[Xapian-tickets] [Xapian] #806: Project does not compile on VS2019 with C++20 turned on

[Xapian]

#806: Project does not compile on VS2019 with C++20 turned on
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 Reporter:  Mateusz Pusz  |             Owner:  Olly Betts
     Type:  defect        |            Status:  new
 Priority:  normal        |         Milestone:  1.4.18
Component:  Other         |           Version:  1.4.16
 Severity:  normal        |        Resolution:
 Keywords:                |        Blocked By:
 Blocking:                |  Operating System:  Microsoft Windows
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Comment (by Olly Betts):

 The system/compiler headers really should be robust to this as it is part
 of the language, but it's certainly plausible there might be bugs in this
 area.

 We do completely avoid `using namespace std;` in public headers, and we
 also aim to avoid it in internal headers, but some older headers do still
 have it.

 Pushing it down to the block level everywhere doesn't seem workable -
 pretty much every function would need `using namespace std;` or explicit
 `std::` qualifiers adding everywhere.  But `using namespace std;` in non-
 header sources should be safe from this issue provided it's done after all
 headers have been included.  There's still potential for clashes from
 additions in new C++ standard versions, but in practice these are rare and
 the trade-off for convenience seems worthwhile.

 Can you work out which use is actually the problem here?  A targetted fix
 seems reasonable, but I worry a mass refactor to eliminate all cases is
 potentially destabilising for a stable release branch.
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Ticket URL: <https://trac.xapian.org/ticket/806#comment:6>
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