Comment by layer8

The musttail return is not allowed to occur within a try block, or within the scope of variables with nontrivial destructors [0]. (In particular, this means that the function parameters cannot have nontrivial destructors.) Otherwise, within a musttail-called function, it's as if the original caller directly called that function, so exceptional stack unwinding is unaffected.

[0] https://www.mail-archive.com/gcc@gcc.gnu.org/msg95265.html