Why does this code using __LINE__ compile under MSVC in Release mode, but not in Debug mode?

Question

Consider this program:

#include <iostream>

template<bool Debug = false, int Line = __LINE__>
constexpr int adds(const int& a, const int& b) { 
    if (Debug)
        std::cout << __FUNCTION__ << " called on line " << Line << '\n';
    return (a + b);
}

int main() {
    std::cout << adds(3, 7) << '\n';
    std::cout << adds<true, __LINE__> (5, 9) << '\n';
    return 0;
}

When I try to compile and build this in Debug mode Visual Studio 2017 is generating these compiler errors:

1>------ Build started: Project: Simulator, Configuration: Debug x64 ------
1>main2.cpp
1>c:\***\main2.cpp(12): error C2672: 'adds': no matching overloaded function found
1>c:\***\main2.cpp(12): error C2975: 'Line': invalid template argument for 'adds', expected compile-time constant expression
1>c:\***\main2.cpp(3): note: see declaration of 'Line'
1>Done building project "Simulator.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

However, when I try this under Release mode: It compiles, builds, runs, and produces the appropriate output:

10
adds called on line 12
14

Is this a potential Visual Studio 2017 bug? If not, why does it work in one mode and not the other?

You can see it compiled here: Compiler Explorer

---

Here's a copy of the command line flags for both debug and release modes:

Debug

/JMC /permissive- /GS /W3 /Zc:wchar_t /Qspectre /ZI /Gm- /Od /sdl /Fd"x64\Debug\vc141.pdb" /Zc:inline /fp:precise /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /RTC1 /Gd /MDd /std:c++latest /FC /Fa"x64\Debug\" /EHsc /nologo /Fo"x64\Debug\" /Fp"x64\Debug\Simulator.pch" /diagnostics:classic 

Release

/permissive- /GS /GL /W3 /Gy /Zc:wchar_t /Qspectre /Zi /Gm- /O2 /sdl /Fd"x64\Release\vc141.pdb" /Zc:inline /fp:precise /D "NDEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /Gd /Oi /MD /std:c++latest /FC /Fa"x64\Release\" /EHsc /nologo /Fo"x64\Release\" /Fp"x64\Release\Simulator.pch" /diagnostics:classic 

Answer 1

Seems like it was reported: __LINE__ cannot be used as an argument for constexpr functions.

We have a known bug for this issue on the C++ team here.
[...]
We have determined that this issue is not a bug. Please refer to comments of Jonathan.

And Jonathan says:

This is a side-effect of the compilers support for Edit-and-Continue (basically we don't want a change to the value of __LINE__ to be considered a 'rude' edit that suppresses Edit-and-Continue): if you compiler with /Zi instead of /ZI then the code should compile (but the executable won't support Edit-and-Continue).
[...]
The bug is considered a feature ...

From MSVC docs:

The /ZI option is similar to /Zi, but it produces a PDB file in a format that supports the Edit and Continue feature. [...] The /ZI option is also incompatible with use of the __LINE__ predefined macro; code compiled with /ZI can't use __LINE__ as a non-type template argument, although __LINE__ can be used in macro expansions.

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However, when I try this under Release mode: It compiles, builds, runs, and produces the appropriate output:

I guess the reason for it is the /ZI vs /Zi flag difference. Your release mode flags have /Zi so it compiles fine.