I looked at the source code of M::B::WithXSpp, and it indeed has a bug. In function ACTION_generate_main_xs:

...
  if (keys(%$xs_files) > 1) {
    # user knows what she's doing, do not generate XS
    $self->log_info("Found custom XS files. Not auto-generating main X
+S file...\n");
    return 1;
  }
...
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The HashRef $xs_files contain all user-write XS files, but also contain the auto generated buildtmp/main.xs. And, it tests whether there are more than 1 xs files. This logic causes two issues:

1. If I have only one manually-written XS file, the builder will still auto-generate the main.xs, which is not the expected behavior.

2. If I run the builder, it will generate a main.xs. Thus the builder's behavior will change unexpectedly, as now the XS file amount is changed.

This logic should be revised, and it looks like the plain XS support of M::B::WithXSpp is not extensively tested. Where can I report this issue?