = requiredClass("scala.reflect.TypeTest")
+  @tu lazy val TypeTestModule_identity: Symbol = TypeTestClass.companionModule.requiredMethod(nme.identity)
 
   @tu lazy val QuotedExprClass: ClassSymbol = requiredClass("scala.quoted.Expr")
   @tu lazy val QuotedExprModule: Symbol = QuotedExprClass.companionModule

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

@@ -1331,7 +1331,7 @@ trait Applications extends Compatibility {
         val result = assignType(cpy.UnApply(tree)(unapplyFn, unapplyImplicits(unapplyApp), unapplyPatterns), ownType)
         unapp.println(s"unapply patterns = $unapplyPatterns")
         if ((ownType eq selType) || ownType.isError) result
-        else tryWithClassTag(Typed(result, TypeTree(ownType)), selType)
+        else tryWithTypeTest(Typed(result, TypeTree(ownType)), selType)
       case tp =>
         val unapplyErr = if (tp.isError) unapplyFn else notAnExtractor(unapplyFn)
         val typedArgsErr = args mapconserve (typed(_, defn.AnyType))

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

@@ -45,6 +45,38 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       case _ => EmptyTree
   end synthesizedClassTag
 
+  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
+          // optimization when we know the typetest will always succeed
+          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[<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(tp2),
+              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 = 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
+    }
+  end synthesizedTypeTest
+
   val synthesizedTupleFunction: SpecialHandler = (formal, span) =>
     formal match
       case AppliedType(_, funArgs @ fun :: tupled :: Nil) =>
@@ -374,6 +406,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
   val specialHandlers = List(
     defn.ClassTagClass        -> synthesizedClassTag,
+    defn.TypeTestClass        -> synthesizedTypeTest,
     defn.EqlClass             -> synthesizedEql,
     defn.TupledFunctionClass  -> synthesizedTupleFunction,
     defn.ValueOfClass         -> synthesizedValueOf,

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

@@ -764,7 +764,7 @@ class Typer extends Namer
             TypeComparer.constrainPatternType(tpt1.tpe, pt)
           }
         // special case for an abstract type that comes with a class tag
-        tryWithClassTag(ascription(tpt1, isWildcard = true), pt)
+        tryWithTypeTest(ascription(tpt1, isWildcard = true), pt)
       }
       cases(
         ifPat = handlePattern,
@@ -773,21 +773,28 @@ class Typer extends Namer
     }
   }
 
-  /** For a typed tree `e: T`, if `T` is an abstract type for which an implicit class tag `ctag`
-   *  exists, rewrite to `ctag(e)`.
+  /** For a typed tree `e: T`, if `T` is an abstract type for which an implicit type test or class tag `tt`
+   *  exists, rewrite to `tt(e)`.
    *  @pre We are in pattern-matching mode (Mode.Pattern)
    */
-  def tryWithClassTag(tree: Typed, pt: Type)(using Context): Tree = tree.tpt.tpe.dealias match {
+  def tryWithTypeTest(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))
-      withoutMode(Mode.Pattern)(
-        inferImplicit(defn.ClassTagClass.typeRef.appliedTo(tref), EmptyTree, tree.tpt.span)
-      ) 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))
+        withoutMode(Mode.Pattern)(
+          inferImplicit(tpe, EmptyTree, tree.tpt.span)
+        ) match
+          case SearchSuccess(clsTag, _, _) =>
+            Some(typed(untpd.Apply(untpd.TypedSplice(clsTag), untpd.TypedSplice(tree.expr)), pt))
+          case _ =>
+            None
       }
+      val tag = withTag(defn.TypeTestClass.typeRef.appliedTo(pt, tref))
+        .orElse(withTag(defn.ClassTagClass.typeRef.appliedTo(tref)))
+        .getOrElse(tree)
+      if tag.symbol.owner == defn.ClassTagClass && config.Feature.sourceVersion.isAtLeast(config.SourceVersion.`3.1`) then
+        report.warning("Use of ClassTag for type testing may be unsound. Consider using `reflect.Typable` instead.", tree.srcPos)
+      tag
     case _ => tree
   }
 
@@ -1835,10 +1842,10 @@ 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]`
-        // was rewritten to `x @ ctag(e)` by `tryWithClassTag`.
-        // Rewrite further to `ctag(x @ e)`
+      if fn.symbol.exists && (fn.symbol.owner.derivesFrom(defn.TypeTestClass) || fn.symbol.owner == defn.ClassTagClass) && !body1.tpe.isError =>
+        // A typed pattern `x @ (e: T)` with an implicit `tt: TypeTest[T]` or `ctag: ClassTag[T]`
+        // was rewritten to `x @ tt(e)` `x @ ctag(e)` by `tryWithTypeTest`.
+        // Rewrite further to `tt(x @ e)` or `ctag(x @ e)`
         tpd.cpy.UnApply(body1)(fn, Nil,
             typed(untpd.Bind(tree.name, untpd.TypedSplice(arg)).withSpan(tree.span), arg.tpe) :: Nil)
       case _ =>

docs/docs/reference/changed-features/pattern-matching.md

@@ -240,4 +240,13 @@ def foo(f: Foo) = f match {
 ```
 
 There are plans for further simplification, in particular to factor out *product
-match* and *name-based match* into a single type of extractor.
+match* and *name-based match* into a single type of extractor.
+
+## Type testing
+
+Abstract type testing with `ClassTag` is replaced with `TypeTest` or the alias `Typeable`.
+
+- pattern `_: X` for an abstract type requires a `TypeTest` in scope
+- pattern `x @ X()` for an unapply that takes an abstract type requires a `TypeTest` in scope
+
+[More details on TypeTest](../other-new-features/type-test.md)

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

@@ -0,0 +1,146 @@
+---
+layout: doc-page
+title: "TypeTest"
+---
+
+TypeTest
+--------
+
+When pattern matching there are two situations where were a runtime type test must be performed.
+The first is kind is an explicit type test using the ascription pattern notation.
+```scala
+(x: X) match
+  case y: Y =>
+```
+The second is when an extractor takes an argument that is not a subtype of the scrutinee type.
+```scala
+(x: X) match
+  case y @ Y(n) =>
+
+object Y:
+  def unapply(x: Y): Some[Int] = ...
+```
+
+In both cases, a class test will be performed at runtime.
+But when the type test is on an abstract type (type parameter or type member), the test cannot be performed because the type is erased at runtime.
+
+A `TypeTest` can be provided to make this test possible.
+
+```scala
+package scala.reflect
+
+trait TypeTest[-S, T]:
+  def unapply(s: S): Option[s.type & T]
+```
+
+It provides an extractor that returns its argument typed as a `T` if the argument is a `T`.
+It can be used to encode a type test.
+```scala
+def f[X, Y](x: X)(using tt: TypeTest[X, Y]): Option[Y] =
+  x match
+    case tt(x @ Y(1)) => Some(x)
+    case tt(x) => Some(x)
+    case _ => None
+```
+
+To avoid the syntactic overhead the compiler will look for a type test automatically if it detects that the type test is on abstract types.
+This means that `x: Y` is transformed to `tt(x)` and `x @ Y(_)` to `tt(x @ Y(_))` if there is a contextual `TypeTest[X, Y]` in scope.
+The previous code is equivalent to
+
+```scala
+def f[X, Y](x: X)(using TypeTest[X, Y]): Option[Y] =
+  x match
+    case x @ Y(1) => Some(x)
+    case x: Y => Some(x)
+    case _ => None
+```
+
+We could create a type test at call site where the type test can be performed with runtime class tests directly as follows
+
+```scala
+val tt: TypeTest[Any, String] =
+  new TypeTest[Any, String]
+    def unapply(s: Any): Option[s.type & String] =
+      s match
+        case s: String => Some(s)
+        case _ => None
+
+f[AnyRef, String]("acb")(using tt)
+```
+
+The compiler will synthesize a new instance of a type test if non is found in scope as
+```scala
+new TypeTest[A, B]:
+  def unapply(s: A): Option[s.type & B] =
+    s match
+      case s: B => Some(s)
+      case _ => None
+```
+If the type tests cannot be done there will be an unchecked warning that will be raised on the `case s: B =>` test.
+
+The most common `TypeTest` are the ones that take any parameters (i.e. `TypeTest[Any, T]`).
+To make it possible to use this directly in context bounds we provide the alias
+```scala
+package scala.reflect
+
+type Typeable[T] = TypeTest[Any, T]
+```
+
+This alias can be used as
+
+```scala
+def f[T: Typeable]: Boolean =
+  "abc" match
+    case x: T => true
+    case _ => false
+
+f[String] // true
+f[Int] // fasle
+```
+
+### TypeTest and ClassTag
+`TypeTest` is a replacemnt for the same functionallity performed by the `ClassTag.unaplly`.
+Using `ClassTag` instances happend to be unsound.
+`TypeTest` fixes that unsoundess and adds extra flexibility with the `S` type.
+`ClassTag` type tests will still be supported but a warining will be emitted after 3.0.
+
+
+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 +1,28 @@
+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:
+
+  /** Trivial type test that always succeeds */
+  def identity[T]: TypeTest[T, T] = Some(_)

library/src/scala/reflect/Typeable.scala

@@ -0,0 +1,12 @@
+package scala.reflect
+
+/** A shorhand for `TypeTest[Any, T]`. A `Typeable[T] contains the logic needed to
+ *  know at runtime if a value can be downcasted to `T`.
+ *
+ *  If a pattern match is performed on a term of type `s: Any` 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 `Typeable[T]` (`TypeTest[Any, T]`) is summoned and used to perform the test.
+ */
+type Typeable[T] = TypeTest[Any, T]

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/classtag-typetest/3_0-migration.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.0-migration`
+import scala.reflect.ClassTag
+
+def f3_0m[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>

tests/neg-custom-args/fatal-warnings/classtag-typetest/3_0.scala

@@ -0,0 +1,6 @@
+import scala.language.`3.0`
+import scala.reflect.ClassTag
+
+def f3_0[T: ClassTag](x: Any): Unit =
+  x match
+    case _: T =>