[FIXED] Why is there no warning from -Wfloat-equal when comparing containers of doubles?

Issue

If I use the compiler option -Wfloat-equal with GCC or Clang, equality comparisons of float/double values cause a warning. However, when comparing containers (like std::vector or std::tuple) of float or double values, no such warning is raised.

Example code (also at https://godbolt.org/z/YP8v8hTs3):

#include <tuple>
#include <vector>
#include <assert.h>

int main() {
    double d = 1.2;

    std::tuple<double, double> t_d{1.2, 3.14};
    std::tuple<double, double> t_d_2{1.2, 3.14};

    std::vector<double> v_d{1.2, 3.14};
    std::vector<double> v_d_2{1.2, 3.14};

    // this causes a warning, like "warning: comparing floating-point with '==' or '!=' is unsafe [-Wfloat-equal]":
    assert(d == 1.2);
    // but why no warning from -Wfloat-equal here?
    assert(t_d == t_d_2);
    // no warning here either:
    assert(v_d == v_d_2);

    // all of these cause warnings as expected:
    assert(std::get<0>(t_d) == 1.2);
    assert(std::get<0>(t_d) == std::get<0>(t_d_2));
    assert(v_d[0] == 1.2);
    assert(v_d[0] == v_d_2[0]);

    return 0;
}

Why are the warnings omitted for these container comparisons? And more importantly, what can I do to actually get these warnings as well?

Solution

GCC doesn't report warnings for system headers by default. The desired behavior may be obtained by adding -Wsystem-header compiler flag.

Quotation from the documentation:

-Wsystem-headers

Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code...

Live demo: https://godbolt.org/z/s6rExszj6

Clang seemingly adopted the same approach, see https://clang.llvm.org/docs/UsersManual.html#controlling-diagnostics-in-system-headers and https://clang.llvm.org/docs/UsersManual.html#options-to-control-error-and-warning-messages.

Live demo: https://godbolt.org/z/n9xY8rcM8

Answered By - Daniel Langr

Answer Checked By - Katrina (PHPFixing Volunteer)