#830 Cannot disable all warnings but one
It's not possible to disable all warnings but one (or n, in the general case). Suppose one wants to enable all warnings except 509. When -Wall is turned on, all other warning filters are ignored, so -Wall -w509 cannot be used.
So, the next approach one might try would be to use -w509,+202,+309,+403,+512,+321,+322, to disable 509 and enable the warnings that swig disables by default. However, because Swig_warnfilter() prepends to the warning filter list, the warn filter starts as
-202,-309,-403,-512,-321,-322
then gets set to
-509,+202,+309,+403,+512,+321,+322,-202,-309,-403,-512,-321,-322
Now, warning 509 does indeed get ignored. However, 202, for example, is still ignored. The reason it is ignored is that Swig_warning scans through the warning filter for all the ones matching the warning number, and uses the *last* match found. So, it uses the -202 filter because it comes after the +202 filter.
One fix would be to break out of the loop in Swig_warning() when the first match is found.
It would be great if there was an option like -Wall that only cleared the default warn filter, instead of disabling all warning filters.
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I think you mean "It's not possible to ENABLE all warnings but one".
-Wall is documented to disable all other warning filters, but that makes it behave differently to GCC's -Wall ("gcc -Wall -Wno-foo" does what you'd expect) so personally I'm not sold on that being the right thing to do.
So I think that the two things -Wall does now should be separated and that -Wall should simply turn on the extra warnings, and there should be a new option (say -Wnofilters) to disable all other warning filters. The manual doesn't say why this is useful, but I guess it's so you can take a look at all the warnings being suppressed when trying to debug a SWIG interface file.
It also sounds like Swig_warning() should break out of the loop at the first match as you suggest (which would be more efficient too!)
But this might be controversial. If you're interested in working on a patch, I'd suggest asking on the swig-dev mailing list (actually, tracker updates are sent to the dev list, but of course people may not actually read them...)
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This is similiar to #1754967 which has been resolved for swig-1.3.32. Please see the updated Warnings.html documentation and the CHANGES entry dated 08/31/2007.