Fix #5546: Disallow comparing opaque types under -language:strictEquality

Disallow comparing different opaque types under -language:strictEquality

Author: odersky

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

@@ -831,11 +831,13 @@ trait Implicits { self: Typer =>
       (other ne NoType) && !other.derivesFrom(defn.AnyValClass)
     }
 
+    // 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 {
       def apply(t: Type): Type = t match {
         case t: TypeRef =>
           t.info match {
-            case TypeBounds(lo, hi) if lo ne hi => apply(hi)
+            case TypeBounds(lo, hi) if lo.ne(hi) && !t.symbol.is(Opaque) => apply(hi)
             case _ => t
           }
         case t: RefinedType =>

tests/neg/i5546.scala

@@ -0,0 +1,16 @@
+import language.strictEquality
+
+object O {
+
+  opaque type Meters = Double
+  object Meters { def apply(d: Double): Meters = d }
+  implicit def eqM: Eq[Meters, Meters] = Eq
+
+  opaque type Feet = Double
+  object Feet { def apply(d: Double): Feet = d }
+  implicit def eqF: Eq[Feet, Feet] = Eq
+
+  def main(args: Array[String]): Unit =
+    println(Feet(3) == Meters(3)) // error: cannot compare
+
+}