[libcxx-dev] How to check for a feature-test macro

[Richard Smith via libcxx-dev]

On Tue, 4 Dec 2018 at 19:34, Marshall Clow via libcxx-dev <
libcxx-dev at lists.llvm.org> wrote:

> In the tests, we have the following usage:
>
> #if TEST_STD_VER > 14
> # if !defined(__cpp_lib_filesystem)
> #  error "__cpp_lib_filesystem is not defined"
> # elif __cpp_lib_filesystem < 201703L
> #  error "__cpp_lib_filesystem has an invalid value"
> # endif
> #endif
>
> I submit that that's non-portable, because some standard libraries may not
> implement all the features (that's the point of the feature-test macro),
> and this test should not fail.
>

Maybe this is lack of imagination on my part, but isn't a test failure for
an unimplemented standard library feature exactly the desired behavior?


> In D55308, I am using the following form:
>
> #if TEST_STD_VER > 17
>   LIBCPP_ASSERT(IS_DEFINED(__cpp_lib_char8_t)); // libc++ implements this
> # if defined(__cpp_lib_char8_t)
> #  if __cpp_lib_char8_t < 201811L
> #   error "__cpp_lib_char8_t has an invalid value"
> #  endif
> # endif
> #endif
>
> Basically it (unconditionally) checks that if the macro is defined, then
> it has a sane value.
> Additionally, since we know that libc++ defines this, we check that.
>
> An alternate formulation - w/o the `IS_DEFINED` trickery.
>
> #if TEST_STD_VER > 17
> # if !defined(__cpp_lib_char8_t)
>   LIBCPP_STATIC_ASSERT(false, "__cpp_lib_char8_t is not defined");
> # else
> #  if __cpp_lib_char8_t < 201811L
> #   error "__cpp_lib_char8_t has an invalid value"
> #  endif
> # endif
> #endif
>
> Comments welcome.
>

A standard library implementation that partially supports a feature may
define the feature test macro to a value lower than the standard one. Your
revised approach would reject that. There are two things that I think are
reasonable:

1) Condition all your tests on the relevant feature test macros, and add

LIBCPP_STATIC_ASSERT(__cpp_lib_char8_t >= 201811L, "libc++ should implement
this");

to make sure that libc++ in particular behaves as expected, or

2) Write tests that test for conformance: the feature test macros should be
correct and the functionality should be available. That way a standard
library implementation that fails to implement a feature (and doesn't
advertise the feature via feature test macros) doesn't get an "all tests
pass" despite not implementing all of the standard library.

I'd lean towards option 2 myself, but maybe vendors of other stdlibs using
the libc++ testsuite might have a different opinion.

-- Marshall
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