Replace std::result_of (removed in C++20) with std::invoke_result

[jsmbennett]

Problem

std::result_of was deprecated in C++17 and removed in C++20. libstdc++ still ships it as an extension, so GCC/Linux builds never notice — but libc++ does not, so the engine currently fails to compile on macOS (and any other libc++ target):

error: no template named 'result_of' in namespace 'std'

Because simulant/signals/signal.h is reachable from nearly every public header, this breaks essentially every translation unit — both in-tree and in downstream projects that link the engine. On AppleClang 21 / macOS 15 arm64 a clean build produces 35 such errors.

Changes

simulant/threads/future.h — the local result_of_t alias is reimplemented on std::invoke_result via a small partial specialization, so the existing result_of_t<Function(Args...)> call sites in async() are untouched.

simulant/coroutines/helpers.h — std::result_of<Func()>::type → std::invoke_result<Func>::type (3 sites, including the two friend declarations).

simulant/signals/signal.h — removes typedef std::result_of<Signature> result;.

This last one is deliberately not a mechanical substitution, and is worth a second look. Signature here is a function type such as void(int), so result_of treats the return type void as the callable rather than the signature's return type. Since C++14 result_of is SFINAE-friendly, so std::result_of<void(int)> is simply an empty struct with no ::type:

std::result_of<void(int)> has ::type = 0
sizeof = 1 (empty struct)

It is unused in-tree (grep finds no reference to signal<...>::result) and can't be meaningfully used out of tree, so removing it seemed better than translating a typedef that never worked. Happy to instead define it as a real return-type trait if you'd prefer to keep the name.

Compatibility

std::invoke_result is C++17, so this preserves the C++17 compatibility added in 2175228. The replacement trait was checked against functors, lambdas, function pointers and void-returning callables under both -std=c++17 and -std=c++20.

Testing

• Engine builds clean on macOS 15 / arm64, AppleClang 21, libc++ (SIMULANT_BUILD_SAMPLES=OFF SIMULANT_BUILD_TESTS=OFF) — previously failed outright.
• A downstream project links the resulting libsimulant.dylib and runs correctly against the GL2X renderer.
• No Linux/GCC regression is expected since std::invoke_result is standard in both libstdc++ and libc++; I don't have a Linux box to hand, so a CI run there would be worth confirming.

🤖 Generated with Claude Code