BUG #15434: NUMERIC without any precision or scale truncates scale to16 digits

The following bug has been logged on the website:

Bug reference:      15434
Logged by:          Andrey L
Email address:      parihaaraka@gmail.com
PostgreSQL version: 10.5
Operating system:   Ubuntu 16.04
Description:

# select 
 1800000000000000001::numeric / (10^18)::numeric truncated, -- truncates to
16 digits after decimal point
 1800000000000000001::numeric(48,18) / (10^18)::numeric correct,
 1800000000000000001::numeric / (10^18)::numeric * (10^18)::numeric
to_be_sure;
     truncated      |       correct        |              to_be_sure
     
--------------------+----------------------+--------------------------------------
 1.8000000000000000 | 1.800000000000000001 |
1800000000000000000.0000000000000000
(1 row)