bug#67455: (Record source position, etc., in doc strings, and use this in *Help* and backtraces.)

[Stefan Monnier]

>> No matter how many extra tests you add to reduce the frequency, you're
>> fundamentally adding a bug :-(
>
> Well, macroexp--expand-all has treated (function (lambda ...)) as a
> function long before I started on this project in November.

Of course it does, and that's correct, because by definition the
argument to `macroexp--expand-all` must be source code expression.

> Would trusting the same thing in backquote-process be any
> more dangerous?

Yup, because backquote has no guarantee that the code it must produce is
one that will build a source code expression.

> How about adding (an) extra arm(s) to the large pcase in
> macroexp--expand-all which would recognise backquote's output for
> "evaluated" lambdas.
>
> What we get back from backquote looks like:
>
> (cons 'lambda (cons plain-args body)) or
> (list 'lambda args)) or maybe one or two other forms.

Same problem:

    (cons 'lambda (cons plain-args body))

constructs something that may look like a function but that may not be
intended to be used as a function.  All we know is that it should build
a list with a `lambda` symbol in it.

It's only when the result of the execution of this code is passed to
`macroexp--expand-all` that we discover that it was meant to build
a list that represents the source code of a function; and only at *that*
point are we allowed to modify that list by macro-expanding expressions
in its body, modifying its docstring, byte-compiling, etc...


        Stefan