Prefer argument name over "$n" for the refname of a plpgsql argument.

If a function argument has a name, use that as the "refname" of the
PLpgSQL_datum representing the argument, instead of $n as before.
This allows better error messages in some cases.

Pavel Stehule, reviewed by Jeevan Chalke

Discussion: https://postgr.es/m/CAFj8pRB9GyU2U1Sb2ssgP26DZ_yq-FYDfpvUvGQ=k4R=yOPVjg@mail.gmail.com

src/pl/plpgsql/src/pl_comp.c

@@ -433,9 +433,14 @@ do_compile(FunctionCallInfo fcinfo,
 							 errmsg("PL/pgSQL functions cannot accept type %s",
 									format_type_be(argtypeid))));
 
-				/* Build variable and add to datum list */
-				argvariable = plpgsql_build_variable(buf, 0,
-													 argdtype, false);
+				/*
+				 * Build variable and add to datum list.  If there's a name
+				 * for the argument, use that as refname, else use $n name.
+				 */
+				argvariable = plpgsql_build_variable((argnames &&
+													  argnames[i][0] != '\0') ?
+													 argnames[i] : buf,
+													 0, argdtype, false);
 
 				if (argvariable->dtype == PLPGSQL_DTYPE_VAR)
 				{

src/test/regress/expected/plpgsql.out

@@ -6029,3 +6029,14 @@ SELECT * FROM list_partitioned_table() AS t;
  2
 (2 rows)
 
+--
+-- Check argument name is used instead of $n in error message
+--
+CREATE FUNCTION fx(x WSlot) RETURNS void AS $$
+BEGIN
+  GET DIAGNOSTICS x = ROW_COUNT;
+  RETURN;
+END; $$ LANGUAGE plpgsql;
+ERROR:  "x" is not a scalar variable
+LINE 3:   GET DIAGNOSTICS x = ROW_COUNT;
+                          ^

src/test/regress/sql/plpgsql.sql

@@ -4811,3 +4811,12 @@ BEGIN
 END; $$ LANGUAGE plpgsql;
 
 SELECT * FROM list_partitioned_table() AS t;
+
+--
+-- Check argument name is used instead of $n in error message
+--
+CREATE FUNCTION fx(x WSlot) RETURNS void AS $$
+BEGIN
+  GET DIAGNOSTICS x = ROW_COUNT;
+  RETURN;
+END; $$ LANGUAGE plpgsql;