This is my suggested regression test for D113868, plus some matching improvements to my existing "robust_against_adl" test.
Also, mark these tests as compile-only. They actually are safe to run — notice that the code "runs" at constexpr-time in C++20, without error — because both of the input ranges are entirely filled with nullptr, so no matter how you shuffle the elements, they remain sorted and partitioned and heapified and everything. But there's no real reason to run them at runtime, so let's just avoid the distraction.
Test cases that fail in trunk right now are commented out with TODO FIXME. D113868 currently fixes some-but-not-all of them.
Since these are new tests maybe clang-format them to LLVM style?