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4601. Unclear error handling for C++ math functions

Section: 29.7.4 [c.math.lerp] Status: Ready Submitter: Jan Schultke Opened: 2026-07-01 Last modified: 2026-07-17

Priority: 3

View all other issues in [c.math.lerp].

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Discussion:

Consider the following example, which (mathematically) attempts to compute ∞+0(∞−∞) . 

#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.

[2026-07-10; LWG telecon.]

Set priority to 3 after LWG telecon. LWG would like blanket wording that says that lerp and 3-arg hypot (and the extended floating-point type overloads in cmath) are also covered by the blanket wording on when C math.h functions raise errors and are constant expressions.

[2026-07-17 LWG telecon; Change status New → Ready.]

Proposed resolution:

This wording is relative to N5046.

  1. Modify 29.7.1 [cmath.syn] as indicated:

    -1- The contents and meaning of the header <cmath> are a subset of the C standard library header <math.h> and only the declarations shown in the synopsis above are present, with the addition of a three-dimensional hypotenuse function (29.7.3 [c.math.hypot3]), a linear interpolation function (29.7.4 [c.math.lerp]), and the mathematical special functions described in 29.7.6 [sf.cmath]. [Note 1: Several functions have additional overloads in this document, but they have the same behavior as in the C standard library. — end note]

    -2- The treatment of error conditions specified in ISO/IEC 9899:2024 7.12.1 and the effect of those errors on constant expressions specified in 16.2 [library.c] apply to all functions described in 29.7 [c.math] except those in 29.7.6 [sf.cmath].

    [Note ?: This applies even to functions and overloads not defined in the C standard library. — end note]