Implement SIP-67: strictEquality pattern matching (fixes #22732)

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -28,6 +28,7 @@ object Feature:
 
   val dependent = experimental("dependent")
   val erasedDefinitions = experimental("erasedDefinitions")
+  val strictEqualityPatternMatching = experimental("strictEqualityPatternMatching")
   val symbolLiterals = deprecated("symbolLiterals")
   val saferExceptions = experimental("saferExceptions")
   val pureFunctions = experimental("pureFunctions")
@@ -58,6 +59,7 @@ object Feature:
     (scala2macros, "Allow Scala 2 macros"),
     (dependent, "Allow dependent method types"),
     (erasedDefinitions, "Allow erased definitions"),
+    (strictEqualityPatternMatching, "relaxed CanEqual checks for ADT pattern matching"),
     (symbolLiterals, "Allow symbol literals"),
     (saferExceptions, "Enable safer exceptions"),
     (pureFunctions, "Enable pure functions for capture checking"),

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

@@ -1338,7 +1338,7 @@ trait Applications extends Compatibility {
           case Apply(fn @ Select(left, _), right :: Nil) if fn.hasType =>
             val op = fn.symbol
             if (op == defn.Any_== || op == defn.Any_!=)
-              checkCanEqual(left.tpe.widen, right.tpe.widen, app.span)
+              checkCanEqual(left, right.tpe.widen, app.span)
           case _ =>
         }
         app

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

@@ -84,6 +84,9 @@ object Implicits:
   def strictEquality(using Context): Boolean =
     ctx.mode.is(Mode.StrictEquality) || Feature.enabled(nme.strictEquality)
 
+  def strictEqualityPatternMatching(using Context): Boolean =
+    Feature.enabled(Feature.strictEqualityPatternMatching)
+
 
   /** A common base class of contextual implicits and of-type implicits which
    *  represents a set of references to implicit definitions.
@@ -1039,7 +1042,7 @@ trait Implicits:
    *   - if one of T, U is a subtype of the lifted version of the other,
    *     unless strict equality is set.
    */
-  def assumedCanEqual(ltp: Type, rtp: Type)(using Context) = {
+  def assumedCanEqual(leftTreeOption: Option[Tree], ltp: Type, rtp: Type)(using Context): Boolean = {
     // Map all non-opaque abstract types to their upper bound.
     // This is done to check whether such types might plausibly be comparable to each other.
     val lift = new TypeMap {
@@ -1062,15 +1065,20 @@ trait Implicits:
 
     ltp.isError
     || rtp.isError
-    || !strictEquality && (ltp <:< lift(rtp) || rtp <:< lift(ltp))
+    || locally:
+      if strictEquality then
+        strictEqualityPatternMatching && leftTreeOption.exists: leftTree =>
+          ltp <:< lift(rtp) && (leftTree.symbol.flags.isAllOf(Flags.EnumValue) || leftTree.symbol.flags.isAllOf(Flags.Module | Flags.Case))
+      else
+        (ltp <:< lift(rtp) || rtp <:< lift(ltp))
   }
 
   /** Check that equality tests between types `ltp` and `rtp` make sense */
-  def checkCanEqual(ltp: Type, rtp: Type, span: Span)(using Context): Unit =
-    if (!ctx.isAfterTyper && !assumedCanEqual(ltp, rtp)) {
+  def checkCanEqual(left: Tree, rtp: Type, span: Span)(using Context): Unit =
+    val ltp = left.tpe.widen
+    if !ctx.isAfterTyper && !assumedCanEqual(Some(left), ltp, rtp) then
       val res = implicitArgTree(defn.CanEqualClass.typeRef.appliedTo(ltp, rtp), span)
       implicits.println(i"CanEqual witness found for $ltp / $rtp: $res: ${res.tpe}")
-    }
 
   object hasSkolem extends TreeAccumulator[Boolean]:
     def apply(x: Boolean, tree: Tree)(using Context): Boolean =

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

@@ -175,7 +175,7 @@ class ReTyper(nestingLevel: Int = 0) extends Typer(nestingLevel) with ReChecking
 
   override def inferView(from: Tree, to: Type)(using Context): Implicits.SearchResult =
     Implicits.NoMatchingImplicitsFailure
-  override def checkCanEqual(ltp: Type, rtp: Type, span: Span)(using Context): Unit = ()
+  override def checkCanEqual(left: Tree, rtp: Type, span: Span)(using Context): Unit = ()
 
   override def widenEnumCase(tree: Tree, pt: Type)(using Context): Tree = tree
 

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

@@ -183,7 +183,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       *  one of `tp1`, `tp2` has a reflexive `CanEqual` instance.
       */
     def validEqAnyArgs(tp1: Type, tp2: Type)(using Context) =
-      typer.assumedCanEqual(tp1, tp2)
+      typer.assumedCanEqual(None, tp1, tp2)
        || withMode(Mode.StrictEquality) {
             !hasEq(tp1) && !hasEq(tp2)
           }

compiler/test/dotty/tools/dotc/typer/SIP67Tests.scala

@@ -0,0 +1,44 @@
+package dotty.tools.dotc.typer
+
+import dotty.tools.DottyTest
+import dotty.tools.dotc.core.Contexts.*
+
+import org.junit.Test
+import org.junit.Assert.fail
+
+class SIP67Tests extends DottyTest:
+
+  private def checkNoErrors(source: String): Unit =
+    val ctx = checkCompile("typer", source)((_, _) => ())
+    if ctx.reporter.hasErrors then
+      fail("Unexpected compilation errors were reported")
+
+  @Test
+  def sip67test1: Unit =
+    checkNoErrors:
+      """
+      import scala.language.strictEquality
+      import scala.language.experimental.strictEqualityPatternMatching
+      enum Foo:
+        case Bar
+
+      val _ =
+        (??? : Foo) match
+          case Foo.Bar =>
+      """
+  @Test
+  def sip67test2: Unit =
+    checkNoErrors:
+      """
+      import scala.language.strictEquality
+      import scala.language.experimental.strictEqualityPatternMatching
+
+      sealed trait Foo
+
+      object Foo:
+        case object Bar extends Foo
+
+      val _ =
+        (??? : Foo) match
+          case Foo.Bar =>
+      """

library/src/scala/runtime/stdLibPatches/language.scala

@@ -50,6 +50,13 @@ object language:
     @compileTimeOnly("`erasedDefinitions` can only be used at compile time in import statements")
     object erasedDefinitions
 
+    /** Experimental support for relaxed CanEqual checks for ADT pattern matching
+     *
+     * @see [[https://github.com/scala/improvement-proposals/pull/97]]
+     */
+    @compileTimeOnly("`strictEqualityPatternMatching` can only be used at compile time in import statements")
+    object strictEqualityPatternMatching
+
     /** Experimental support for using indentation for arguments
      */
     @compileTimeOnly("`fewerBraces` can only be used at compile time in import statements")