Simon Peyton Jones authored Feb 18, 2019 and Marge Bot committed Feb 20, 2019

There was a missing case in the very simple optimiser,
CoreOpt.simpleOptExpr, which led to Trac #13208 comment:2.

In particular, in simple_app, if we find a Let, we should
just float it outwards. Otherwise we leave behind some
easy-to-reduce beta-redexes.