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4601. Unclear error handling for C++ math functions

Section: 29.7.4 [c.math.lerp] Status: New Submitter: Jan Schultke Opened: 2026-07-01 Last modified: 2026-07-04

Priority: Not Prioritized

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Discussion:

Consider the following example, which (mathematically) attempts to compute $\mathrm{\infty }+0(\mathrm{\infty }-\mathrm{\infty })$.

#include <cmath>
#include <limits>

constexpr float inf = std::numeric_limits<float>::infinity();
constexpr float x = std::lerp(inf, inf, 0.f);

MSVC accepts this, but GCC and Clang reject it due to floating-point arithmetic that produces a NaN, within the implementation of std::lerp.

16.2 [library.c] states that "call to a C standard library function is a non-constant library call if it raises a floating-point exception other than FE_INEXACT", but this does not apply to std::lerp because it is not a C standard library function. Since this means there exists no specification of when std::lerp raises which floating-point exceptions, all calls to std::lerp are constant expressions (?), which is inconsistent with the other functions in <cmath>.

Similarly, the three-dimensional hypotenuse function is missing a specification of when any errors would occur, which is addressed only partially by LWG 3172⁽ⁱ⁾ by saying that a "range error may occur" without any specification of when exactly that would happen or when a domain error occurs.

A possible direction is to ensure that C23 §7.21.1 Treatment of error conditions applies to three-dimension std::hypot and std::lerp as well, despite those not being C standard library functions.

Proposed resolution: