On Jan 9, 2014, at 2:51 PM, Richard Smith <[email protected]> wrote:
> On Thu, Jan 9, 2014 at 2:39 PM, Marshall Clow <[email protected]> wrote: > On Jan 9, 2014, at 2:20 PM, Richard Smith <[email protected]> wrote: > >> On Thu, Jan 9, 2014 at 7:25 AM, Alp Toker <[email protected]> wrote: >> >> It's not fair on the user or the system header maintainers to bring up >> unexpected errors with this flag. >> >> I don't think this is clear. If the code in the system header is ill-formed, >> I would expect the system header maintainers to be *grateful* that we flag >> it as an error and not just as a warning. But I'll wait to hear what they >> have to say about that. > > A few points, in no particular order: > > * If we have ill-formed code in system headers, I would expect it to fail to > compile whether the user specifies -Wsystem-headers or not. > > And yet in some cases we don't, because we deliberately support some flavors > of broken system headers. > Given that, would you want -Wsystem-headers to make such code produce an > error or only a warning? Eww. First - is any of that code in libc++? Second, there’s two audiences that I think are affected here, and they have different needs: * People trying to use the system headers. These people are, in general, unable/unwilling to change them. Giving them warnings/errors is just noise. * People who are working on the system headers. They need to see the warnings. > > * If a user has a clean build, and then rebuilds with -Wsystem-headers, I > would expect to get warnings - not errors. [ Isn’t that what PR18327 is all > about? ] > > Do you expect this because the flag starts with -W? Yes. > (Maybe that's the problem here -- it's *not* a warning flag in the sense that > the other -W flags are. But that's not unprecedented -- nor is -Werror.) > * There are some “interesting” language features which are only enabled for > system headers, and cause warnings if used in user code. > [ User-defined suffixes that do not start with an underscore, for example. ] > > This is a really great example, thanks. Note that putting something like this in your source file gives you a warning; std::complex<long double> operator"" ilx(long double __im) { return { 0.0l, __im }; } $ clang11 junk.cpp junk.cpp:5:31: warning: user-defined literal suffixes not starting with '_' are reserved; no literal will invoke this operator [-Wuser-defined-literals] std::complex<long double> operator"" ilx(long double __im) ^ 1 warning generated. > So it seems there are at least three different classes of errors that we > might think about producing in system headers: > > 1) warnings that the user has turned into errors with -Werror or -Werror=foo > 2) errors for ill-formed code that we suppress by default in system headers > as a workaround for a system header bug > 3) errors for code that is ill-formed outside system headers but valid > within system headers (using reserved names, adding names to namespace std, > that sort of thing) > > With -Wsystem-headers enabled, I think (1) should be an error, (3) should > remain suppressed (not even a warning), and (2) should be either a warning or > an error (and your first two bullets don't give me a clear idea of which way > these cases should go). Agreed for #1 and #3. And for #2, I’d like to see a warning; “This is code we wouldn’t allow outside a system header, but we do so here” I don’t see the point of making #2 an error. For most people (who can’t fix it), it’s just a hassle. For system developers, they can read warning output just as well as error output. — Marshall
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