Evan Rutledge Borden authored Nov 21, 2017 and Ben Gamari committed Nov 21, 2017

Many industrial users have aligned around the idea that implicit exports
are an anti-pattern. They lead to namespace pollution and byzantine
naming schemes. They also prevent GHC's dead code analysis and create
more obstacles to optimization. This warning allows teams/projects to
warn on or enforce via -Werror explicit export lists.

This warning also serves as a complement to warn-missing-import-lists.

This was originally discussed here:
https://github.com/ghc-proposals/ghc-proposals/pull/93

Test Plan: Three new minimal tests have been added to the type checker.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4197