Hi,
I've run into some compile errors after updating to 4.9 -- usually getting undefined references to symbols defined in helper static libraries.
It turns out this is triggered by gcc -flto now creating slim object files by default (-ffat-lto-objects "fixes" it) - but I think it is actually an ld bug that should be fixed at some point. ld (regardless of whether I use -fuse-linker-plugin, -fuse-ld=gold or -fuse-ld=bfd) doesn't seem to see LTO bytecode in object files that are inside an ar wrapper.
It deals with the library just fine if I use "ar x" to extract its object files and link to them individually as opposed to the .a file.
I've attached a small test case to demonstrate ("make broken" shows the error, "make works" shows the workaround).