Summary:
The code for mkAtDefault is as follows.

    mkATDefault (L loc (TyFamInstDecl { tfid_eqn = L _ e }))
          | TyFamEqn { tfe_tycon = tc, tfe_pats = pats, tfe_rhs = rhs } <- e
          = do { tvs <- checkTyVars (ptext (sLit "default")) equalsDots tc (hswb_cts pats)
               ; return (L loc (TyFamEqn { tfe_tycon = tc
                                         , tfe_pats = tvs
                                         , tfe_rhs = rhs })) }

An associated type in a class of the form

    type FoldableConstraint t x = ()

has an AnnEqual attached to the location in tfid_eqn. Since the location
is discarded, this annotation is then disconnected from the AST.

Test Plan: ./validate

Reviewers: hvr, austin

Reviewed By: austin

Subscribers: bgamari, thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D842

GHC Trac Issues: #10307