[Bug libstdc++/67996] New: std::ios_base::seekdir raises -Wswitch with Clang

[kim.grasman at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67996

Bug ID: 67996
   Summary: std::ios_base::seekdir raises -Wswitch with Clang
   Product: gcc
   Version: 4.6.3
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: libstdc++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: kim.grasman at gmail dot com
  Target Milestone: ---

When using libstdc++ with Clang, we can't seem to form a fully-covered switch
over std::ios_base::seekdir:

  void f(std::ios_base::seekdir way)
switch(way) {
  case std::ios_base::beg:
// ...
break;

  case std::ios_base::cur:
// ...
break;

  case std::ios_base::end:
// ...
break;
}
  }

...t.cc:87:12: warning: enumeration value
'_S_ios_seekdir_end' not handled in switch [-Wswitch]
switch (way)

This looks like it was discussed long ago in #17922, so I don't know if this
has regressed or if it's something about Clang's implementation of this
diagnostic that is different from GCC.

A discussion on the Clang list is available here:
http://article.gmane.org/gmane.comp.compilers.clang.devel/45198/match=overeager

What's the motivation for _S_ios_seekdir_end? Any chance it could be removed?

Thanks!

[Bug libstdc++/67996] std::ios_base::seekdir raises -Wswitch with Clang

[kim.grasman at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67996

--- Comment #2 from Kim Gräsman  ---
Thanks, I figured that was why.

The warning is not a problem if I'm using  directly, then I can just
disable it locally.

But we ran into this with a third-party library having the code above in one of
their headers. This makes it nigh-impossible for us to enable -Wswitch for our
own code, as the third-party header could be indirectly included in any
translation unit.

  ios:
 enum seekdir { beg, cur, end, _S_ios_seekdir_end };

  third-party.h:
switch(seekdir) {
case beg: break;
case cur: break;
case end: break;
}

  us.cpp:
#include "third-party.h"  // WARN

I wasn't sure if the extra enumerator was allowed by the standard, so thanks
for confirming that.

We've now worked around it by patching the third-party to use an if/else chain
instead, but I thought I'd raise it anyway because it creates a messy situation
when the warning "leaks" like this.

[Bug libstdc++/67996] std::ios_base::seekdir raises -Wswitch with Clang

[kim.grasman at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67996

--- Comment #4 from Kim Gräsman  ---
FWIW, it turns out that GCC warns, too:

$ g++ -Iinclude -I/ssd/code/poco/CppUnit/include
-I/ssd/code/poco/CppUnit/WinTestRunner/include
-I/ssd/code/poco/Foundation/include -I/ssd/code/poco/XML/include
-I/ssd/code/poco/Util/include -I/ssd/code/poco/Net/include
-I/ssd/code/poco/Crypto/include -I/ssd/code/poco/NetSSL_OpenSSL/include
-I/ssd/code/poco/Data/include -I/ssd/code/poco/Data/SQLite/include
-I/ssd/code/poco/Data/ODBC/include -I/ssd/code/poco/Data/MySQL/include
-I/ssd/code/poco/Zip/include -I/ssd/code/poco/PageCompiler/include
-I/ssd/code/poco/PageCompiler/File2Page/include -I/ssd/code/poco/PDF/include
-I/ssd/code/poco/CppParser/include -I/ssd/code/poco/MongoDB/include
-I/ssd/code/poco/PocoDoc/include -I/ssd/code/poco/ProGen/include -Wall
-Wno-sign-compare -DPOCO_UTIL_NO_JSONCONFIGURATION   -DPOCO_HAVE_IPv6
-D_XOPEN_SOURCE=500 -D_REENTRANT -D_THREAD_SAFE -D_FILE_OFFSET_BITS=64
-D_LARGEFILE64_SOURCE -DPOCO_HAVE_FD_EPOLL  -O2 -DNDEBUG -fPIC -c
src/MemoryStream.cpp -o
/ssd/code/poco/Foundation/obj/Linux/x86_64/release_shared/MemoryStream.o
In file included from src/MemoryStream.cpp:17:0:
include/Poco/MemoryStream.h: In instantiation of
‘Poco::BasicMemoryStreamBuf::pos_type Poco::BasicMemoryStreamBuf::seekoff(Poco::BasicMemoryStreamBuf::off_type,
std::ios_base::seekdir, std::ios_base::openmode) [with ch = char; tr =
std::char_traits; Poco::BasicMemoryStreamBuf::pos_type =
std::fpos<__mbstate_t>; Poco::BasicMemoryStreamBuf::off_type = long
int; std::ios_base::seekdir = std::_Ios_Seekdir; std::ios_base::openmode =
std::_Ios_Openmode]’:
src/MemoryStream.cpp:59:1:   required from here
include/Poco/MemoryStream.h:87:4: warning: enumeration value
‘_S_ios_seekdir_end’ not handled in switch [-Wswitch]
switch (way)
^
include/Poco/MemoryStream.h:113:4: warning: enumeration value
‘_S_ios_seekdir_end’ not handled in switch [-Wswitch]
switch (way)
^

This is the age-old GCC 4.6.3 that we're stuck on, however, later versions may
be more forgiving.