Re: CAST from numeric(18,3) to numeric doesnt work, posgresql 13.3

=?UTF-8?B?SsOhbiBQZWNzxZFr?= <jan(dot)pecsok(at)gmail(dot)com> writes:
> Where conversion from numeric (18,3) to numeric didnt work as expected.

Hmm, interesting. The cause of this is that coerce_type_typmod
supposes that casting to a -1 typmod is a total no-op. Which
is true so far as the run-time behavior is concerned, but not
if something inspects the exposed type of the expression.

The attached seems to be enough to fix it, though I wonder if
we've made the same assumption anywhere else.

Although this changes no existing regression test results,
I'm still a bit hesitant to back-patch it, because I am not
real sure that nothing out there is depending on the current
behavior (which has stood for decades). I think it'd be all
right to put into HEAD, and maybe it's not too late for v14.

regards, tom lane