Related to bug#29619.

The function documentation of xref-find-references says:

    Find references to the identifier at point. With prefix argument,
prompt for
    the identifier.

The Emacs manual (Node: Identifier Search) says:

    ‘M-?’ finds all the references for the identifier at point. If there’s
no
    identifier at point, or when invoked with a prefix argument, the command
    prompts for the identifier, with completion.


Inconsistent to both, the actual behaviour of xref-find-references and M-?
is to
*always* prompt for the identifier from the user (irrespective of whether
it is
invoked with or without the prefix-arg). The only way to get the documented
behaviour is by customizing xref-prompt-for-identifier, and there is no way
of
discovering this without reading the source code.