On Wed, May 10, 2023 at 06:30:40PM +0300, Eli Zaretskii wrote:
> > Date: Wed, 10 May 2023 16:22:26 +0200
> > From: Jakub Jelinek <ja...@redhat.com>
> > Cc: Gabriel Ravier <gabrav...@gmail.com>, jwakely....@gmail.com,
> > fwei...@redhat.com, gcc@gcc.gnu.org, ar...@aarsen.me
> >
> > > > Are you seriously saying that no accepts-invalid bug should ever be
> > > > fixed under any circumstances on the basis that some programmers might
> > > > rely on code exploiting that bug ??
> > >
> > > Sorry, I'm afraid I don't understand the question. What are
> > > "accepts-invalid bugs"?
> >
> > They are bugs where compiler accepts something that isn't valid in
> > the selected language nor considered valid extension.
> > So, after the fix we reject something that has been accepted before.
>
> If some program is plainly invalid, not just because the criteria of
> validity have shifted, then yes, such a program should be rejected.
Many of the accepts-invalid cases are when something used to be valid in some
older standard and is not valid in a newer standard, often even changes
meaning completely in even newer standard.
Examples include e.g. the auto keyword, which means something completely
different in C++11 and later than what it meant in C++98, or say comma in
array reference in C++17 vs. C++20 vs. C++23 (a[1, 2] is the same as a[(1, 2)]
in C++17, got deprecated in C++20 and is ill-formed or changed meaning
in C++23 (multi-dimensional array operator).
Or any time something that wasn't a keyword in older standard version
and is a keyword in a newer standard.
alignas/alignof/nullptr/static_assert/thread_local in C++11 and C23,
char16_t/char32_t/constexpr/decltype/noexcept in C++11,
constinit/consteval in C++20,
bool/false/true/typeof_unqual in C23.
int bool = 1;
is completely valid C17 if one doesn't include <stdbool.h> header,
or
int static_assert = 2;
valid C17 if one doesn't include <assert.h>
etc. These used to compile and will no any longer wheen using -std=c2x or
in a few years when -std=gnu23 becomes the default will not compile by
default, even when it used to be valid C17.
And here are talking about code that wasn't valid already in C99...
Jakub
