Function add_parameter_name

src/pl/plpgsql/src/pl_comp.c:1029–1049

* Add a name for a function parameter to the function's namespace */

 * Add a name for a function parameter to the function's namespace
 */
static void
add_parameter_name(PLpgSQL_nsitem_type itemtype, int itemno, const char *name)
{
	/*
	 * Before adding the name, check for duplicates.  We need this even though
	 * functioncmds.c has a similar check, because that code explicitly
	 * doesn't complain about conflicting IN and OUT parameter names.  In
	 * plpgsql, such names are in the same namespace, so there is no way to
	 * disambiguate.
	 */
	if (plpgsql_ns_lookup(plpgsql_ns_top(), true,
						  name, NULL, NULL,
						  NULL) != NULL)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
				 errmsg("parameter name \"%s\" used more than once",
						name)));

	/* OK, add the name */
	plpgsql_ns_additem(itemtype, itemno, name);
}

/*
 * Add a dummy RETURN statement to the given function's body