noreturn

This __declspec attribute tells the compiler that a function does not return. As a consequence, the compiler knows that the code following a call to a __declspec(noreturn) function is unreachable.

If the compiler finds a function with a control path that does not return a value, it generates a warning (C4715) or error message (C2202). If the control path cannot be reached due to a function that never returns, you can use __declspec(noreturn) to prevent this warning or error.

Note
Adding __declspec(noreturn) to a function that is expected to return can result in undefined behavior.

In the following sample,the else clause does not contain a return statement. Declaring fatal as __declspec(noreturn) avoids an error or warning message.

// noreturn2.cpp
__declspec(noreturn) extern void fatal () {}

int main() {
   if(1)
     return 1;
   else if(0)
     return 0;
   else
     fatal();
}

Reference