[commit: ghc] master: Warn about simplifiable class constraints (9421b0c)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/9421b0c77122d40bf72665ea9f90dca64a0a0ae2/ghc

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

commit 9421b0c77122d40bf72665ea9f90dca64a0a0ae2
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Apr 21 13:06:54 2016 +0100

    Warn about simplifiable class constraints
    
    Provoked by Trac #11948, this patch adds a new warning to GHC
    
      -Wsimplifiable-class-constraints
    
    It warns if you write a class constraint in a type signature that
    can be simplified by an existing instance declaration.  Almost always
    this means you should simplify it right now; type inference is very
    fragile without it, as #11948 shows.
    
    I've put the warning as on-by-default, but I suppose that if there are
    howls of protest we can move it out (as happened for -Wredundant-constraints.
    
    It actually found an example of an over-complicated context in CmmNode.
    
    Quite a few tests use these weird contexts to trigger something else,
    so I had to suppress the warning in those.
    
    The 'haskeline' library has a few occurrences of the warning (which
    I think should be fixed), so I switched it off for that library in
    warnings.mk.
    
    The warning itself is done in TcValidity.check_class_pred.
    
    HOWEVER, when type inference fails we get a type error; and the error
    suppresses the (informative) warning.  So as things stand, the warning
    only happens when it doesn't cause a problem.  Not sure what to do
    about this, but this patch takes us forward, I think.


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

9421b0c77122d40bf72665ea9f90dca64a0a0ae2
 compiler/basicTypes/BasicTypes.hs                  |  8 ++-
 compiler/cmm/CmmNode.hs                            |  2 +-
 compiler/main/DynFlags.hs                          |  5 +-
 compiler/typecheck/TcInteract.hs                   | 15 +++++-
 compiler/typecheck/TcType.hs                       |  2 +-
 compiler/typecheck/TcValidity.hs                   | 63 +++++++++++++++++-----
 compiler/types/InstEnv.hs                          | 45 ++++++++--------
 docs/users_guide/using-warnings.rst                | 22 ++++++++
 mk/warnings.mk                                     |  1 +
 .../dependent/should_compile/dynamic-paper.hs      |  5 +-
 testsuite/tests/ghci/scripts/ghci047.script        |  4 +-
 testsuite/tests/ghci/scripts/ghci047.stderr        |  4 +-
 .../tests/indexed-types/should_compile/Gentle.hs   |  3 +-
 .../indexed-types/should_compile/NonLinearLHS.hs   |  4 +-
 .../tests/indexed-types/should_compile/T11067.hs   |  5 ++
 .../tests/indexed-types/should_compile/T5002.hs    |  6 ++-
 .../should_compile/SomethingShowable.stderr        |  6 ++-
 testsuite/tests/polykinds/T6020a.hs                |  4 +-
 .../tests/simplCore/should_compile/simpl014.hs     |  5 +-
 testsuite/tests/th/T3100.hs                        |  3 +-
 .../typecheck/should_compile/GivenOverlapping.hs   |  3 +-
 testsuite/tests/typecheck/should_compile/T10195.hs |  4 +-
 testsuite/tests/typecheck/should_compile/T3108.hs  |  3 +-
 testsuite/tests/typecheck/should_compile/T4361.hs  |  2 +-
 testsuite/tests/typecheck/should_compile/T6055.hs  |  4 +-
 testsuite/tests/typecheck/should_compile/T7541.hs  |  3 +-
 testsuite/tests/typecheck/should_compile/T7875.hs  |  3 +-
 testsuite/tests/typecheck/should_compile/tc229.hs  |  4 +-
 testsuite/tests/typecheck/should_fail/T11948.hs    | 22 ++++++++
 .../tests/typecheck/should_fail/T11948.stderr      | 10 ++++
 testsuite/tests/typecheck/should_fail/all.T        |  1 +
 31 files changed, 209 insertions(+), 62 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 9421b0c77122d40bf72665ea9f90dca64a0a0ae2