From 1c9ffbe64003cef8a686ff1e29aae160e314775e Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Wed, 2 Apr 2008 13:20:57 +0000
Subject: [PATCH] Fix Trac #2141: invalid record update

See Note [Record field lookup] in TcEnv.  The fix here
is quite straightforward.
---
 compiler/typecheck/TcEnv.lhs |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index aa27a43..1f9d305 100644
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -134,10 +134,22 @@ tcLookupGlobal name
 
 tcLookupField :: Name -> TcM Id		-- Returns the selector Id
 tcLookupField name = do
-    thing <- tcLookupGlobal name
+    thing <- tcLookup name	-- Note [Record field lookup]
     case thing of
-	AnId id -> return id
-	other	-> wrongThingErr "field name" (AGlobal thing) name
+	AGlobal (AnId id) -> return id
+	thing -> wrongThingErr "field name" thing name
+
+{- Note [Record field lookup]
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~
+You might think we should have tcLookupGlobal here, since record fields
+are always top level.  But consider
+	f = e { f = True }
+Then the renamer (which does not keep track of what is a record selector
+and what is not) will rename the definition thus
+	f_7 = e { f_7 = True }
+Now the type checker will find f_7 in the *local* type environment, not
+the global one. It's wrong, of course, but we want to report a tidy
+error, not in TcEnv.notFound.  -}
 
 tcLookupDataCon :: Name -> TcM DataCon
 tcLookupDataCon name = do
-- 
1.7.10.4