std:: unreachable

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Program support utilities
Defined in header <utility>
[ [ noreturn ] ] void unreachable ( ) ;
(since C++23)

Invokes undefined behavior at a given point.

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

[[ assume ( expression )]] (C++23) specifies that the expression will always evaluate to true at a given point
(attribute specifier)
(C++20)
informs the compiler that a pointer is aligned
(function template)
C documentation for unreachable

External Links

1. GCC docs: __builtin_unreachable
2. Clang docs: __builtin_unreachable
3. MSVC docs: __assume
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