Comment 0 for bug 1611676

gccgo-6 fills /usr/lib/gcc/x86_64-linux-gnu/6 which triggers clang to select gcc6 stdlib folders. But the gcc6 installation is incomplete so all compilations will fail with not finding headers or standard libraries.

E.g.
#include <cstdio>
int main() {}

fails with cannot find cstdio

See the verbose output where it selects gcc 6 as base.
clang++ -v file.cc

removing gccgo-6 makes it functional again.