This is an auto-filed bug because sci-mathematics/msieve calls ar directly.
The issue was originally discovered on amd64, but it may be reproducible on other arches as well.
If you think that a different summary clarifies the issue better, feel free to change it.
Attached build log and emerge --info.
If you think it doesn't make sense fix these type of issues, I'd like to point out that won't be possible use a different AR implementation (like llvm-ar) by setting the AR variable. So this issue has been reproduced by setting the AR variable to x86_64-pc-linux-gnu-ar and by removing the /usr/bin/ar binary.
Created attachment 642040 [details]
build log and emerge --info