= return pstate -- Common case: no equational constraints
refineAlt con pstate ex_tvs co_vars pat_ty
- = do { opt_gadt <- doptM Opt_GADTs -- No type-refinement unless GADTs are on
- ; if (not opt_gadt) then return pstate
- else do
-
- { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
+ = -- See Note [Flags and equational constraints]
+ do { checkTc (isRigidTy pat_ty) (nonRigidMatch con)
-- We are matching against a GADT constructor with non-trivial
-- constraints, but pattern type is wobbly. For now we fail.
-- We can make sense of this, however:
vcat [ ppr con <+> ppr ex_tvs,
ppr [(v, tyVarKind v) | v <- co_vars],
ppr reft]
- } } }
+ } }
\end{code}
+Note [Flags and equational constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If there are equational constraints, we take account of them
+regardless of flag settings; -XGADTs etc applies only to the
+*definition* of a data type.
+
+An alternative would be also to reject a program that *used*
+constructors with equational constraints. But want we should avoid at
+all costs is simply to *ignore* the constraints, since that gives
+incomprehensible errors (Trac #2004).
+
%************************************************************************
%* *
nonRigidMatch con
= hang (ptext SLIT("GADT pattern match in non-rigid context for") <+> quotes (ppr con))
- 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
+ 2 (ptext SLIT("Solution: add a type signature"))
nonRigidResult res_ty
= hang (ptext SLIT("GADT pattern match with non-rigid result type") <+> quotes (ppr res_ty))
- 2 (ptext SLIT("Tell GHC HQ if you'd like this to unify the context"))
+ 2 (ptext SLIT("Solution: add a type signature"))
inaccessibleAlt msg
= hang (ptext SLIT("Inaccessible case alternative:")) 2 msg