Peng Liu b350bc2c0b
[libc++] Verify forward_list self-merging is a no-op (#129985)
https://wg21.link/LWG3088 requires that `forward_list::merge()` is a no-op when passed
`*this`, which aligns with the behavior of `list::merge`. Although libc++'s implementation
of `forward_list::merge()` already meets this requirement, there were no tests to verify
this behavior. This patch adds the necessary tests to ensure that self-merging remains a
no-op and prevents any future regressions on this.

Closes #104942.
2025-03-19 12:28:11 -04:00
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