Here is the clang bug report with the testcase: https://bugs.llvm.org/show_bug.cgi?id=36915
This affects the port audio/x42-plugins-lv2 on 12 that has clang60.
The error message:
In file included from src/tuna.c:70:
src/spectr.c:136:32: error: implicit conversion from '_Complex int' to 'double' is not permitted in C++
complex_t p = cos (omega) + _I * sin (omega);
Upstream seems to think that this is programmer error and that what changed was that clang generated an error instead of miscompiling the code. Given a lack of progress on the suggested compatibility support upstream, I'm closing this for now and assuming that we'll get improvements (if any) in a future clang/llvm release.