Fix excessive comparisons in minmax algorithms

[mirion-dev]

According to [alg.min.max#32], the algorithms should perform at most max(floor(3/2 (N−1)),0) comparisons, but the current implementations of std::{ranges::}minmax_element violate this requirement when N is even. This can be reproduced with the added test cases in this PR.

Although std::ranges::minmax does not violate it because its upper bound is (3/2) N [alg.min.max#23], I have optimized it as well.

[mirion-dev]

It's weird that the failed test P0811R3_midpoint_lerp works on my machine and contains no minmax.

[StephanTLavavej]

@zacklj89 had also encountered lerp(1, 2, 2) == 3; expected 3 on ARM64 internally. I think this test is now flaky and I'll need to separately add workarounds to it.

[Copilot]

Pull request overview

This PR updates the STL implementations of std::minmax_element / std::ranges::minmax_element to meet the C++ standard comparison-count upper bound for even-sized inputs, and adds targeted tests that reproduce/regress the issue. It also slightly optimizes std::ranges::minmax.

Changes:

• Adjust std::minmax_element and std::ranges::minmax_element initialization/loop structure to avoid the extra comparison when N is even.
• Optimize std::ranges::minmax initialization to reduce comparisons.
• Add new comparison-count regression tests for both legacy and ranges algorithms.

Show a summary per file

File	Description
stl/inc/algorithm	Fixes minmax-element comparison counts for even N; also optimizes ranges::minmax initialization.
tests/std/tests/P0896R4_ranges_alg_minmax/test.cpp	Adds constexpr + runtime comparison-count tests for ranges::minmax and ranges::minmax_element, including an input-range case.
tests/std/tests/Dev11_0532622_minmax_element/test.cpp	Adds legacy std::minmax_element comparison-count regression tests (guarded to avoid debug-predicate effects).

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• Files reviewed: 3/3 changed files
• Comments generated: 0

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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[mirion-dev]

Failed on P0811R3_midpoint_lerp again.

[StephanTLavavej]

Thanks (I will deal with the midpoint lerp failures, somehow).