$ cat test.cc
namespace {
void f() {}
}
$ gcc -Wunused-functions test.cc
$ clang -Wunused-functions test.cc
test.cc:2:6: warning: unused function 'f' [-Wunused-function]
void f() {}
^
1 warning generated.

clang r129794 suggests that clang tries to be compatible with gcc here, so this
warning should be moved to -Wunneeded-internal-declaration.

r129794 is more gcc compatible in that there is no -Wunused-function warning emitted if you reference the function anywhere, e.g:

static void foo() {}

template
void tf() {
foo();
}

clang would show that 'foo' is unused while gcc would show nothing. After r129794 this becomes -Wunneeded-internal-declaration which is much more clear. Also warning for these cases can be disabled separate from -Wunused-function.

In general, warning about unused functions in unnamed namespaces is works-as-expected IMO, unless there is a compelling argument against it.

Fair enough.

If -Wunneeded-internal-declaration wasn't introduced to make -Wunused-function gcc-compatible, then the difference between these two flags is pretty subtle imho. Oh well :-)