In this commit
    commit 02ac2974
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Wed Nov 16 10:37:47 2011 +0000

    Fix CaseIdentity optimisaion

    In fixing one bug I'd introduced another;
       case x of { T -> T; F -> F }
    wasn't getting optmised!  Trivial to fix.

I introduced yet another!  This line of code in SimplUtils.mkCase1

    check_eq (Var v)    (DataAlt con) []   = v == dataConWorkId con
                                             -- Optimisation only

is patently false when arg_tys is non-empty.  Astonishing that it
has not shown up before now.

Easily fixed though.  This was all shown up by Trac #13417, which is
now fixed.

Merge to 8.0, 8.2.