Build fails on Apple clang 16: alias-template CTAD in Negentropy call sites

[kojira]

Summary

Current master fails to build with Apple clang 16 (the compiler in the Xcode Command Line Tools shipped with macOS 15):

src/apps/relay/RelayNegentropy.cpp:99:24: error: alias template 'Negentropy' requires template arguments; argument deduction only allowed for class templates

The same error appears at four call sites in src/apps/mesh/cmd_sync.cpp.

Cause

Negentropy ne(storage, ...) relies on class template argument deduction through an alias template:

template<typename T>
using Negentropy = negentropy::Negentropy<T>;   // external/negentropy/cpp/negentropy.h

CTAD for alias templates is a C++20 feature (P1814). Apple clang 16 does not implement it; Apple clang 17 does.

Why CI doesn't catch it

The macos.yml workflow runs on macos-latest, which currently carries clang 17, so the job stays green. Anyone on a stock macOS 15 install with the standard CLT hits the error.

Reproduction

Platform	Compiler	Result
macOS 15.2, x86_64	Apple clang 16.0.0 (clang-1600.0.26.6)	❌ error above, pristine master c65da49
macOS, arm64	Apple clang 17.0.0 (clang-1700.0.13.5)	✅ builds

Fix

Naming the template argument explicitly at the five call sites is enough:

Negentropy<std::remove_reference_t<decltype(storage)>> ne(storage, ...);

It's a no-op wherever deduction already worked, and needs no per-site knowledge of the storage type. Verified: with this change master builds and runs on the clang 16 machine, and still builds unchanged on clang 17.

I have this ready as a small PR and will open it referencing this issue.

Thanks for the recent macOS work (#190, #210) — with those in, this was the only thing left between master and a clean build on a current stock macOS install.

[hoytech]

Can you please try the master branch? Instead of using decltype as in your PR, I decided to just explicitly indicate it at each call-site:

4cd3cf6