Issue an error for pattern synonyms defined in a local scope (#8757)

This also fixes the internal crash when using pattern synonyms
in GHCi (#8749)

compiler/rename/RnBinds.lhs

@@ -434,9 +434,16 @@ rnBindLHS name_maker _ bind@(FunBind { fun_id = name@(L nameLoc _) })
        ; return (bind { fun_id = L nameLoc newname }) } 
 
 rnBindLHS name_maker _ bind@(PatSynBind{ patsyn_id = rdrname@(L nameLoc _) })
-  = do { addLocM checkConName rdrname
+  = do { unless (isTopRecNameMaker name_maker) $
+           addErr localPatternSynonymErr
+       ; addLocM checkConName rdrname
        ; name <- applyNameMaker name_maker rdrname
        ; return (bind{ patsyn_id = L nameLoc name }) }
+  where
+    localPatternSynonymErr :: SDoc
+    localPatternSynonymErr
+      = hang (ptext (sLit "Illegal pattern synonym declaration"))
+           2 (ptext (sLit "Pattern synonym declarations are only valid in the top-level scope"))
 
 rnBindLHS _ _ b = pprPanic "rnBindHS" (ppr b)
 

compiler/rename/RnPat.lhs

@@ -23,6 +23,7 @@ module RnPat (-- main entry points
               NameMaker, applyNameMaker,     -- a utility for making names:
               localRecNameMaker, topRecNameMaker,  --   sometimes we want to make local names,
                                              --   sometimes we want to make top (qualified) names.
+              isTopRecNameMaker,
 
               rnHsRecFields1, HsRecFieldContext(..),
 
@@ -193,6 +194,10 @@ data NameMaker
 topRecNameMaker :: MiniFixityEnv -> NameMaker
 topRecNameMaker fix_env = LetMk TopLevel fix_env
 
+isTopRecNameMaker :: NameMaker -> Bool
+isTopRecNameMaker (LetMk TopLevel _) = True
+isTopRecNameMaker _ = False
+
 localRecNameMaker :: MiniFixityEnv -> NameMaker
 localRecNameMaker fix_env = LetMk NotTopLevel fix_env 
 

testsuite/tests/patsyn/should_fail/all.T

 
 test('mono', normal, compile_fail, [''])
 test('unidir', normal, compile_fail, [''])
+test('local', normal, compile_fail, [''])