[commit: ghc] ghc-8.2: Allow associated types to pattern-match in non-class-bound variables (82366c4)

[git at git.haskell.org]

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

On branch  : ghc-8.2
Link       : http://ghc.haskell.org/trac/ghc/changeset/82366c4c1501d57dd8490818ef6e0709aca8836f/ghc

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

commit 82366c4c1501d57dd8490818ef6e0709aca8836f
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Tue Mar 14 10:58:58 2017 -0400

    Allow associated types to pattern-match in non-class-bound variables
    
    Summary:
    After 8136a5cbfcd24647f897a2fae9fcbda0b1624035 (#11450), if you have
    a class with an associated type:
    
    ```
    class C a where
      type T a b
    ```
    
    And you try to create an instance of `C` like this:
    
    ```
    instance C Int where
      type T Int Char = Bool
    ```
    
    Then it is rejected, since you're trying to instantiate the variable ``b`` with
    something other than a type variable. But this restriction proves quite
    onerous in practice, as it prevents you from doing things like this:
    
    ```
    class C a where
      type T a (b :: Identity c) :: c
    
    instance C Int where
      type T Int ('Identity x) = x
    ```
    
    You have to resort to an auxiliary type family in order to define this now,
    which becomes extremely tiring. This lifts this restriction and fixes #13398,
    in which it was discovered that adding this restriction broke code in the wild.
    
    Test Plan: ./validate
    
    Reviewers: simonpj, bgamari, austin
    
    Reviewed By: simonpj
    
    Subscribers: rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3302
    
    (cherry picked from commit 67345ccf51538acf2b6452c738ba641b119f5a5e)


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

82366c4c1501d57dd8490818ef6e0709aca8836f
 compiler/typecheck/TcValidity.hs                   | 60 +++++++++++---------
 docs/users_guide/8.2.1-notes.rst                   | 19 +++++++
 docs/users_guide/glasgow_exts.rst                  | 66 ++++++----------------
 testsuite/tests/ghci/scripts/T4175.hs              |  4 +-
 testsuite/tests/ghci/scripts/T4175.stdout          |  9 +--
 testsuite/tests/ghci/scripts/T6018ghcifail.stderr  | 13 ++---
 .../tests/indexed-types/should_compile/T13398a.hs  | 19 +++++++
 .../tests/indexed-types/should_compile/T13398b.hs  | 12 ++++
 testsuite/tests/indexed-types/should_compile/all.T |  2 +
 .../indexed-types/should_fail/SimpleFail10.hs      |  3 +-
 .../indexed-types/should_fail/SimpleFail10.stderr  | 11 ----
 testsuite/tests/indexed-types/should_fail/all.T    |  2 +-
 testsuite/tests/th/T9692.hs                        |  2 +-
 testsuite/tests/th/T9692.stderr                    |  4 +-
 .../tests/typecheck/should_fail/T6018fail.stderr   | 13 ++---
 15 files changed, 125 insertions(+), 114 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 82366c4c1501d57dd8490818ef6e0709aca8836f