Compilation Error: offset of on non-standard layout type

[theduke]

I'm trying to compile a simple example using the pre-built code, but I'm stumbling over a compilation error.

Environment:
wasi-sdk 12.0
CXX and LD flags are taken directly from the Fastly runtime Makefile.

/host/spidermonkey/include/js/HeapAPI.h:251:38: error: offset of on non-standard-layout type 'js::gc::TenuredChunkBase' [-Werror,-Winvalid-offsetof]
const size_t ChunkMarkBitmapOffset = offsetof(TenuredChunkBase, markBits);
                                     ^                          ~~~~~~~~
/host/wasi-sdk-12.0/lib/clang/11.0.0/include/stddef.h:104:24: note: expanded from macro 'offsetof'
#define offsetof(t, d) __builtin_offsetof(t, d)

[theduke]

Note that things seem to work just fine when ignoring with -Wno-invalid-offsetof.