Fix empty do-block error call

data/15-regressions.blt

@@ -1072,3 +1072,12 @@ data ReformatParenMode
   = ReformatParenModeKeep  -- don't modify parens at all
   | ReformatParenModeClean -- remove unnecessary parens
   | ReformatParenModeAll   -- add superfluous parens everywhere
+
+#test empty do block error
+func = process $ do
+  it "some long description to fill this line" $ do
+  ( aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+    + bbbbbbbbbbbbbbbbbb
+    + cccccccccccccccccccccccccccccccccccccccccccccccccc
+    )
+    `shouldReturn` thing

source/library/Language/Haskell/Brittany/Internal/ToBriDoc/Expr.hs

@@ -613,6 +613,17 @@ layoutExpr lexpr@(L _ expr) = do
           , expDoc1
           ]
       -- docSeq [appSep $ docLit "let in", expDoc1]
+    HsDo epAnn stmtCtx (L (SrcSpanAnn stmtEpAnn _) []) ->
+      case stmtCtx of
+        DoExpr _ ->
+          docHandleComms epAnn $ docHandleComms stmtEpAnn $ docLitS "do"
+        MDoExpr _ ->
+          docHandleComms epAnn $ docHandleComms stmtEpAnn $ docLitS "mdo"
+        ListComp ->
+          error "brittany internal error: ListCompo with null statements"
+        MonadComp ->
+          error "brittany internal error: ListCompo with null statements"
+        _ -> unknownNodeError "HsDo{} unknown stmtCtx" lexpr
     HsDo epAnn stmtCtx (L (SrcSpanAnn stmtEpAnn _) stmts) ->
       docHandleComms epAnn $ do
         case stmtCtx of