thomasanderson added a comment.
In https://reviews.llvm.org/D35388#1167305, @ldionne wrote:
> ABI-wise, I think this change is OK. Indeed, if `__type_visibility__` is not
> supported, we're already marking the base template as
> `__visibility__("default")`, so marking the extern template declaration with
> `__visibility__("default")` is not a problem. If `__type_visibility__` is
> supported, then there's no change in behavior. So I think there is no change
> in behavior either way, and I'm actually wondering why @thomasanderson wants
> this change in if the behavior is always the same. Perhaps I am missing
> something?
It's been a while, but IIRC this fixes the Chromium build with gcc and libc++,
since std::vector_base_common<true>::__throw_length_error() wasn't getting
exported otherwise.
See the comment in the Chromium source code:
https://cs.chromium.org/chromium/src/buildtools/third_party/libc%2B%2B/BUILD.gn?rcl=0dd5c6f980d22be96b728155249df2da355989d9&l=88
And also this CL https://reviews.llvm.org/D35326
> As a diversion, I'm confused by the very fact that we need to apply
> `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` to the _extern template declaration_. I
> would have expected instead that, if anything, we need to specify the
> visibility on the _explicit instantiation_ in the dylib.
https://reviews.llvm.org/D35388
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