C++20 doesn't have std::unreachable() - here's what to use instead
26 January 2026
Today I learned that std::unreachable() is a C++23 feature, not C++20. I hit this when trying to fix MSVC warning C4715 ("not all control paths return a value") in a switch statement's default case.
The Problem
I had switch statements like this that needed to indicate the default case was unreachable:
inline constexpr auto opcodeext_alu(e_instruction_id id) {
switch (id) {
case e_instruction_id::add: return e_opcode_alu_extension::add;
case e_instruction_id::sub: return e_opcode_alu_extension::sub;
// ... other cases ...
default:
std::unreachable(); // C++23 only!
}
}
GCC and Clang were happy, but MSVC complained about std::unreachable() not being available.
The Fix
Use compiler intrinsics directly:
namespace detail {
[[noreturn]] inline void unreachable_impl() {
#if defined(_MSC_VER) && !defined(__clang__)
__assume(false);
#else
__builtin_unreachable();
#endif
}
}
Then use detail::unreachable_impl() in your switch defaults.
Why not just suppress the warning?
Using #pragma warning(disable: 4715) or pragma clang diagnostic push/pop works, but it's hiding a potentially real bug. The explicit unreachable marker documents intent and helps the optimizer.
Bonus: constexpr context
If you need this in a constexpr function and want a compile-time error if the unreachable path is actually taken, you might think to add:
if (std::is_constant_evaluated()) {
throw "unreachable code in constexpr";
}
But MSVC doesn't like throw in constexpr functions (even though C++20 technically allows it). The simpler version without the constexpr check works fine - the compiler will still catch invalid paths at compile time because the function won't return a value.