boost.json won't compile with visual studio and `/Zc:implicitNoexcept-`

[tru]

Version of Boost

1.76.0

Steps necessary to reproduce the problem

include boost/json.hpp anywhere and compile your program with /Zc:implicitNoexcept-

All relevant compiler information

Visual Studio 2019

We need this flag for our project to avoid a bigger refactoring, but with boost.json included we get the following error:

C:\.conan\c5dbf7\1\include\boost/json/pilfer.hpp(191): error C2338:
C:\.conan\c5dbf7\1\include\boost/json/value.hpp(3522): note: see reference to function template instantiation 'boost::json::value &&boost::json::pilfer<boost::json::value&>(T) noexcept' being compiled
        with
        [
            T=boost::json::value &
        ]

I understand this is non-standard - so I am just looking for a workaround we can apply to our boost sources until we can get rid of the implicitNoexcept flag.

Thanks.

[grisumbras]

I don't think we should support a flag to enable non-standard behaviour. Even if we change stuff, so that it will work, there's no guarantee we'll introduce something else that doesn't work with this flag.

[tru]

Hi,

I understand that sentiment. On the other hand we have been building boost with this flag since version 1.23 or something similar and boost.json is the first module it breaks on. So maybe there is a simple fix here that can make it work?

[grisumbras]

Boost 1.23 is from 2001, there wasn't noexcept at the time and so there wasn't this flag in MSVS either.

At first, I was considering accepting a patch to accommodate this flag. But the issue is, we aren't testing with this flag and we certainly aren't planning to. So, each new class in the library will potentially break your build again.

As an alternative, if you don't use Boost.Json, you can disable it. In your logs above I see that you use Conan. Json can be disabled with boost:without_json=True in your profile.

[pdimov]

I think that only a few destructors need to be made noexcept for this to be fixed, and maybe just one, ~value.

[vinniefalco]

if we can get it working we need to adjust the test matrix or add a new entry with /Zc:implicitNoexcept-

[pdimov]

It's actually more than just ~value :-)

Specifically, ~value, ~array, ~object, ~string, ~key_value_pair, and ~storage_ptr. (I.e. only those classes on which pilfer is used.)

[vinniefalco]

Those can easily be noexcept since they don't throw

[pdimov]

They are already noexcept by default. The option disables the implicit noexcept they get.

[vinniefalco]

So what change must be made?

[pdimov]

This one: #636

[grisumbras]

@tru can you check if this PR does indeed solve your issue?

[tru]

Hi. Thanks for this. But when we updated to 1.77 recently we had to drop this flag and work on the root cause since several modules in boost failed with this flag. So it's not something we need anymore.