[commit: ghc] master: Make error when deriving an instance for a typeclass less misleading (d5a4e49)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/d5a4e49d657682eeb6e86ae464d281974ce2f5e2/ghc

>---------------------------------------------------------------

commit d5a4e49d657682eeb6e86ae464d281974ce2f5e2
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Fri Oct 14 10:40:56 2016 -0400

    Make error when deriving an instance for a typeclass less misleading
    
    Before, when you attempted to derive an instance for a typeclass,
    e.g.,
    
    ```
    class C1 (a :: Constraint) where
    class C2 where
    
    deriving instance C1 C2
    ```
    
    GHC would complain that `C2`'s data constructors aren't in scope. But
    that
    makes no sense, since typeclasses don't have constructors! By refining
    the
    checks that GHC performs when deriving, we can make the error message a
    little more sensible.
    
    This also cleans up a related `DeriveAnyClass` infelicity. Before, you
    wouldn't have been able to compile code like this:
    
    ```
    import System.IO (Handle)
    class C a
    deriving instance C Handle
    ```
    
    Since GHC was requiring that all data constructors of `Handle` be in
    scope. But `DeriveAnyClass` doesn't even generate code that mentions
    any data constructors, so this requirement is silly!
    
    Fixes #11509.
    
    Test Plan: make test TEST=T11509
    
    Reviewers: simonpj, austin, bgamari
    
    Reviewed By: simonpj, bgamari
    
    Subscribers: thomie, simonpj
    
    Differential Revision: https://phabricator.haskell.org/D2558
    
    GHC Trac Issues: #11509


>---------------------------------------------------------------

d5a4e49d657682eeb6e86ae464d281974ce2f5e2
 compiler/typecheck/TcDeriv.hs                      | 134 +++++++++++++++------
 compiler/typecheck/TcDerivUtils.hs                 |  41 +++++--
 docs/users_guide/8.2.1-notes.rst                   |  14 +++
 docs/users_guide/glasgow_exts.rst                  |  11 ++
 .../tests/deriving/should_compile/T11509_2.hs      |  13 ++
 .../tests/deriving/should_compile/T11509_3.hs      |   9 ++
 testsuite/tests/deriving/should_compile/all.T      |   2 +
 testsuite/tests/deriving/should_fail/T11509_1.hs   |  52 ++++++++
 .../tests/deriving/should_fail/T11509_1.stderr     |   7 ++
 testsuite/tests/deriving/should_fail/all.T         |   2 +
 10 files changed, 242 insertions(+), 43 deletions(-)

Diff suppressed because of size. To see it, use:

    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc d5a4e49d657682eeb6e86ae464d281974ce2f5e2