format (and friends) should reject volatile arguments

[CaseyCarter]

... for now, at least. This corrects a defect in our implementation of [format.args]/4's requirement that:

typename Context::template formatter_type<remove_cvref_t<T>>()
  .format(declval<T&>(), declval<Context&>())

be a valid expression. We were incorrectly stripping cv-qualifiers from T.

[CaseyCarter]

I'm open to suggestions for test coverage. I did verify that this program:

#include <format>
#include <iostream>

int main() {
    volatile int v = 42;
    std::cout << std::format("{}\n", v);
}

was accepted before and rejected after the change, but it's hard to do "correctly fails to compile" tests internally.

[fsb4000]

Should your PR also close the issue #2427 ?

Also the issue says that

volatile double v = 42;
std::cout << std::format("{}\n", v);

works.

Should it be rejected too?

[miscco]

I guess we have to wait for libc++ for compile.fail tests

[StephanTLavavej]

Looks good to me. After reading through the surrounding code, I saw that:

STL/stl/inc/format

Lines 172 to 175 in 7b2c54b

	template <class _Ty, class _Context>
	concept _Has_formatter = requires(_Ty& _Val, _Context& _Ctx) {
	_STD declval<typename _Context::template formatter_type<remove_cvref_t<_Ty>>>().format(_Val, _Ctx);
	};

still implements the remove_cvref_t<T> mentioned in https://eel.is/c++draft/format.arg#4 .

[AlexGuteniev]

See also #2246

[CaseyCarter]

Should your PR also close the issue #2427 ?

Also the issue says that

volatile double v = 42;
std::cout << std::format("{}\n", v);

works.

Should it be rejected too?

I believe that behavior is consistent with the wording in the working draft; we should accept volatile float/double/long double/const char_type*/nullptr_t/cv-void* which are copied into non-volatile storage before formatting, otherwise only volatile program-defined class types whose formatter accepts volatile objects. I don't necessarily agree that the wording is sensible, but that is what it now says.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for handling this highly volatile issue! 🧪 🥼 😹