X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcPat.lhs;h=defe6fb6aa85607eea1bb08b1a0e5a81ebf412ce;hb=b25d67d4c0398dbceca0165c3b43df0625e1e06a;hp=0d62b6bf81b59d432382fb5ece549dc04e7cf367;hpb=d31018bce01934b8399a15dbcae35be0827b45d1;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index 0d62b6b..defe6fb 100644
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -867,11 +867,8 @@ refineAlt con pstate ex_tvs [] pat_ty
   = 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:
@@ -899,9 +896,20 @@ refineAlt con pstate ex_tvs co_vars pat_ty
 				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).
+
 
 %************************************************************************
 %*									*
@@ -1107,11 +1115,11 @@ lazyPatErr pat tvs
 
 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