Fix #6987: don't generate assignment for dropped fields

Previously we only check whether an accessor is dropped.
We should check whether the corresponding field is dropped or not.

Author: liufengyun

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

@@ -212,7 +212,6 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
         }
         splitStats(stats1)
       case Nil =>
-        (Nil, Nil)
     }
     splitStats(tree.body)
 
@@ -222,6 +221,10 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
         dropped += acc
         Nil
       }
+      else if (!isRetained(acc.field)) { // It may happen for unit fields, tests/run/i6987.scala
+        dropped += acc.field
+        Nil
+      }
       else {
         if (acc.hasAnnotation(defn.TransientParamAnnot))
           ctx.error(em"transient parameter $acc is retained as field in class ${acc.owner}", acc.sourcePos)

tests/run/i6987b.scala

@@ -0,0 +1,18 @@
+enum SingleCase {
+  case TheCase1(u: Unit)
+}
+
+case class TheCase2(u: Unit)
+
+case class TheCase3(s: String, u: Unit)
+
+class TheCase4(val u: Unit)
+
+abstract class TheCase5(val u: Unit)
+
+@main def Test =
+  SingleCase.TheCase1(())
+  TheCase2(())
+  TheCase3("", ())
+  TheCase4(())
+  new TheCase5(()) {}