FIX Trac #1935: generate superclass constraints for derived classes

This bug only reports a problem with phantom types, but actually
there was quite a long-standing and significant omission in the
constraint generation for derived classes.  See
Note [Superclasses of derived instance] in TcDeriv.

The test deriving-1935 tests both cases.

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 859b988..e79318b 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -530,21 +530,24 @@ mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
                                dataConInstOrigArgTys data_con rep_tc_args,
                    not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
 
+                       -- See Note [Superclasses of derived instance]
+             sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
+                                         (classSCTheta cls)
+             inst_tys =  [mkTyConApp tycon tc_args]
+
              stupid_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args
              stupid_constraints = substTheta stupid_subst (tyConStupidTheta rep_tc)
-             all_constraints = stupid_constraints ++ ordinary_constraints
-                        -- see Note [Data decl contexts] above
+             all_constraints = stupid_constraints ++ sc_constraints ++ ordinary_constraints
 
              spec = DS { ds_loc = loc, ds_orig = orig
                        , ds_name = dfun_name, ds_tvs = tvs 
-                       , ds_cls = cls, ds_tys = [mkTyConApp tycon tc_args]
+                       , ds_cls = cls, ds_tys = inst_tys
                        , ds_theta =  mtheta `orElse` all_constraints
                        , ds_newtype = False }
 
        ; return (if isJust mtheta then Just (Right spec)       -- Specified context
                                   else Just (Left spec)) }     -- Infer context
 
-
 mk_typeable_eqn orig tvs cls tycon tc_args rep_tc _rep_tc_args mtheta
        -- The Typeable class is special in several ways
        --        data T a b = ... deriving( Typeable )
@@ -695,6 +698,30 @@ new_dfun_name clas tycon   -- Just a simple wrapper
        -- a suitable string; hence the empty type arg list
 \end{code}
 
+Note [Superclasses of derived instance] 
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In general, a derived instance decl needs the superclasses of the derived
+class too.  So if we have
+       data T a = ...deriving( Ord )
+then the initial context for Ord (T a) should include Eq (T a).  Often this is 
+redundant; we'll also generate an Ord constraint for each constructor argument,
+and that will probably generate enough constraints to make the Eq (T a) constraint 
+be satisfied too.  But not always; consider:
+
+ data S a = S
+ instance Eq (S a)
+ instance Ord (S a)
+
+ data T a = MkT (S a) deriving( Ord )
+ instance Num a => Eq (T a)
+
+The derived instance for (Ord (T a)) must have a (Num a) constraint!
+Similarly consider:
+       data T a = MkT deriving( Data, Typeable )
+Here there *is* no argument field, but we must nevertheless generate
+a context for the Data instances:
+       instance Typable a => Data (T a) where ...
+
 
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