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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 11:34:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106248

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
To put it another way, in C++17 and earlier, writing the buffer stops becau=
se
we reach EOF when reading from the istream. If it *didn't* stop there, it w=
ould
overflow the buffer and have undefined behaviour. But luckily the program u=
sed
the correct buffer size, and EOF happens at just the right time to prevent
overflow. This is the behaviour required by the C++17 standard.

In C++20, writing to the buffer stops because we know the buffer is full and
will not allow it to overflow. That means we don't detect that the input st=
ream
reached EOF, because we don't even try to read any more characters. This is=
 the
behaviour required by the C++20 standard.

The "bug" is that for C++17 mode we use GCC's ability to detect buffer size=
s to
provide the safer C++20 behaviour, but that's only possible when optimizing=
.=