Fix #14114 by checking for duplicate vars on pattern synonym RHSes

Summary:
Because we weren't checking for duplicate variables on the right-hand
sides of pattern synonyms, bogus definitions like this one passed the renamer:

```lang=haskell
pattern Foo a <- (a,a)
```

Luckily, the fix is simple.

Test Plan: make test TEST=T14114

Reviewers: mpickering, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #14114

Differential Revision: https://phabricator.haskell.org/D3866

1 files changed, 8 insertions, 7 deletions

diff --git a/compiler/rename/RnPat.hs b/compiler/rename/RnPat.hs
index 320e4f3d12..9b439a7b19 100644
--- a/compiler/rename/RnPat.hs
+++ b/compiler/rename/RnPat.hs
@@ -47,8 +47,8 @@ import RnEnv
 import RnFixity
 import RnUtils             ( HsDocContext(..), newLocalBndrRn, bindLocalNames
                            , warnUnusedMatches, newLocalBndrRn
-                           , checkDupAndShadowedNames, checkTupSize
-                           , unknownSubordinateErr )
+                           , checkDupNames, checkDupAndShadowedNames
+                           , checkTupSize , unknownSubordinateErr )
 import RnTypes
 import PrelNames
 import TyCon               ( tyConName )
@@ -67,7 +67,7 @@ import TysWiredIn          ( nilDataCon )
 import DataCon
 import qualified GHC.LanguageExtensions as LangExt
 
-import Control.Monad       ( when, liftM, ap, unless )
+import Control.Monad       ( when, liftM, ap )
 import qualified Data.List.NonEmpty as NE
 import Data.Ratio
 
@@ -321,10 +321,11 @@ rnPats ctxt pats thing_inside
           --    complain *twice* about duplicates e.g. f (x,x) = ...
           --
           -- See note [Don't report shadowing for pattern synonyms]
-        ; unless (isPatSynCtxt ctxt)
-              (addErrCtxt doc_pat $
-                checkDupAndShadowedNames envs_before $
-                collectPatsBinders pats')
+        ; let bndrs = collectPatsBinders pats'
+        ; addErrCtxt doc_pat $
+          if isPatSynCtxt ctxt
+             then checkDupNames bndrs
+             else checkDupAndShadowedNames envs_before bndrs
         ; thing_inside pats' } }
   where
     doc_pat = text "In" <+> pprMatchContext ctxt