Fix Trac #3012: allow more free-wheeling in standalone deriving

[ghc-hetmet.git] / compiler / typecheck / TcRnTypes.lhs

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index 19432fa..fd7e954 100644 (file)
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -333,7 +333,7 @@ data TcLclEnv               -- Changes as we move inside an expression
                        -- Discarded after typecheck/rename; not passed on to desugarer
   = TcLclEnv {
        tcl_loc  :: SrcSpan,            -- Source span
-       tcl_ctxt :: ErrCtxt,            -- Error context
+       tcl_ctxt :: [ErrCtxt],          -- Error context, innermost on top
        tcl_errs :: TcRef Messages,     -- Place to accumulate errors
 
        tcl_th_ctxt    :: ThStage,            -- Template Haskell context
@@ -516,10 +516,13 @@ instance Outputable RefinementVisibility where
 \end{code}
 
 \begin{code}
-type ErrCtxt = [TidyEnv -> TcM (TidyEnv, Message)]     
-                       -- Innermost first.  Monadic so that we have a chance
-                       -- to deal with bound type variables just before error
-                       -- message construction
+type ErrCtxt = (Bool, TidyEnv -> TcM (TidyEnv, Message))
+       -- Monadic so that we have a chance
+       -- to deal with bound type variables just before error
+       -- message construction
+
+       -- Bool:  True <=> this is a landmark context; do not
+       --                 discard it when trimming for display
 \end{code}
 
 
@@ -876,7 +879,7 @@ functions that deal with it.
 
 \begin{code}
 -------------------------------------------
-data InstLoc = InstLoc InstOrigin SrcSpan ErrCtxt
+data InstLoc = InstLoc InstOrigin SrcSpan [ErrCtxt]
 
 instLoc :: Inst -> InstLoc
 instLoc inst = tci_loc inst