C++ `__enzyme_register_derivative` with return types

I'm trying to register a custom forward-mode derivative using `__enzyme_register_derivative` in C++, building on the [customfwd](https://github.com/EnzymeAD/Enzyme/blob/main/enzyme/test/Integration/ForwardMode/customfwd.c) test/example from the Enzyme repository.

Depending on the return type of the differentiated function, I encounter different assertion failures during compilation:
- `int`: Assertion failed: (returnUsed), function CreateForwardDiff, file EnzymeLogic.cpp, line 4790.
- `double` :  Assertion failed: (Ty && "Invalid GetElementPtrInst indices for type!"), function checkGEPType, file Instructions.h, line 942.

Here is the minimal reproducible example:
```cpp
#include
#include

// Assertion failed: (returnUsed), function CreateForwardDiff, file EnzymeLogic.cpp, line 4790.
// using T = int;

// Assertion failed: (Ty && "Invalid GetElementPtrInst indices for type!"), function checkGEPType, file Instructions.h, line 942.
using T = double;

T __enzyme_fwddiff(T (*)(T *), T *, T *);

T square(T *x) { return (*x) * (*x); }

int derivative = 0;
T derivative_square(T *x, T *dx) { derivative++; return (T) 100; }

void* __enzyme_register_derivative_square[] =
{
(void*)square,
(void*)derivative_square,
};

T dsquare(T *x, T *dx) { return __enzyme_fwddiff((void*)square, enzyme_dup, x, dx); }

int main()
{
T x = 3, dx = 1;
T res = dsquare(&x, &dx);

assert(derivative == 1);
assert(res == 100);

return EXIT_SUCCESS;
}