AS_IF: empty run-if-false causes invalid code.

[Van de Bugger]

Hi,

I am using Autoconf 2.69 on Fedora Linux. Let us consider an trivial
example:

$ cat configure.ac 
AC_INIT([foo],[0.001])
AS_IF([true],[
    # Comment
])

$ autoreconf -i .

$ ./configure 

$ cat ./configure | tail -n 10
ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS
conftest.$ac_ext $LIBS >&5'
ac_compiler_gnu=$ac_cv_c_compiler_gnu


if true; then :

    # Comment

fi

Everything is ok. If the second argument of AS_IF does not include any
executable commands, it is perfectly acceptable and generates valid
code because AS_IF uses : command immediatelly after "then" keyword.

Let us add the third argument to AS_IF:

$ cat configure.ac 
AC_INIT([foo],[0.001])
AS_IF([true],[
    # Comment
],[
    # Another comment
])

$ autoreconf -i .

$ ./configure 
./configure: line 1670: syntax error near unexpected token `fi'
./configure: line 1670: `fi'

$ cat ./configure | tail -n 10

if true; then :

    # Comment

else

    # Another comment

fi

You see, generated code is not correct: there is no command in "else"
part, bash does not like it.

It is not a big problem, but it is quite confusing for newbie user. And
it seems it can be easily avoided by generating ":" command in the
"else" part, like AS_IF does it in "then" part of conditional
statement.