I dug more into how #14158 started working. I temporarily reverted the
patch that "fixed" it, namely

    commit a6c448b4
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Mon Aug 28 17:33:59 2017 +0100

    Small refactor of getRuntimeRep

Sure enough, there was a real bug, described in the new
TcExpr Note [Visible type application zonk]

In general, syntactic substituion should be kind-preserving!
Maybe we should check that invariant...