Compilation failure when using enzyme_out with matrix derivatives

[error_message.txt](https://github.com/user-attachments/files/24036995/error_message.txt)

Hi,

I'm working on using Enzyme to compute derivatives involving matrices, and I noticed an issue when using the `enzyme_out` annotation. Compilation fails in this case, but using `enzyme_dup` compiles successfully and produces correct results. I’m wondering whether this is a bug in Enzyme or related to Clang 16.

I've attached anexample below to reproduce the issue:

Code to reproduce

```cpp

#include
#include
#include
int enzyme_dup;
int enzyme_dupnoneed;
int enzyme_out;
int enzyme_const;

template < typename return_type, typename ... T >
return_type __enzyme_fwddiff(void*, T ... );

template < typename return_type, typename ... T >
return_type __enzyme_autodiff(void*, T ... );

double objective(Eigen::Matrix& x) {

Eigen::Matrix a = Eigen::Matrix::Ones();

a.array() += x[0];

Eigen::Matrix b = Eigen::Matrix::Ones();

b.array() += x[1];

Eigen::Matrix c;

c = b * a;
return c.sum();

}

void objective_ref(Eigen::Matrix& x, double& result) {

Eigen::Matrix a = Eigen::Matrix::Ones();

a.array() += x[0];

Eigen::Matrix b = Eigen::Matrix::Ones();

b.array() += x[1];

Eigen::Matrix c;
c = b * a;
result = c.sum();

}

struct double3{
double x, y, z;
};

void return_by_ref_reverse(){

Eigen::Matrix x;
x << 1.0, 3.0, 42.0;

double lambda = 1;
double z = 0;

// Fail to compile if using enzyme_out
double3 res = __enzyme_autodiff((void*)objective_ref,
enzyme_out, x,
enzyme_dup, &z, &lambda);
std::cout << "Return by reference reverse mode:" << std::endl;
std::cout << "d_x: "<< res.x << ", " << res.y << ", " << res.z << std::endl;

}

void return_by_value_reverse(){

Eigen::Matrix x;
x << 1.0, 3.0, 42.0;

Eigen::Matrix dx;
dx << 0.0, 0.0, 0.0;

// Fail to compile if using enzyme_out
auto res = __enzyme_autodiff((void*)objective,
enzyme_out, x);

std::cout << "Return-by-value reverse mode 2:" << std::endl;
std::cout << res << std::endl;
}

int main() {
return_by_value_reverse();
return_by_ref_reverse();

return 0;
}

```

The full error message is included in `error_message.txt`.

Thanks!