Fix Trac #3943: incorrect unused-variable warning

[ghc-hetmet.git] / compiler / rename / RnPat.lhs

diff --git a/compiler/rename/RnPat.lhs b/compiler/rename/RnPat.lhs
index 813f39b..58c2c34 100644 (file)
--- a/compiler/rename/RnPat.lhs
+++ b/compiler/rename/RnPat.lhs
@@ -167,10 +167,12 @@ newName (LetMk mb_top fix_env) rdr_name
         do { name <- case mb_top of
                        Nothing  -> newLocalBndrRn rdr_name
                        Just mod -> newTopSrcBinder mod rdr_name
-          ; bindLocalNamesFV_WithFixities [name] fix_env $
+          ; bindLocalName name $       -- Do *not* use bindLocalNameFV here
+                                       -- See Note [View pattern usage]
+             addLocalFixities fix_env [name] $
             thing_inside name })
                          
-    -- Note: the bindLocalNamesFV_WithFixities is somewhat suspicious 
+    -- Note: the bindLocalName is somewhat suspicious
     --       because it binds a top-level name as a local name.
     --       however, this binding seems to work, and it only exists for
     --       the duration of the patterns and the continuation;
@@ -178,6 +180,14 @@ newName (LetMk mb_top fix_env) rdr_name
     --       before going on to the RHSes (see RnSource.lhs).
 \end{code}
 
+Note [View pattern usage]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+  let (r, (r -> x)) = x in ...
+Here the pattern binds 'r', and then uses it *only* in the view pattern.
+We want to "see" this use, and in let-bindings we collect all uses and
+report unused variables at the binding level. So we must use bindLocalName
+here, *not* bindLocalNameFV.  Trac #3943.
 
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