Create: Better debug output

For tracking down #505

1 files changed, 4 insertions, 1 deletions

diff --git a/haddock-api/src/Haddock/Interface/Create.hs b/haddock-api/src/Haddock/Interface/Create.hs
index a351c0dc..8d561d68 100644
--- a/haddock-api/src/Haddock/Interface/Create.hs
+++ b/haddock-api/src/Haddock/Interface/Create.hs
@@ -789,7 +789,10 @@ extractDecl name mdl decl
           [s0] -> let (n, tyvar_names) = (tcdName d, tyClDeclTyVars d)
                       L pos sig = addClassContext n tyvar_names s0
                   in L pos (SigD sig)
-          _ -> error "internal: extractDecl (ClassDecl)"
+          _ -> O.pprPanic "extractDecl" (O.text "Ambiguous decl for" O.<+> O.ppr name O.<+> O.text "in class:"
+                                         O.$$ O.nest 4 (O.ppr d)
+                                         O.$$ O.text "Matches:"
+                                         O.$$ O.nest 4 (O.ppr matches))
       TyClD d@DataDecl {} ->
         let (n, tyvar_tys) = (tcdName d, lHsQTyVarsToTypes (tyClDeclTyVars d))
         in SigD <$> extractRecSel name mdl n tyvar_tys (dd_cons (tcdDataDefn d))