[libcxx] [libcxxabi] [llvm] [libc++] Upgrade to GCC 16 (PR #206235)

[Nikolas Klauser via llvm-commits]

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@@ -12,6 +12,9 @@
 
 // T& unique_ptr::operator[](size_t) const
 
+// FIXME: Why does this start failing with GCC 16?
+// XFAIL: gcc-16 && std-at-least-c++20
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philnik777 wrote:

#215497

https://github.com/llvm/llvm-project/pull/206235