GCC feature request: warn on "if (function_name)"

[Jamie Lokier]

Ross Dickson wrote:
> The fix is to put the brackets back on "!need_resched()"  so that we call
> the function and test its return value - not just test the function pointer!

[ Ross' bug was writing "if (!need_resched)" instead of
"if (!need_resched())" ]

I'm very surprised GCC doesn't warn about that.  A quick test confirms
GCC 3.2.2 at least doesn't.

So, this is a feature request:

    - Warn when a function name is tested in a boolean context.
      (A function pointer variable or expression should not be warned for).

      By boolean context I mean any place where a function name is
      used as a value and tested against zero.  Some examples:

          if (function_name)
          if (function_name && ( <some other expression> ))
          if (function_name != 0)
          if (function_name == 0)
          if (!function_name)
          x = function_name ? a : b;

    - Don't warn if there are two levels of parantheses.

I know it's occasionally useful to test the NULL-ness of a functin
name, of weak symbols.  In most cases, though, it's a bug.  If you
really want to check a weak symbol, just write "if ((symbol))".  That
syntax is already well known for testing the result of an assignment,
as in "if ((x = 1))" does not yield a warning but "if (x = 1)" does.

Perhaps a later GCC than 3.2.2 already has this test; if someone is
able to check, that would be nice.

Thanks muchly :)
-- Jamie
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