C++26: more constexpr in the standard library

Summary

Last week, we discussed language features that are becoming constexpr in C++26. Today, let’s turn our attention to the standard library features that will soon be usable at compile time. One topic is missing: exceptions. As they need both core language and library changes, I thought they deserved their own post.P2562R1: constexpr stable sortingThis paper proposes making std::stable_sort, std::stable_partition, std::inplace_merge, and their ranges counterparts usable in constant expressions. While many algorithms have become constexpr over the years, this family related to stable sorting had remained exceptions — until now.The recent introduction of constexpr containers gives extra motivation for this proposal. If you can construct a container at compile time, it’s only natural to want to sort it there, too. More importantly, a constexpr std::vector can now support efficient, stable sorting algorithms.A key question is whether the algorithm can meet its computational complexity requirements under the constraints of constant evaluation. Fortunately, std::is_constant_evaluated() provides an escape hatch for implementations. For deeper details, check out the proposal itself.P1383R2: More constexpr for <cmath> and <complex>While P0533 made many <cmath> and <cstdlib> functions constexpr-friendly in C++23, it only addressed functions with trivial behavior — those no more complex than the basic arithmetic operators.Floating-point computations can yield different results depending on compiler settings, optimization levels, and hardware platforms. For instance, calculating std::sin(1e100) may produce varying outcomes due to the intricacies of floating-point arithmetic at such scales. The paper discusses these challenges and suggests that some variability in results is acceptable, given the nature of floating-point computations.The proposal accepts the need for a balance between strict determinism and practical flexibility. It suggests that while some functions should produce ...