Description
Describe the issue
In https://github.com/abseil/abseil-cpp/blob/master/absl/hash/internal/hash.h#L883-L889 , abseil defines AbslHashValue for absl::optional whenever T is hashable.
Depending on ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE, absl::optional may or may not be a typedef for std::optional.
Downstream code uses absl::flat_hash_map with a key that is a std::optional and the build succeeds on Linux machines and fails on macOS where ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE is true. That is why Google or-tools doesn't build for me, I encountered this issue at https://github.com/google/or-tools/blob/stable/ortools/sat/intervals.cc#L98 .
When defining the hasher for these types, when the macro is set, please also define them for the std:: versions of those types.
Steps to reproduce the problem
There is a problem with producing a minimized example of the real problem right now due to google/or-tools#4642 .
What you can observe is that static_assert of is_hashable<operations_research::sat::Literal> is true, and is_hashable<std::option<int>> is true (due to the std::hash fallback), but is_hashable<std::option<operations_research::sat::Literal>> is false.
What version of Abseil are you using?
https://registry.bazel.build/modules/abseil-cpp/20240722.0.bcr.2
It was pulled in through bazel_dep(name = "or-tools", version = "9.12")
What operating system and version are you using?
macOS Sequoia 15.4.1
What compiler and version are you using?
Apple clang version 17.0.0 (clang-1700.0.13.3)
Target: arm64-apple-darwin24.4.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
What build system are you using?
bazel 7.5.0
Additional context
No response