On Fri, Dec 9, 2022 at 4:12 PM Eli Zaretskii <eliz@gnu.org> wrote:

Thanks, but all this still doesn't answer my questions,
unfortunately. You-all are describing something that I cannot wrap my
head around, because that basic question was not answered yet.

Would someone please take me through the problem step by step?

There is a MELPA package depending on `pcase'. It contains a `map' pattern:

(pcase-let* ((`(,_name . ,(map filename outline-path id front-context-string indirectp))
...)

The support for `map' pattern is not built into `pcase.el'. Instead, it is
enabled through `pcase--get-macroexpander', which is implemented as follows:

(defun pcase--get-macroexpander (s)
"Return the macroexpander for pcase pattern head S, or nil."
(get s 'pcase-macroexpander))

It returns `pcase-macroexpander' property of the symbol (i.e. `map' in this
case). The property should be available at compile time. If not, the library
will fail to byte-compile.

To set the property of an additional pattern, `rx', `map', `seq', etc. all use
`pcase-defmacro'. I think autoloading the `pcase-defmacro' form in `map.el' can
prevent the error as well as possible future errors in other libraries. This is
already done in `rx.el'.