[GHC] #11824: GHC error in desugarer lookup

[Simon Peyton Jones]

| The problem is this: we must generate and typecheck record selectors on
| a per-group basis, since they may be used by later groups.

Why?  Record selectors are value bindings which aren't used when typechecking data types.

| Future plans aside,, would you be okay with pushing D2108 to the ghc-8.0
| branch as-is in the interest of moving the release along?

I guess so.  But at least prominently comment the bit I missed.

Simon