[Bug c++/90590] enumeration value not handled in switch warning for std::ios_base::seek_dir

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90590

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-05-23
          Component|libstdc++                   |c++
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In theory we could do this to avoid the warning:

--- a/libstdc++-v3/include/bits/ios_base.h
+++ b/libstdc++-v3/include/bits/ios_base.h
@@ -189,13 +189,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   operator^=(_Ios_Iostate& __a, _Ios_Iostate __b)
   { return __a = __a ^ __b; }

+  enum _Ios_Seekdir_range
+    {
+      _S_ios_seekdir_beg_ = 0,
+      _S_ios_seekdir_end = 1L << 16
+    };

-  enum _Ios_Seekdir 
+  enum _Ios_Seekdir : __underlying_type(_Ios_Seekdir_range)
     { 
       _S_beg = 0,
       _S_cur = _GLIBCXX_STDIO_SEEK_CUR,
-      _S_end = _GLIBCXX_STDIO_SEEK_END,
-      _S_ios_seekdir_end = 1L << 16 
+      _S_end = _GLIBCXX_STDIO_SEEK_END
     };


But giving the enum a fixed underlying type changes its semantics, as now any
value of the underlying type is a valid value of the enumeration type.

As I said in the Clang bug, I think a better approach would be to suppress the
warning for enumerators that are defined in system headers and that use
reserved names.

So I'm changing this to a compiler enhancement request, to suppress this
warning for cases like this.