K.Prasad [off-list ref] wrote:

quoted

My understanding is weak function definitions must appear in a different C
file than their call sites to work on some toolchains.

Atleast, there are quite a few precedents inside the Linux kernel for
__weak functions being invoked from the file in which they are defined
(arch_hwblk_init, arch_enable_nonboot_cpus_begin and hw_perf_disable to
name a few).
Moreover the online GCC docs haven't any such constraints mentioned.

I've seen problems in this area.  gcc sometimes inlines a weak function that's
in the same file as the call point.

David