Re-resolve ==, != after expanding opaque types

compiler/src/dotty/tools/dotc/transform/ElimOpaque.scala

@@ -1,4 +1,5 @@
-package dotty.tools.dotc
+package dotty.tools
+package dotc
 package transform
 
 import core._
@@ -12,13 +13,17 @@ import Denotations.{SingleDenotation, NonSymSingleDenotation}
 import SymDenotations.SymDenotation
 import DenotTransformers._
 import TypeUtils._
+import Names._
+import ast.Trees._
 
 object ElimOpaque {
   val name: String = "elimOpaque"
 }
 
 /** Rewrites opaque type aliases to normal alias types */
 class ElimOpaque extends MiniPhase with DenotTransformer {
+  thisPhase =>
+  import ast.tpd._
 
   override def phaseName: String = ElimOpaque.name
 
@@ -52,4 +57,19 @@ class ElimOpaque extends MiniPhase with DenotTransformer {
         ref
     }
   }
+
+  /** Resolve overloading of `==` and `!=` methods with the representation
+   *  types in order to avoid boxing.
+   */
+  override def transformApply(tree: Apply)(using Context): Tree =
+    val sym = tree.symbol
+    if sym == defn.Any_== || sym == defn.Any_!= then
+      tree match
+        case Apply(Select(receiver, name: TermName), args)
+        if atPhase(thisPhase)(receiver.tpe.widen.typeSymbol.isOpaqueAlias) =>
+          applyOverloaded(receiver, name, args, Nil, defn.BooleanType)
+        case _ =>
+          tree
+    else
+      tree
 }

docs/docs/reference/other-new-features/opaques-details.md

@@ -46,6 +46,21 @@ object o {
 def id(x: o.T): o.T = x
 ```
 
+### Translation of Equality
+
+Comparing two values of opaque type with `==` or `!=` normally uses universal equality,
+unless another overloaded `==` or `!=` operator is defined for the type. To avoid
+boxing, the operation is mapped after type checking to the (in-)equality operator
+defined on the underlying type. For instance,
+```scala
+  opaque type T = Int
+
+  ...
+  val x: T
+  val y: T
+  x == y    // uses Int equality for the comparison.
+```
+
 ### Toplevel Opaque Types
 
 An opaque type alias on the toplevel is transparent in all other toplevel definitions in the sourcefile where it appears, but is opaque in nested

tests/pos/opaque.scala

@@ -35,6 +35,8 @@ object usesites {
                    // as a contextual implicit this takes precedence over the
                    // implicit scope implicit LogarithmOps.
                    // TODO: Remove any2stringadd
+  assert(l == Logarithm(1.0))
+  assert(l != l2)
   val d = l3.toDouble
   val l5: Logarithm = (1.0).asInstanceOf[Logarithm]
 }