select * from json_populate_record(null::x, '{"a":1,"b":2}')

-- p_some_num of type int
select * from json_populate_record( null:my_record_type, '{"a":'||p_some_num||'',"b":2}' )

ERROR: function json_populate_record(my_record_type, text) does not exist

select * from json_populate_record(
  null:my_record_type, ('{"a":'||p_some_num||'',"b":2}')::json )

CREATE TABLE x(a int,b int);

CREATE TYPE x(a int,b int);