{"ast_node_type":"other","tokens":[{"end":72,"kind":"keyword","start":0,"text":"///"}]}
    /// actually requiring `a <: b`. This is of course a valid coercion,
    /// to `subtype_predicate` -- that is, "coercing" `a` to `b` winds up
    /// but it's not as flexible as `FnCtxt::coerce` would be.