Matthew Pickering authored May 10, 2016 and Ben Gamari committed May 10, 2016

In the long discussion on #11432, it was decided that when a type
constructor is parsed as a variable ((--.->) is one example) then in
order to export the type constructor then the user should be required to
use the ExplicitNamespaces keyword.

This was implemented in quite an indirect manner in the renamer. It
is much more direct to enforce this in the parser at the expense of
slighty worse error messages. Further to this, the check in the
renamer was actually slightly wrong. If the variable was in scope
then no error was raised, this was causing panics, see #12026 for an
example.

Reviewers: austin, bgamari

Subscribers: davean, skvadrik, thomie

Differential Revision: https://phabricator.haskell.org/D2181

GHC Trac Issues: #12026