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

[Richard Smith via libcxx-dev]

On Thu, 6 Dec 2018 at 09:58, Marshall Clow <mclow.lists at gmail.com> wrote:

> On Wed, Dec 5, 2018 at 5:21 PM Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> 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?
>>
>
> No. The test here is for the correct definition of the macro in question.
> With the adoption of feature test macros, the committee has explicitly
> sanctioned the idea of incomplete implementations.
>

Maybe I'm misunderstanding what you mean, but I don't agree. An incomplete
implementation is non-conforming, just as it always was. No part of the
language or library is optional. All parts are required to be implemented,
and all feature test macros are required to be defined, by a conforming
implementation. The existence of feature test macros do not sanction the
absence of features in a conforming implementation, they merely provide a
mechanism by which a user of an implementation can query whether they're
using an incomplete (and therefore non-conforming) implementation, and if
so, which parts are available and which parts are still work in progress.

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.
>>
>
> It would; I haven't considered 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.
>> <http://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev>
>>
>
> I'd rather avoid that; long-term test failures are not in anyones
> best-interest.
> It leads to people ignoring the test results "oh, yeah we always have test
> failures"
>

Test failures on tests for features that you don't implement seem like
exactly the right behavior to me. Isn't that already the status quo for the
rest of libc++'s test suite? If someone doesn't implement, say,
std::unique_ptr, shouldn't all the unique_ptr tests fail for their
implementation?
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