Weird behaviour of M-; (`comment-dwim`) in Autoconf-mode

Question

When editing an Autoconf (configure.ac) file I observe strange behaviour when using M-; (comment-dwim) to uncomment a selected region. When a region with comments is selected comment-dwim calls uncomment-region, so maybe the problem is in that function.

Commenting a region works as expected:

# ACML
AC_SEARCH_LIBS(dgemm, acml_mp,
   [found_blas=1 AM_CPPFLAGS="-D_acml_ $AM_CPPFLAGS"],
   [AC_MSG_NOTICE([NOT using AMD the ACML library])],
   [-lgfortran]
)

becomes:

dnl # ACML
dnl AC_SEARCH_LIBS(dgemm, acml_mp,
dnl    [found_blas=1 AM_CPPFLAGS="-D_acml_ $AM_CPPFLAGS"],
dnl    [AC_MSG_NOTICE([NOT using AMD the ACML library])],
dnl    [-lgfortran]
dnl )

However, when trying to uncomment the region, the result is:

dnl ACML
dnl AC_SEARCH_LIBS(dgemm, acml_mp,
   [found_blas=1 AM_CPPFLAGS="-D_acml_ $AM_CPPFLAGS"],
dnl    [AC_MSG_NOTICE([NOT using AMD the ACML library])],
   [-lgfortran]
dnl )

Did I miss something or have I hit a bug?

Answer 1

Works in Emacs 23.4, reproduced in Emacs 24.3.

This is Bug #14046, with no resolution as I write.

I think it's a duplicate of Bug #15822, which is fixed by revision 115027 to autoconf.el released in Emacs 24.4.