E2500 error in C++20 build (MSVC) format.h line 2748 (fmt v8.0.1)

[cliffdiamond]

This could be minor but the more I read up about it, the more it's looking like there's an issue here. Maybe you can code around it.

I'm getting an Intellisense-only error in format.h at line 2748 when building with VS2022. It seems to relate to the use of fixed_string in the template parameters.

The full error is:

E2500 a literal operator template must have a template parameter list equivalent to '<char ...>'

I'm using fmt V8.0.1 with headers only (FMT_HEADER_ONLY), with my language standard option set to /std:c++20. I've also tried /std:c++latest but the problem persists. It disappears when switching to /std:c++17.

I found this article (https://stackoverflow.com/questions/53789050/template-parameter-list-for-literal-operator) which seems to relate to the issue but may be out-of-date as it's a couple of years old. It seems to be saying that gcc is wrong in allowing non-char parameters, and quotes the following in the C++ standard (though it doesn't say which version of the standard):

"The declaration of a literal operator template shall have an empty parameter-declaration-clause and its template-parameter-list shall have a single template-parameter that is a non-type template parameter pack with element type char."

Please advise if this is something that should be changed to be more compliant.

Regards

Cliff

[alexezeder]

It's actually string literal operator template and it works thanks to C++20 non-type template parameter, without any GCC or Clang extensions.

Looks like there is the same issue - #2536.

[cliffdiamond]

I've reported this to the VS developers.

[lifeinaglasshouse]

This bug still happen after a year. It seem like Microsoft still didn't fix it. I cannot find any other solution other than changing to C++17

Hope this issue could be solved.

[mpayrits]

Still no change with Visual Studio 17.5.0, but this can be worked around by defining FMT_USE_NONTYPE_TEMPLATE_ARGS=0 on the cl.exe command line or just before including fmt headers. We managed to continue using fmt in MSVC with C++20 this way without further issues.
Note: this applies to fmt v9.x. For v8.x (and possibly earlier), the macro name is different and one has to define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS=0.
Note 2: our use case involves CUDA and to get that to work with newer versions of nvcc and MSVC with C++20 and fmt v9.x, we also had to define FMT_USE_CONSTEXPR=1. v8.x works without that extra define and it's probably not necessary for straight C++ either.