After installing libstdc++-14-dev, it fixed itself, on both clang++, and clang++ built from upstream.
- Bug#1063701: clang: When invoking "clang++", &qu... kritomas
- Bug#1063701: clang: When invoking "clang++"... Sylvestre Ledru
- Bug#1063701: Re: Bug#1063701: clang: When invoking &q... kritomas
- Bug#1063701: Fixed it kritomas
