__LINE__ is not constexpr in MSVC

Question

ok, the latest VS 2019 Community, local "all defaults" C++ console project:

int main()
{
    // cl Version 19.21.27702.2 for x86
    // 
    constexpr auto MSCVER = _MSC_VER; // 1921
    constexpr auto MSCFULLVER = _MSC_FULL_VER; //192127702
    constexpr auto MSCBUILD = _MSC_BUILD; // 2
    /*
    : error C2131:  expression did not evaluate to a constant
    : message :  failure was caused by non-constant arguments or reference to a non-constant symbol
    : message :  see usage of '__LINE__Var'
    */
    constexpr auto LINE = __LINE__;
}

But. Seemingly the same compiler on Godbolt compiles this ok. As ever before.

https://godbolt.org/z/rn44rN

Any idea?

The Status as of 2019-07-22

Apparently, this is a bug which is a feature. Ugh. And there is a macro which is almost perfect, besides the fact it casts to int, and the type of the LINE is long. Here is my version:

#define _DBJ_CONCATENATE_(a, b) a ## b
#define _DBJ_CONCATENATE(a, b)  _DBJ_CONCATENATE_(a, b)

#define CONSTEXPR_LINE long(_DBJ_CONCATENATE(__LINE__,U)) 

Original is here. I had almost the same solution, but I was adding a zero instead of U. Perhaps because I spent hours trying to figure out what is going on.

I am sorry, but the reasoning of the MSVC team on that page is just strange. I am wondering is there a detail in the standard which resolves this issue.

Many thanks to commenters for pointing me in the right direction. But there is this remaining mistery: How come Godbolt + MSVC, has no problems with this same code?

Answer 1

MSVC 团队在该链接中指出，如果将“调试信息格式”从“/ZI”（用于编辑并继续的程序数据库）更改为“/Zi”（程序数据库），这也会修复该问题（但会禁用编辑并继续） ）。这对我有用。

如果您不使用“编辑并继续”，这似乎是正确的解决方案。

Answer 2

显然，这已经是MSVC 社区的一个已知问题：

我们的 C++ 团队在此问题上有一个已知错误。该开发者社区项目的状态将在检查该错误时更新。再次感谢您向我们报告此事。

显然，它被认为是编译器的“功能”：

该错误被认为是一个功能：“编辑并继续”拥有一小部分但直言不讳且热情的用户群体（主要是游戏开发者）

链接线程中的用户提供了以下解决方法：

#define CAT(X,Y) CAT2(X,Y)
#define CAT2(X,Y) X##Y

#define USABLE_LINE int(CAT(__LINE__,U)) 
//appending 'U' shouldn't change much as __LINE__ is supposedly non-negative anyway