[commit: ghc] master: Consider equality contexts exotic, uninferrable by "deriving" (02b4845)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/02b4845e07ef7110b2f735f323eb8748903330ff/ghc

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

commit 02b4845e07ef7110b2f735f323eb8748903330ff
Author: Richard Eisenberg <eir at cis.upenn.edu>
Date:   Tue Dec 16 17:15:49 2014 -0500

    Consider equality contexts exotic, uninferrable by "deriving"
    
    See comments in #8984. This takes back the fix for #6088.


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

02b4845e07ef7110b2f735f323eb8748903330ff
 compiler/typecheck/TcValidity.hs                                      | 4 +---
 testsuite/tests/indexed-types/should_compile/all.T                    | 1 -
 .../tests/indexed-types/{should_compile => should_fail}/T6088.hs      | 0
 testsuite/tests/indexed-types/should_fail/T6088.stderr                | 4 ++++
 testsuite/tests/indexed-types/should_fail/all.T                       | 2 ++
 5 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/compiler/typecheck/TcValidity.hs b/compiler/typecheck/TcValidity.hs
index 8575cf8..ca8b63a 100644
--- a/compiler/typecheck/TcValidity.hs
+++ b/compiler/typecheck/TcValidity.hs
@@ -878,10 +878,8 @@ validDerivPred :: TyVarSet -> PredType -> Bool
 validDerivPred tv_set pred
   = case classifyPredType pred of
        ClassPred _ tys       -> check_tys tys
-                  -- EqPred ReprEq is a Coercible constraint; treat
-                  -- like a class
-       EqPred ReprEq ty1 ty2 -> check_tys [ty1, ty2]
        TuplePred ps          -> all (validDerivPred tv_set) ps
+       EqPred {}             -> False  -- reject equality constraints
        _                     -> True   -- Non-class predicates are ok
   where
     check_tys tys = hasNoDups fvs
diff --git a/testsuite/tests/indexed-types/should_compile/all.T b/testsuite/tests/indexed-types/should_compile/all.T
index ae15c27..928a70d 100644
--- a/testsuite/tests/indexed-types/should_compile/all.T
+++ b/testsuite/tests/indexed-types/should_compile/all.T
@@ -194,7 +194,6 @@ test('T6152',
      run_command,
      ['$MAKE -s --no-print-directory T6152'])
 
-test('T6088', normal, compile, [''])
 test('T7082', normal, compile, [''])
 
 test('Overlap1', normal, compile, [''])
diff --git a/testsuite/tests/indexed-types/should_compile/T6088.hs b/testsuite/tests/indexed-types/should_fail/T6088.hs
similarity index 100%
rename from testsuite/tests/indexed-types/should_compile/T6088.hs
rename to testsuite/tests/indexed-types/should_fail/T6088.hs
diff --git a/testsuite/tests/indexed-types/should_fail/T6088.stderr b/testsuite/tests/indexed-types/should_fail/T6088.stderr
new file mode 100644
index 0000000..221dd32
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T6088.stderr
@@ -0,0 +1,4 @@
+
+T6088.hs:16:33:
+    Couldn't match type ‘Pos n’ with ‘True’
+    When deriving the instance for (C (B n))
diff --git a/testsuite/tests/indexed-types/should_fail/all.T b/testsuite/tests/indexed-types/should_fail/all.T
index 821342c..a52e621 100644
--- a/testsuite/tests/indexed-types/should_fail/all.T
+++ b/testsuite/tests/indexed-types/should_fail/all.T
@@ -132,3 +132,5 @@ test('T9580', normal, multimod_compile_fail, ['T9580', ''])
 test('T9662', normal, compile_fail, [''])
 test('T7862', normal, compile_fail, [''])
 test('T9896', normal, compile_fail, [''])
+test('T6088', normal, compile_fail, [''])
+