For the record, along the way I notice a) that metacpan does not know your module depends on ExtUtils::Depends and Test2::Suite, and b) in the module you appear to invoke Scalar::Util::weaken without loading Scalar::Util.

After confusing myself a few times, I've come to the conclusion that you've hit an unfortunate case - the EXTEND macro evaluates its arguments several times. Along the way it invokes EXTEND_HWM_SET, which looks like this for a debugging build:

#  define EXTEND_HWM_SET(p, n)                      \
        STMT_START {                                \
            SSize_t ix = (p) - PL_stack_base + (n); \
            if (ix > PL_curstackinfo->si_stack_hwm) \
                PL_curstackinfo->si_stack_hwm = ix; \
        } STMT_END

[download]

So given that n here is the string ix==3 ? 2*n : n, it is quite reasonable things are going to go wrong. If you use some variable other than ix, or (for example) duplicate the EXTEND call inside the if/else blocks to avoid the conditional, then the error goes away.

Update: #19818

HTH, Hugo

In reply to Re: XS debugging "failed to extend arg stack" by hv
in thread [Solved] XS debugging "failed to extend arg stack" by NERDVANA

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