Fix some duplication in the parser

D3673 experienced reduce/reduce conflicts when trying to use
opt_instance for associated data families.

That was probably because the author tried to use it for
Haskell98-syntax without also applying it to GADT-syntax, which actually
leads to a reduce/reduce conflict. Consider the following state:

```
data . T = T
data . T where T :: T
```

The parser must decide at this point whether or not to reduce an empty
`opt_instance`. But doing so would also commit to either
Haskell98 or GADT syntax! Good thing we also accept an optional
"instance" for GADT syntax, so the `opt_instance` is there in both
productions and there's no reduce/reduce conflict anymore.

Also no need to inline `opt_instance`, how it used to be.

1 files changed, 4 insertions, 20 deletions

diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y
index e61ff31844..276fcb1c5b 100644
--- a/compiler/parser/Parser.y
+++ b/compiler/parser/Parser.y
@@ -1320,36 +1320,20 @@ at_decl_inst :: { LInstDecl GhcPs }
                         (mj AnnType $1:$2++(fst $ unLoc $3)) }
 
         -- data/newtype instance declaration, with optional 'instance' keyword
-        -- (can't use opt_instance because you get reduce/reduce errors)
-        | data_or_newtype capi_ctype tycl_hdr_inst constrs maybe_derivings
-               {% amms (mkDataFamInst (comb4 $1 $3 $4 $5) (snd $ unLoc $1) $2 (snd $ unLoc $3)
-                                    Nothing (reverse (snd $ unLoc $4))
-                                            (fmap reverse $5))
-                       ((fst $ unLoc $1):(fst $ unLoc $3) ++ (fst $ unLoc $4)) }
-
-        | data_or_newtype 'instance' capi_ctype tycl_hdr_inst constrs maybe_derivings
+        | data_or_newtype opt_instance capi_ctype tycl_hdr_inst constrs maybe_derivings
                {% amms (mkDataFamInst (comb4 $1 $4 $5 $6) (snd $ unLoc $1) $3 (snd $ unLoc $4)
                                     Nothing (reverse (snd $ unLoc $5))
                                             (fmap reverse $6))
-                       ((fst $ unLoc $1):mj AnnInstance $2:(fst $ unLoc $4)++(fst $ unLoc $5)) }
+                       ((fst $ unLoc $1):$2++(fst $ unLoc $4)++(fst $ unLoc $5)) }
 
         -- GADT instance declaration, with optional 'instance' keyword
-        -- (can't use opt_instance because you get reduce/reduce errors)
-        | data_or_newtype capi_ctype tycl_hdr_inst opt_kind_sig
-                 gadt_constrlist
-                 maybe_derivings
-                {% amms (mkDataFamInst (comb4 $1 $3 $5 $6) (snd $ unLoc $1) $2
-                                (snd $ unLoc $3) (snd $ unLoc $4) (snd $ unLoc $5)
-                                (fmap reverse $6))
-                        ((fst $ unLoc $1):(fst $ unLoc $3)++(fst $ unLoc $4)++(fst $ unLoc $5)) }
-
-        | data_or_newtype 'instance' capi_ctype tycl_hdr_inst opt_kind_sig
+        | data_or_newtype opt_instance capi_ctype tycl_hdr_inst opt_kind_sig
                  gadt_constrlist
                  maybe_derivings
                 {% amms (mkDataFamInst (comb4 $1 $4 $6 $7) (snd $ unLoc $1) $3
                                 (snd $ unLoc $4) (snd $ unLoc $5) (snd $ unLoc $6)
                                 (fmap reverse $7))
-                        ((fst $ unLoc $1):mj AnnInstance $2:(fst $ unLoc $4)++(fst $ unLoc $5)++(fst $ unLoc $6)) }
+                        ((fst $ unLoc $1):$2++(fst $ unLoc $4)++(fst $ unLoc $5)++(fst $ unLoc $6)) }
 
 data_or_newtype :: { Located (AddAnn, NewOrData) }
         : 'data'        { sL1 $1 (mj AnnData    $1,DataType) }