Document AS_IF(..., ..., [[]]) glitch

* doc/autoconf.texi (Common Shell Constructs):
Note the if-false arg problem reported by Jannick in:
https://lists.gnu.org/r/autoconf/2020-03/msg00045.html

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 594547c0..f4520bba 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -13750,6 +13750,13 @@ AS_IF([test "x$foo" = xyes], [HANDLE_FOO([yes])],
 @noindent
 ensures any required macros of @code{HANDLE_FOO}
 are expanded before the first test.
+
+The @var{run-if-false} argument should either consist entirely of
+blanks, or expand to a nonempty shell command.  For example,
+@code{AS_IF([:], [:], [[]])} is invalid because its @var{run-if-false}
+argument contains the nonblank characters @code{[]} which expand to
+nothing.  This restriction on @var{run-if-false} also applies to other
+macros with ``if-false'' arguments denoting shell commands.
 @end defmac
 
 @defmac AS_MKDIR_P (@var{file-name})