为何std::isnan在C++20中仍非constexpr？是否存在技术原因？

Great question! Your hunch about it being tied to C header heritage isn't far off, but there are a few more technical and standardization-specific reasons behind this:

• C Standard Compatibility Constraints
  std::isnan originates from C's <math.h> (imported into C++ via <cmath>). For a long time, the C standard didn't mark floating-point classification functions like isnan as constexpr—this only changed in C23. C++ has a history of aligning closely with C's definitions for cross-compatibility, so it couldn't unilaterally make std::isnan constexpr before C had done the same. This "wait for C to lead" approach avoids creating inconsistencies between the two languages.

• Implementation Complexity Across Platforms
  Unlike the constexpr functions in <limits> (which mostly expose static type properties like maximum values), isnan requires inspecting the binary representation of a floating-point value (e.g., checking for all-1 exponents and non-zero mantissa in IEEE 754). Early on, compilers and hardware had limited support for constexpr operations that directly manipulate floating-point bits, especially across non-IEEE 754 compliant platforms. While modern toolchains can handle this easily now, the standardization lagged behind the implementation capability.

• Standard Committee Prioritization
  The C++ standards committee has a huge backlog of features and fixes to address. "Retrofitting" constexpr onto existing C-compatible functions like std::isnan wasn't a top priority compared to new language features (like concepts in C++20) or critical bug fixes. Additionally, numeric_limits in <limits> focuses on static type traits, not value-specific checks like isnan—so it's not an oversight that numeric_limits doesn't include this functionality; it's just outside the scope of what that class is designed for.

Good news though: with C23 now formalizing constexpr versions of these math functions, we can expect C++26 to follow suit and make std::isnan constexpr at last.

内容的提问来源于stack exchange，提问作者NoSenseEtAl