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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/106248] [11/12/13 Regression]
 operator>>std::basic_istream at boundary condition behave differently in
 different opt levels
Date: Mon, 11 Jul 2022 11:20:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106248

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-07-11
             Status|UNCONFIRMED                 |NEW
            Summary|operator>>std::basic_istrea |[11/12/13 Regression]
                   |m at boundary condition     |operator>>std::basic_istrea
                   |behave differently in       |m at boundary condition
                   |different opt levels        |behave differently in
                   |                            |different opt levels
      Known to fail|                            |11.1.0
     Ever confirmed|0                           |1
      Known to work|                            |10.4.0

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In C++20 mode the operator>> overload has changed to one that binds to char
(&)[10] and so knows the size of the output buffer. The loop stops reading =
when
it has written as many chars as will fit in the buffer, and does not set
eofbit:

              if (__extracted < __num - 1
                  && __traits_type::eq_int_type(__c, __eof))
                __err |=3D ios_base::eofbit;

In pre-C++20 modes, operator>> just binds to a const char* and will overflo=
w it
if the buffer is not big enough. However, libstdc++ now uses
__builtin_object_size to detect the size of the buffer, and so will stop
writing when the buffer is full. That detection using __builtin_object_size
only works when optimization is enabled, which is why we stop before setting
the eofbit when optimizing.

I don't want to just remove the size detection, because it prevents undefin=
ed
behaviour. But we need a way to prevent overflow without altering the
observable behaviour for C++17 and earlier.=