This is an auto-filled bug because dev-lang/mlton calls cc 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 CC implementation (like clang) by setting the CC variable. So this issue has been reproduced by setting the CC variable to x86_64-pc-linux-gnu-gcc and by removing the /usr/bin/cc - /usr/bin/gcc binaries.
Created attachment 638808 [details]
build log and emerge --info