Moin Peter,
On 2020-02-16 14:18, Peter John Acklam via RT wrote:

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> Sun Feb 16 08:18:23 2020: Request 131827 was acted upon.
> Transaction: Ticket created by pjacklam
> Queue: Math-BigInt
> Subject: Mixing accuracy and precision creates NaN
> Broken in: 1.999818
> Severity: (no value)
> Owner: Nobody
> Requestors: pjacklam@gmail.com
> Status: new
> Ticket <URL:

https://rt.cpan.org/Ticket/Display.html?id=131827 >
>
>
> Mixing operand where one has accuracy set and the other has precision
> set, returns NaN:
>
> use strict;
> use warnings;
> use Math::BigFloat;
>
> my $x = Math::BigFloat -> new(6);
> $x -> accuracy(5);
>
> my $y = Math::BigFloat -> new(4);
> $y -> precision(-3);
>
> my $zadd = $x + $y; # = NaN
> my $zmul = $x * $y; # = NaN
> my $zsub = $x - $y; # = NaN
> my $zdiv = $x / $y; # = NaN
> my $zpow = $x ** $y; # = NaN
>
> The actual code that creates the NaN is in Math::BigInt::round(),
> where a comment says "set A and set P is an fatal error".
>
> It should be possible to get a meaningful result, even when both
> accuracy and precision is set. I consider the current behaviour to be
> a bug.
>
> If I remember correctly, IEEE 754-2008 states that the rounding
> parameters are determined by the first operand.

When I wrote that code, mixing A and P did not make sense to me, so the
design
decision was taken. It looks it never made it into the documentation,
tho.
I'm not opposed to a change here, though. There are of course multiple
ways:
* follow the first operand (as you propose)
* follow the operand with the higher (or lower) accuracy?
* always use the operand which has A (or P) set?
For instance, both "3 + 3.5" and "3.5 + 3 " create "6.5" under bignum,
not 6, as a
"follow first operand" would indicate. So care must be taken to create
meaningful
operations and results.
Having "$a + $b" be the same as "$b + $a" can be useful. So, what should
the
following example:
$ perl -MMath::BigFloat -wle '$y = Math::BigFloat->new(14);
$y->precision(-3); print $y," ",$y+1.5," ",$y+Math::BigFloat->new(1,5),"
",Math::BigFloat->new(1,5)+$y'
4.000 5.500 NaN NaN
print as the last two strings?
Best regards,
Tels