[PATCH] D35388: [libc++] Give extern templates default visibility on gcc

[Louis Dionne via Phabricator via cfe-commits]

ldionne added a comment.

In https://reviews.llvm.org/D35388#1167315, @thomasanderson wrote:

> 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.


Ah, well of course! That's because I assumed that `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` was always used on extern template declarations where the base template had been declared with `_LIBCPP_TEMPLATE_VIS`, which isn't true for `__vector_base_common`.

> 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

I would think that both this change and https://reviews.llvm.org/D35326 are good.


https://reviews.llvm.org/D35388