commit 4bbdd7f83187901fbf6bc2eb23b48d8749ef8dc9
author Yifei Teng <yifeit@google.com> Thu Feb 09 20:07:32 2023 +0000
committer CQ Bot <fuchsia-internal-scoped@luci-project-accounts.iam.gserviceaccount.com> Thu Feb 09 20:07:32 2023 +0000
tree 2ff1511918b0846e138de04760b6ff87ca108c8b
parent 8dce40608114e62fefa678c95f77a8d8b0b004ba

[fidl][cpp] Don't use ToUnderlying when converting enums

When the new C++ bindings is used in Chromium we found that
HLCPP/natural enum conversion breaks when an enum-less library is
included in front of the library of interest. The reason is that we have
multiple overloads of `fidl::ToUnderlying`. There is one overload for
HLCPP strict enums, and separate overloads defined for natural strict &
flexble enums. Due to [1], a templated class that uses ToUnderlying can
only use ToUnderlying overloads that were already included before it.
Thus sometimes the natural strict & flexible overloads aren't visible to
the compiler.

In user code, people can call `fidl::ToUnderlying` and all the necessary
overloads are already included. However, inside FIDL library code such
as `lib/fidl/cpp/hlcpp_conversion.h`, not all `fidl::ToUnderlying` are
defined at the point when that header is included.

The most straightforward solution is to not use `fidl::ToUnderlying` in
FIDL runtime libraries.

[1]: https://en.cppreference.com/w/cpp/language/dependent_name

Change-Id: I8c1058cb4b1eae5e5d3b9f38a769c19aad0c855a
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/800049
Reviewed-by: Ian McKellar <ianloic@google.com>
Commit-Queue: Yifei Teng <yifeit@google.com>
API-Review: Ian McKellar <ianloic@google.com>