Switching to alpaka::Complex broke TWTS field background

[sbastrakov]

After switching to alpaka::Complex<> in #4114 TWTS fast field background got runtime failures with 'cudaErrorLaunchFailure': 'unspecified launch failure'!. From previous experience that normally means that somehow a host version of a host-device function gets called from the device side.

Probably TWTS has the same issue. When switching, I tested the transition radiation, so in principle complex numbers do work on device side in PIConGPU. I am now suspecting that something wrong is happening with selecting math functions like exp as they are probably only used here for complex numbers. Not sure why exactly that happens here, but there are not that many suspects.

[sbastrakov]

So far it appears that the problem is with some but not all cupla-provided math functions of complex numbers. E.g. using exp() works fine, but once sqrt() is called for a complex number this error occurs. I guess somehow it does not find the correct implementation (not clear why) and so uses a generic alpaka one which in turn tries to fall back on std:: and that fails on the device side.

[sbastrakov]

Discussed offline with @psychocoderHPC , it seems to be a bug in cupla. We think we found a fix, I will try implementing it.

[sbastrakov]

I made a fix in cupla alpaka-group/cupla#235 and checked that it solves this issue. Will keep it open till we get the fixed cupla to dev (we planned to update it anyhow, just will need to add the fix first).

[sbastrakov]

Oh, there is also an unrelated issue in alpaka alpaka-group/alpaka#1731 which also stops TWTS from running with alpaka::Complex. Fix is on its way.

[sbastrakov]

Fixed with #4166 (and fixes to alpaka and cupla that it brings to PIConGPU)