Optimise away erased phantom statements

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/localopt/Simplify.scala

@@ -60,6 +60,7 @@ class Simplify extends MiniPhaseTransform with IdentityDenotTransformer {
     new Devalify                    ::
     new Jumpjump                    ::
     new DropGoodCasts               ::
+    new DropNoEffects(this)         ::
     new ConstantFold(this)          ::
     Nil
 
@@ -168,13 +169,16 @@ object Simplify {
   }
 
   def isImmutableAccessor(t: Tree)(implicit ctx: Context): Boolean = {
-    val isImmutableGetter = t.symbol.isGetter && !t.symbol.is(Mutable | Lazy)
-    val isCaseAccessor    = t.symbol.is(CaseAccessor) && !t.symbol.is(Mutable | Lazy)
-    val isProductAccessor = t.symbol.exists                               &&
-                            t.symbol.owner.derivesFrom(defn.ProductClass) &&
-                            t.symbol.owner.is(CaseClass)                  &&
-                            t.symbol.name.isSelectorName                  &&
-                            !t.symbol.info.decls.exists(_.is(Mutable | Lazy)) // Conservatively covers case class A(var x: Int)
-    isImmutableGetter || isCaseAccessor || isProductAccessor
+    val sym = t.symbol
+    val isImmutableGetter = sym.isGetter && !sym.is(Mutable | Lazy)
+    val isCaseAccessor    = sym.is(CaseAccessor) && !sym.is(Mutable | Lazy)
+    val isProductAccessor = sym.exists                               &&
+                            sym.owner.derivesFrom(defn.ProductClass) &&
+                            sym.owner.is(CaseClass)                  &&
+                            sym.name.isSelectorName                  &&
+                            !sym.info.decls.exists(_.is(Mutable | Lazy)) // Conservatively covers case class A(var x: Int)
+    val isBoxedUnit       = (sym eq defn.BoxedUnit_UNIT) ||
+                            (sym eq defn.ErasedPhantom_UNIT)
+    isImmutableGetter || isCaseAccessor || isProductAccessor || isBoxedUnit
   }
 }