This PR fixes a bug in `std::mersenne_twister_engine::seed(_Sseq&)`
where the initialization twist is bypassed when the vectorized engine
optimization (`_LIBCPP_ABI_VECTORIZED_MERSENNE_TWISTER_ENGINE`) is
enabled.
In the vectorized implementation introduced in #206423, the engine state
must be pre-twisted during initialization by calling
`__update_all_states()` at the end of the `seed` functions.
However, in `seed(_Sseq& __q)`, the all-zero state check contains an
early `return;` when a non-zero state is verified. This early return
bypasses the trailing `__update_all_states()` call. Consequently, the
generator is initialized with raw, untwisted seed values (tempered)
rather than the standard twisted values.
This only manifests when compiled with unstable ABI settings
(`_LIBCPP_ABI_VERSION >= 2`) which enables the vectorized
implementation.
I replaced the early `return;` inside the all-zero state verification
block with a loop break, and conditionalized the zero-state fix using
the loop index `__i`, ensuring `__update_all_states()` is always
executed at the end of the function. Added regression tests to
`seed_sseq.pass.cpp` asserting the exact deterministic outputs for both
32-bit and 64-bit engines.
66d049cdad