std::unreachable(3)

NAME
std::unreachable - std::unreachable

Synopsis
   Defined in header <utility>
   [[noreturn]] void unreachable(); (since C++23)
   Invokes undefined behavior. An implementation may use this to optimize impossible
   code branches away (typically, in optimized builds) or to trap them to prevent
   further execution (typically, in debug builds).

Notes
    Feature-test macro Value Std Feature
   __cpp_lib_unreachable 202202L (C++23) std::unreachable

Possible implementation
   [[noreturn]] inline void unreachable()
   {
       // Uses compiler specific extensions if possible.
       // Even if no extension is used, undefined behavior is still raised by
       // an empty function body and the noreturn attribute.
   #if defined(_MSC_VER) && !defined(__clang__) // MSVC
       __assume(false);
   #else // GCC, Clang
       __builtin_unreachable();
   #endif
   }

Example
// Run this code
 #include <cassert>
 #include <cstddef>
 #include <cstdint>
 #include <utility>
 #include <vector>
 struct Color { std::uint8_t r, g, b, a; };
 // Assume that only restricted set of texture caps is supported.
 void generate_texture(std::vector<Color>& tex, std::size_t xy)
 {
     switch (xy)
     {
     case 128: [[fallthrough]];
     case 256: [[fallthrough]];
     case 512: /* ... */
         tex.clear();
         tex.resize(xy * xy, Color{0, 0, 0, 0});
         break;
     default:
         std::unreachable();
     }
 }
 int main()
 {
     std::vector<Color> tex;
     generate_texture(tex, 128); // OK
     assert(tex.size() == 128 * 128);
     generate_texture(tex, 32); // Results in undefined behavior
 }

Possible output:
 Segmentation fault

See also
                        specifies that the expression will always evaluate to
   [[assume(expression)]](C++23) true at a given point
                        (attribute specifier)
   assume_aligned informs the compiler that a pointer is aligned
   (C++20) (function template)
   C documentation for
   unreachable
   External Links
   1. GCC docs: __builtin_unreachable
   2. Clang docs: __builtin_unreachable
   3. MSVC docs: __assume