hide SIP-67 behaviour behind experimental feature flag

Author: mberndt123

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/core/Mode.scala

@@ -31,7 +31,7 @@ case class Mode(val bits: Int) extends AnyVal {
 object Mode {
   val None: Mode = Mode(0)
 
-  private val modeName = new Array[String](32)
+  private val modeName = new Array[String](33)
 
   def newMode(bit: Int, name: String): Mode = {
     modeName(bit) = name
@@ -206,4 +206,7 @@ object Mode {
 
   /** Skip inlining of methods. */
   val NoInline: Mode = newMode(31, "NoInline")
+
+  /** strictEquality pattern matching (SIP-67) */
+  val StrictEqualityPatternMatching: Mode = newMode(32, "strictEqualityPatternMatching")
 }

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 =
+    ctx.mode.is(Mode.StrictEqualityPatternMatching) || Feature.enabled(Feature.strictEqualityPatternMatching)
+
 
   /** A common base class of contextual implicits and of-type implicits which
    *  represents a set of references to implicit definitions.
@@ -1065,7 +1068,9 @@ trait Implicits:
     || locally:
       if strictEquality then
         leftTreeOption.exists: leftTree =>
-          leftTree.symbol.flags.isAllOf(Flags.EnumValue) || (leftTree.symbol.flags.isAllOf(Flags.Module | Flags.Case))
+          strictEqualityPatternMatching && (
+            leftTree.symbol.flags.isAllOf(Flags.EnumValue) || (leftTree.symbol.flags.isAllOf(Flags.Module | Flags.Case))
+          )
       else
         (ltp <:< lift(rtp) || rtp <:< lift(ltp))
   }

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

@@ -12,6 +12,7 @@ class SIP67Tests extends DottyTest:
   def sip67test1: Unit =
     val source = """
     import scala.language.strictEquality
+    import scala.language.experimental.strictEqualityPatternMatching
     enum Foo:
       case Bar
 
@@ -28,6 +29,7 @@ class SIP67Tests extends DottyTest:
   def sip67test2: Unit =
     val source = """
     import scala.language.strictEquality
+    import scala.language.experimental.strictEqualityPatternMatching
 
     sealed trait Foo
 

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")