differences between <> and != when using signed values on the right hand side

When using a signed comparison != does not behave like <> if the right hand side comparator is signed and there is no space between the comparator and the sign

<>+1
<>-1
Both bahave as the inverse to
=+1
=-1
But
!=-1
!=+1
Throw errors.

Tested with 16.3,13.15 and 9.6.24

postgres=# select version();
version
---------------------------------------------------------------------------------------------------------------------------
PostgreSQL 16.3 (Debian 16.3-1.pgdg120+1) on aarch64-unknown-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit
(1 row)

postgres=# select 1<>-1.0;
?column?
----------
t
(1 row)

postgres=# select 1<>-1;
?column?
----------
t
(1 row)

postgres=# select 1!=-1.0;
ERROR: operator does not exist: integer !=- numeric
LINE 1: select 1!=-1.0;
^
HINT: No operator matches the given name and argument types. You might need to add explicit type casts.
postgres=# select 1!=-1;
ERROR: operator does not exist: integer !=- integer
LINE 1: select 1!=-1;
^
HINT: No operator matches the given name and argument types. You might need to add explicit type casts.

Adding a space does produce the expected answer

select 1!= -1.0;
?column?
----------
t
(1 row)