I've created a bug sample project on github: https://github.com/csabahruska/ghc-bug-sample
Follow the readme to reproduce the bug.

Cheers,
Csaba

On Tue, Mar 10, 2020 at 12:31 PM Csaba Hruska <csaba.hruska@gmail.com> wrote:
Hello,

GHC HEAD (9668781a36941e7552fcec38f6d4e1d5ec3ef6d1) compiled with the Quickest flavour segfaults when compiles optparse-applicative-0.15.1.0.
But it compiles fine with the Quick or the default flavours. 
image.png
Everything compiles fine (with vanilla Quickest GHC HEAD) until the Options.Applicative.Help.Core module where GHC segfaults. (as I understand the error code)

Is this a known bug?
If not then I'll submit an issue.

Thanks,
Csaba