Do I see correctly that the standard says
nothing about the precision of mathematical
functions provided by <cmath>?

Why not?

Could this change in the future?

What is the point of adding ever more functions
to <cmath> (like Bessel functions in c++17) if
this comes without any guarantee of accuracy?
As a user, I was better off depending on
a dedicated third-party library than relying on
a poor implementation in the C++ standard lib.

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Dr. Joachim Wuttke
group leader Scientific Computing
Forschungszentrum Jülich GmbH
Jülich Centre for Neutron Science at MLZ
+49 89 158860 715
https://computing.mlz-garching.de
https://jugit.fz-juelich.de/mlz