Fix build for the clang-10 / libstdc++-9 couple

[jcelerier]

Fixes #2490

[vitaut]

Looks like this crashes clang for some reason: https://github.com/jcelerier/fmt/runs/3534703685

[jcelerier]

that's weird, the patch makes constexpr more restrictive... and the crash seems to happen at parsing stage ?! :throw_table:

[jcelerier]

/me spins up a 18.04 docker...

[jcelerier]

of course it works there....

from a ubuntu:18.04 image:

$ apt update
$ apt install git clang-9 gcc cmake build-essential
$ git clone https://github.com/jcelerier/fmt
$ mkdir build
$ cd build
$ CC=clang-9 CXX=clang++-9 cmake -DCMAKE_BUILD_TYPE=Debug  -DBUILD_SHARED_LIBS=ON -DCMAKE_CXX_STANDARD=11 -DFMT_DOC=OFF -DCMAKE_CXX_VISIBILITY_PRESET=hidden -DCMAKE_VISIBILITY_INLINES_HIDDEN=ON  -DFMT_PEDANTIC=ON -DFMT_WERROR=ON /fmt
$ make -j

do you see anything different I could be forgetting ?

[jcelerier]

also it seems to work here... https://github.com/fmtlib/fmt/runs/3534712060?check_suite_focus=true

I'm usually not the one to reach for the "bitflip-inducing cosmic ray" theory but... (or just ub in clang maybe)

[vitaut]

Could be a transient issue.

[vitaut]

Merged, thanks!