Given something like this:

class MyGen {
  ...  
  Input<Buffer<float, 2>> arg_{"arg"};
  ...
};
void MyGen::generate() {
     for (int i = ...) {
      Expr arg_0 = arg_(i, 0);
      Expr arg_1 = arg_(i, 1);
      ...
      Func foo;
      foo.define_extern("foo", {arg_0, arg_1, ...}, ...);
}

...you'll fail to build with something like Symbol not found: arg_im.min.0.

After a bit of sleuthing, I think the issue here is that while we do correctly inline the arg_im function in schedule_functions(), we don't take into account that foo.extern_arguments() contains Exprs that refer to inlined functions. While validate_schedule() attempts to validate that none of the extern args are inlined, it only does so for Funcs... not for Exprs that reference inlined Funcs, like we have here.

There are ways to hack this into working (e.g. wrap arg_0 in a compute_root()'ed Func), but they are awkward.

(Genuinely surprised we've never seen this one before!)