From d1eaeadb08c1412c1572124efaf341bdc0976ccb Mon Sep 17 00:00:00 2001
From: Simon Peyton Jones <simonpj@microsoft.com>
Date: Tue, 24 Oct 2017 11:12:43 +0100
Subject: Temporary fix to Trac #14380

This fix replaces an utterly bogus error message with a decent one,
rejecting a pattern synonym with a list pattern and rebindable syntax.

Not hard to fix properly, but I'm going to wait for a willing volunteer
and/or more user pressure.
---
 compiler/typecheck/TcPatSyn.hs | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

(limited to 'compiler')

diff --git a/compiler/typecheck/TcPatSyn.hs b/compiler/typecheck/TcPatSyn.hs
index d234fd5ca1..58d1506812 100644
--- a/compiler/typecheck/TcPatSyn.hs
+++ b/compiler/typecheck/TcPatSyn.hs
@@ -672,8 +672,10 @@ tcPatToExpr name args pat = go pat
     go1 (ParPat pat)                = fmap HsPar $ go pat
     go1 (PArrPat pats ptt)          = do { exprs <- mapM go pats
                                          ; return $ ExplicitPArr ptt exprs }
-    go1 (ListPat pats ptt reb)      = do { exprs <- mapM go pats
-                                         ; return $ ExplicitList ptt (fmap snd reb) exprs }
+    go1 p@(ListPat pats ptt reb)
+      | Nothing <- reb              = do { exprs <- mapM go pats
+                                         ; return $ ExplicitList ptt Nothing exprs }
+      | otherwise                   = notInvertibleListPat p
     go1 (TuplePat pats box _)       = do { exprs <- mapM go pats
                                          ; return $ ExplicitTuple
                                               (map (noLoc . Present) exprs) box }
@@ -702,8 +704,10 @@ tcPatToExpr name args pat = go pat
     go1 p@(SplicePat (HsUntypedSplice {})) = notInvertible p
     go1 p@(SplicePat (HsQuasiQuote {}))    = notInvertible p
 
-    notInvertible p = Left $
-          text "Pattern" <+> quotes (ppr p) <+> text "is not invertible"
+    notInvertible p = Left (not_invertible_msg p)
+
+    not_invertible_msg p
+      =   text "Pattern" <+> quotes (ppr p) <+> text "is not invertible"
       $+$ hang (text "Suggestion: instead use an explicitly bidirectional"
                 <+> text "pattern synonym, e.g.")
              2 (hang (text "pattern" <+> pp_name <+> pp_args <+> larrow
@@ -713,6 +717,15 @@ tcPatToExpr name args pat = go pat
         pp_name = ppr name
         pp_args = hsep (map ppr args)
 
+    -- We should really be able to invert list patterns, even when
+    -- rebindable syntax is on, but doing so involves a bit of
+    -- refactoring; see Trac #14380.  Until then we reject with a
+    -- helpful error message.
+    notInvertibleListPat p
+      = Left (vcat [ not_invertible_msg p
+                   , text "Reason: rebindable syntax is on."
+                   , text "This is fixable: add use-case to Trac #14380" ])
+
 {- Note [Builder for a bidirectional pattern synonym]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 For a bidirectional pattern synonym we need to produce an /expression/
-- 
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