fix kernel launch failure when sender expressions can throw

[ericniebler]

Description

the following code will fail with an "invalid device function" cuda error:

namespace ex = cuda::experimental::execution;
auto ctx     = ex::stream_context{cuda::experimental::device_ref(0)};
auto gpu     = ctx.get_scheduler();

auto sndr    = ex::on(gpu, ex::just()) | ex::then([]{ return 42; });
auto [value] = ex::sync_wait(std::move(sndr));

however, the following nearly identical code will work:

namespace ex = cuda::experimental::execution;
auto ctx     = ex::stream_context{cuda::experimental::device_ref(0)};
auto gpu     = ctx.get_scheduler();

auto sndr    = ex::on(gpu, ex::just()) | ex::then([]() noexcept { return 42; });
auto [value] = ex::sync_wait(std::move(sndr));

the difference is that the lambda passed to ex::then is noexcept. when it is not noexcept, the sender has a ex::set_error_t(std::exception_ptr) completion signature -- but only in host code.

this results in host code and device code computing different intermediate types, and instantiating kernels with those different types. since the mangled names of the kernels differ, the kernel launch fails.

Fix

the problem stems from the fact that cudax::execution uses some nothrow type traits that are unconditionally true in device code (since device code never throws). the fix is to stop defining the nothrow traits differently for device code.

but that is an incomplete fix, because with that change, device code now needs to be aware of std::exception_ptr. the standard exception_ptr APIs are host-only and so cannot be called from device code.

the fix is to polyfill the exception_ptr APIs for device code, and use the std:: APIs on host.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[ericniebler]

/ok to test d8905b0

[ericniebler]

/ok to test b69591a

[ericniebler]

/ok to test 9f65d45

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