This appears to be a bug in gcc at least. The following code:
#pragma GCC diagnostic ignored "-Wunknown-pragmas"
#pragma GCC diagnostic ignored "-Wuninitialized"
int fn(void) {
#pragma xyzzy
int x;
return x;
}
int main (void) {
return fn();
}
has no problems ignoring the uninitialised x value but still complains about the pragma (without the uninitialized pragma, it generates a warning for x as you'd expect).
If you change the command line options to be -Wall -Wno-unknown-pragmas, then it ignores it just fine. That's okay for your specific case since you want it applied over your entire translation unit but it won't allow the fine-grained control that you would get from the #pragma method (if it worked).
I went to raise a bug report on GCC but found that it already exists (#53431).
While that specific bug has to do with -Wundef, a snippet in one of the comments indicates that it probably applies to all variants affecting the preprocessor (slightly modified for emphasis):
The C++ parser lexes (and preprocesses) before handling the pragmas, whereas the C parser processes the pragmas as it sees them.
We must somehow parse these pragmas also in cp/parser.c:631. Maybe one can do something similar to what we do for cp_parser_initial_pragma, but within the loop and only handling pragma diagnostic. Surely, it will need some trial and error to get it right. If any of you wants to give it a try and need some help, just ask here or in the mailing list.
That explains why we don't see the same problem with -Wuninitialized, because it's detected during later stages of the compilation process, after the pragmas have been activated at the end of preprocessing.
So, if you want to see it fixed in a more timely manner (it was raised over three years ago), I'd suggest (as I have) hassling the GCC bugzilla site to try get some exposure.
