Re: expand-before-require bug

[Bruno Haible]

Eric,

I wrote:
>   - provide a documented way to avoid the warning for a particular macro,
>     I much prefer to have a macro which can be both invoked and required,
>     even if it requires one more line of code to write it down, than to have
>     a macro with restricted use.

Here's a concrete proposal: Introduce a new macro-defining-macro
AC_DEFUN_IDEMPOTENT, such that:

  - macros defined with AC_DEFUN_IDEMPOTENT are *known* to be expandable
    any number of times, hence they may be both invoked and required,

  - macros defined with AC_DEFUN_ONCE are *known* to be expandable only
    once, hence the recommendation for them is to AC_REQUIRE them,

  - about macros defined with AC_DEFUN nothing is known.

So for your warning about required-after-expanded it means:
  - the warning should apply always to AC_DEFUN_ONCEd macros,
  - the warning should never apply to AC_DEFUN_IDEMPOTENTd macros,
  - the warning should apply to AC_DEFUNd macros only when the warning
    level is higher than the default. The warning should indicate that
    either the double expansion is a problem, or the macro should be
    changed to be defined with AC_DEFUN_IDEMPOTENT if it is not a problem.

Bruno