refactor: clarify Scalar type

It now contains the type of the scalar. This way we can discriminate the
void type in a more obvious way.

src/PostgREST/SchemaCache.hs

@@ -247,7 +247,6 @@ decodeFuncs =
                   <*> column HD.text
                   <*> column HD.bool
                   <*> column HD.bool
-                  <*> column HD.bool
                   <*> column HD.bool)
               <*> (parseVolatility <$> column HD.char)
               <*> column HD.bool
@@ -255,16 +254,15 @@ decodeFuncs =
     addKey :: Routine -> (QualifiedIdentifier, Routine)
     addKey pd = (QualifiedIdentifier (pdSchema pd) (pdName pd), pd)
 
-    parseRetType :: Text -> Text -> Bool -> Bool -> Bool -> Bool -> RetType
-    parseRetType schema name isSetOf isComposite isVoid isCompositeAlias
-      | isVoid    = Single $ Scalar True
+    parseRetType :: Text -> Text -> Bool -> Bool -> Bool -> RetType
+    parseRetType schema name isSetOf isComposite isCompositeAlias
       | isSetOf   = SetOf pgType
       | otherwise = Single pgType
       where
         qi = QualifiedIdentifier schema name
         pgType
           | isComposite = Composite qi isCompositeAlias
-          | otherwise   = Scalar False
+          | otherwise   = Scalar qi
 
     parseVolatility :: Char -> FuncVolatility
     parseVolatility v | v == 'i' = Immutable
@@ -339,7 +337,6 @@ funcsSqlQuery pgVer = [q|
      -- if any TABLE, INOUT or OUT arguments present, treat as composite
      or COALESCE(proargmodes::text[] && '{t,b,o}', false)
     ) AS rettype_is_composite,
-    ('void'::regtype = t.oid) AS rettype_is_void,
     bt.oid <> bt.base as rettype_is_composite_alias,
     p.provolatile,
     p.provariadic > 0 as hasvariadic