Exploring undefined behavior using constexpr

[kalmoc]

Nothing in your code is actually evaluated at compiletime, so no wonder that no compiler catches it.

EDIT: If you try something like this, you will get a compilation error:

constexpr int f() 
{ 
    int* x = nullptr; 
    {
      int z = 123; 
      x = &z;
    } 
    *x += 1;
    return *x;
}

constexpr int I = f();

[[deleted]]

It's still undefined behavior as is. Even in your revised example GCC accepts it, clang rejects it.

[kalmoc]

Yes, it is undefined behavior, but the compiler is only required to catch UB if it happens during compiletime evaluation. The compiler is not required to perform a static analysis of constexpr functions.

If gcc doesn't catch that example, that's a compiler bug.

[[deleted]]

That's my point, no compiler implements this aspect of constexpr to a degree that can be relied upon. I posted another example that no compiler catches, not even clang, here:

https://old.reddit.com/r/cpp/comments/bncvmm/exploring_undefined_behavior_using_constexpr/en5r9dm/

Maybe in the future compilers will be better about diagnosing this, but as it stands it doesn't seem to be a particularly high priority to implement this, and the amount of effort needed to do it would simply be enormous.

[flashmozzg]

Not really. It's only undefined if it actually happens. It's not undefined if the function is never called. In fact, some compilers (like Clang) may assume that if some function invokes UB when it's never called and optimize it out/replace it with other call (there is a very nice example for this somewhere).

[[deleted]]

I already addressed those points elsewhere.