Templated Variables Bug With Lambdas in Visual Studio?

Question

c++14 provides variable templates Which work fine in visual-studio-2017, but within lambdas they seem to fall apart. For example:

template <typename T>
const auto PI = std::acos(static_cast<T>(-1));

int main() {
  auto func = []() { cout << PI<float> << endl; };

  func();
}

On gcc 6.3 this outputs:

3.14159

On Visual Studio 2017 this outputs:

0.0

Isn't this an obvious compiler bug? I mean, imagine if this was intended behavior *shudder* — Rakete1111, Mar 05 '18 at 18:40
Interesting. Confirmed with MSVC 19.00.23918 (Visual Studio 2017 desktop). — Robinson, Mar 05 '18 at 18:46
It is the `auto` in template that is failing, if changed to `T` it works as expected — Killzone Kid, Mar 05 '18 at 18:48
@KillzoneKid Wow, that would make an excellent workaround answer. — Jonathan Mee, Mar 05 '18 at 18:51
@JonathanMee I don't know why it fails though, so maybe someone can take over — Killzone Kid, Mar 05 '18 at 18:52
@Justin thank you I did in fact mean to paste that part of the code into the question. — Jonathan Mee, Mar 05 '18 at 18:56
@JonathanMee Actually I seem to remember there was a question about `static_cast` failing under similar circumstance. So maybe nothing to do with `auto`, it just cannot deduce the correct type. — Killzone Kid, Mar 05 '18 at 18:59
Looks like it may be an optimizer issue? Does `-Od` work for you? — AndyG, Mar 05 '18 at 18:59
@AndyG Yes indeed great comment. Putting it in the capture of the lambda also works. — Jonathan Mee, Mar 05 '18 at 19:02

Killzone Kid · Accepted Answer · 2018-03-05T21:18:17.713

3

Wierd bug, but it seems to have a reliable workaround, which works for both, this case and the related case. In both cases forcefully activating template seems to do the job in VS2017:

template <typename T>
const auto PI = std::acos(static_cast<T>(-1));

int main() 
{
  PI<float>; // <------ this
  auto func = []() { std::cout << PI<float> << std::endl; };

  func();
}

GCC 6.3 for example: https://ideone.com/9UdwBT

edited Mar 05 '18 at 21:18

answered Mar 05 '18 at 21:02

Killzone Kid

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And those people tried to pursue the Committee to incorporate their compiler's features into the Standard. – bipll Mar 05 '18 at 21:14
1

@bipll Low hopes, they cannot even fix the [code submission format](https://developercommunity.visualstudio.com/content/problem/207741/template-needs-to-be-force-instantiated-vs2017-bug.html) on their bug tacker – Killzone Kid Mar 05 '18 at 21:39
Can you please post this in the linked question as well. It is the correct solution as it identifies the actual problem, namely that the `PI` template doesn't seem to initialize automatically. There's an answer over there, which gets lucky and solves the problem but doesn't address the underlying issue and I'm loath to accept it. – Jonathan Mee Mar 06 '18 at 13:11
@JonathanMee [Sure thing!](https://stackoverflow.com/a/49132236/8746007) – Killzone Kid Mar 06 '18 at 13:41

Templated Variables Bug With Lambdas in Visual Studio?

1 Answers1

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