"stack install hakyll" on nightly - "stack invariant violated"

[mrkgnao]

General summary/comments (optional)

Attempting to install Hakyll with --resolver nightly gives a stack invariant violated error.

Steps to reproduce

<*> stack install hakyll --resolver nightly
Selected resolver: nightly-2017-07-31

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for hakyll-4.9.8.0:
    pandoc must match >=1.14 && <1.20, but the stack configuration has no specified version
           (latest applicable is 1.19.2.1)
    pandoc-citeproc must match >=0.10.5 && <0.11, but the stack configuration has no specified version
                    (latest applicable is 0.10.5.1)
    process-1.6.1.0 must match >=1.0 && <1.6 (latest applicable is 1.5.0.0)
    time-1.8.0.2 must match >=1.4 && <1.8 (latest applicable is 1.7.0.1)
needed for unknown reason - stack invariant violated.

Recommended action: try adding the following to your extra-deps in /home/mrkgnao/.stack/global-project/stack.yaml:
- pandoc-1.19.2.1
- pandoc-citeproc-0.10.5.1

Expected

I don't think needed for unknown reason is normal behavior -- I've never seen it before.

Stack version

$ stack --version
Version 1.5.0 x86_64

Method of installation

Arch Linux's stack package.

[decentral1se]

I don't think needed for unknown reason is normal behavior -- I've never seen it before.

This seems to be specified behaviour (although I can't quite grok what the source is doing) but it certainly seems like this error message does not explain clearly what is happening here. Maybe we can improve it once someone can clear up what is happening.