\bool_set:Nn not accepting a boolean to be set to its own value

[eg9]

Source: https://tex.stackexchange.com/q/634629/4427

\ExplSyntaxOn

\bool_new:N \l_test_bool
\bool_set_true:N \l_test_bool

\tl_show:x { \bool_if_p:n { \l_test_bool } }

\bool_set:Nn \l_test_bool { \l_test_bool }

\ExplSyntaxOff

would fail with “Missing number, treated as zero” because \tex_chardef:D makes \l_test_bool equivalent to \relax until the assignment is actually performed and the evaluation of the given boolean expression only leaves \l_test_bool, as demonstrated by the \tl_show:x line.

A workaround is to use a nontrivial boolean expression such as \l_test_bool && \c_true_bool, but it should be either documented or fixed.