Fix compilation under -Ydebug

Review by @xeno-by

Before this patch:

```
% scalac -Ydebug test/files/pos/t3999b.scala
..
warning: !!! ... hit cycle trying to complete class FT
test/files/pos/t3999b.scala:5: error: illegal cyclic reference involving class FT
  case class FT(f : Float) extends Score
             ^
locals (2 total):
  constructor FT
  value f
locals (6 total):
  class FT
  module FT
  value <local FT>
  value f
  constructor FT
  value f
```