change SIP-67 implementation based on @odersky's review

Author: mberndt123

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

@@ -1039,7 +1039,7 @@ 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 +1062,20 @@ trait Implicits:
 
     ltp.isError
     || rtp.isError
-    || !strictEquality && (ltp <:< lift(rtp) || rtp <:< lift(ltp))
+    || locally:
+      if strictEquality then
+        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/src/dotty/tools/dotc/typer/Typer.scala

@@ -5052,12 +5052,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
         Linter.warnOnImplausiblePattern(tree, pt)
 
-        if ! (tree.tpe <:< pt && (tree.symbol.flags.isAllOf(Flags.EnumValue) || (tree.symbol.flags.isAllOf(Flags.Module | Flags.Case)))) then
-          val cmp =
-            untpd.Apply(
-              untpd.Select(untpd.TypedSplice(tree), nme.EQ),
-              untpd.TypedSplice(dummyTreeOfType(pt)))
-          typedExpr(cmp, defn.BooleanType)
+        val cmp =
+          untpd.Apply(
+            untpd.Select(untpd.TypedSplice(tree), nme.EQ),
+            untpd.TypedSplice(dummyTreeOfType(pt)))
+        typedExpr(cmp, defn.BooleanType)
       case _ =>
 
   private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol, isUnitExpr: Boolean = false)(using Context): Unit =