Before you spend time on it, see:
https://lists.llvm.org/pipermail/llvm-dev/2019-October/136380.html

We can fix this on our end too [1], but I'm waiting for what LLVM will do.

-Johan

[1] For example:
if(NOT (LDC_LLVM_VER LESS 1000))
    set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
    set(CMAKE_CXX_STANDARD_REQUIRED YES)
    set(CMAKE_CXX_EXTENSIONS NO)
endif()

On 2019-10-30, Johan Engelen wrote:
>Before you spend time on it, see:
>https://lists.llvm.org/pipermail/llvm-dev/2019-October/136380.html
>
>We can fix this on our end too [1], but I'm waiting for what LLVM will do.
>
>-Johan
>
>[1] For example:
>if(NOT (LDC_LLVM_VER LESS 1000))
>    set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform 
>to")
>    set(CMAKE_CXX_STANDARD_REQUIRED YES)
>    set(CMAKE_CXX_EXTENSIONS NO)
>endif()
>

I don't think llvm-config should print -std=c++14, because users may
want -std=c++17, -std=c++2a or -std=gnu++2a. Providing a default
CMAKE_CXX_STANDARD value should be ok but overriding it is probably not
a good idea.

On Sunday, 1 December 2019 at 02:00:14 UTC, Fangrui Song wrote:
> On 2019-10-30, Johan Engelen wrote:
>>Before you spend time on it, see:
>>https://lists.llvm.org/pipermail/llvm-dev/2019-October/136380.html
>>
>>We can fix this on our end too [1], but I'm waiting for what LLVM will do.
>
> I don't think llvm-config should print -std=c++14, because users may want -std=c++17, -std=c++2a or -std=gnu++2a.

Changing C++ standard may change the ABI (aswell as the API), hence it is necessary that llvm-config outputs which c++ standard was used to build LLVM. It was fixed fairly quickly after my forum post btw, so there is no build problem any more.

-Johan