[cfe-dev] 8.0 Regression with __builtin_constant_p.

[Bill Wendling via cfe-dev]

Hi Eric,

The attached patch may help matters, though it now fails because "test2"
doesn't exist (it's not enabled).

-bw



On Fri, Mar 8, 2019 at 11:38 AM Eric Fiselier via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi All,
>
> I know we're late in the release process, but I've discovered a regression
> I believe is serious enough to hold the release [1].
>
> The regression is in usages of `__builtin_constant_p` applied to a
> dereferenced pointer in a constant expression. In certain cases Clang now
> rejects this as a non-constant expression [2].
>
> This regression will have a larger impact than it initially appears.
>
> libstdc++ 7.1 and newer use `__builtin_constant_p` to implement C++17
> `std::char_traits` [4]. This means that any usage of `std::string_view` in
> a constant expression has the possibility to break. `__builtin_constant_p`
> may be rare, but `std::string_view` is not.
>
> This regression was first found because it breaks `absl::StrFormat` [4].
> This bug will break Abseil's LTS w/ libstdc++. Abseil cannot fully work
> around this bug.
>
> I would implore us to fix this bug in the upcoming 8 release.
>
> /Eric
>
> [1] https://reviews.llvm.org/D59038
> [2] https://godbolt.org/z/VOCCJF
> [3] https://github.com/abseil/abseil-cpp/issues/271
> [4]
> https://github.com/gcc-mirror/gcc/blob/gcc-8_1_0-release/libstdc++-v3/include/bits/char_traits.h#L229
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