This bug, discovered by Trac #15325, has been lurking since

  commit 1c9fd3f1
  Author: Simon Peyton Jones <simonpj@microsoft.com>
  Date:   Thu Dec 3 12:57:54 2015 +0000

    Case-of-empty-alts is trivial (Trac #11155)

I'd forgotttnen to modify getIdFromTrivialExpr when I
modified exprIsTrivial.   Easy to fix, though.