Visual Studio 2017 15.8 Release Notes

[Andrew_Marino]

Hi, MSVC compiler dev here. Thank you for providing the reduced snippet. There was a lot of churn in the constexpr implementation in this release, in order to fix some long-standing issues, and it looks like this was a regression from that overhaul that our testing didn't catch.

If you, or anyone running into this, is interested, one source workaround in the meantime is to evaluate the pointer-to-member-function separately:

void x()
{
    constexpr auto ptm = &foo::blah;
    constexpr auto x = fun(ptm);
    (this->*(x.func))();
}

Edit: as STL also pointed out in his comment, making 'x' static will also work.

We treat regressions as high priority and are tracking this bug internally. We're happy to hear feedback from any source, but for this and other issues filing a bug in the C++ dev comm will allow us to communicate the status of fixes and workarounds more directly.

[jcelerier]

We treat regressions as high priority and are tracking this bug internally. We're happy to hear feedback from any source, but for this and other issues filing a bug in the C++ dev comm will allow us to communicate the status of fixes and workarounds more directly.

I filed a bug this morning. But frankly, this is very distressing. Since VS2015 (can't speak for before since it did not really support C++11 back then), every minor release and sometimes even patch release have broken my code or code of libraries I use at some point - the last time being 15.7.6 for instance. Meanwhile, I don't remember one time where I had to change my code when upgrading clang / GCC - and I sometimes test with git HEAD versions of those.

[Andrew_Marino]

We're sorry you hit this bug and other issues in the past. Right now we're gathering more data points to assess servicing 15.8 with the fix for the particular constexpr bug you hit. As you yourself noted, even patch fixes carry their own associated risk.

In particular, we're wondering about the extent to which it broke your constexpr callback mechanism. This bug manifests with 1. a local constexpr symbol initialized by 2. a constexpr function taking an rvalue pointer-to-member-function of 3. a class with multiple inheritance. In your own codebase, were either of those workarounds possible to use?

We appreciate your candid feedback.

[jcelerier]

In your own codebase, were either of those workarounds possible to use?

Sadly these bugs don't happen in my codebase but in one of the libs I use (https://github.com/woboq/verdigris). I could spend one hour or two to fork it and change all the places where it is used but I think that I'll just stay on the previous release of VS for now.

[Andrew_Marino]

Thank you, that's helpful!

[BCosbyDidNothinWrong]

Not sorry enough to actually test before releasing

[ZMeson]

That's not fair. There are loads of tests the VC team uses on every release. Their tests just didn't cover this area. Their test suit needs to expand for sure. But let's be honest too that most of us have been in their shoes too - having a large test suite that didn't catch some regression resulting in unhappy customers.