LVRET detects if a sub is called as an lvalue (or something like that). I have no problem putting an SV on the stack, the problem is getting the following to compile

sub foo :lvalue { lvalue_xs_sub() }
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I could avoid it with something like

sub foo :lvalue { out_param_xs_sub(my $x); $x }
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but the point is to create a test for a bug I want to fix so I can fix another bug (delayed object destruction) in substr, pos and vec.