Incorrect result when variable self assignment is used

[cpizzolato]

I have noticed the following behaviour when assigning the result of an operation to a variable used in the operation itself:

#include <autodiff/forward.hpp>

int main() {

	using DoubleT = dual;
	DoubleT a = 1;
	a = a - 2*a;
	cout << "it's " << a << " should be -1\n"; // dual returns -4

	a = 1;
	a = a + 2 * a;
	cout << "it's " << a << " should be 3\n";  // dual returns 4

	a = 1;
	a = a - a;
	cout << "it's " << a << " should be 0\n";  // dual returns -2

	a = 1;
	a = a + a;
	cout << "it's " << a << " should be 2\n"; // dual returns 2

	a = 1;
	a = 2*a - a;
	cout << "it's " << a << " should be 1\n";  // dual returns -3

	a = 1;
	a = 2*a + a;
	cout << "it's " << a << " should be 3\n";  // dual returns 3

	a = 1;
	a = DoubleT{ 2 * a - a };
	cout << "it's " << a << " should be 1\n";  // dual correctly returns 1
}

[allanleal]

There is some aliasing happening there. dual is assuming by default that there is no aliasing (that is, it is not used on the right-hand side). In Eigen, one needs to explicitly state that there is no alias so that a temporary is not created. If we follow Eigen's convention, then we would need to write:

a.noalias() = 2*x - y; // a is not used on the right-hand side and thus we can avoid a temporary
a = 2*x - y; // this also works, but a temporary is created to hold 2*x - y before assigning to a
a = 2*a - a; // a is used on the right-hand side, but now a temporary holding the result prevents a from changing in the middle of the evaluation.

[allanleal]

Just submitted a pull request with the fix. It should be merged to the master branch soon.

[allanleal]

I'll close this issue now that a bug fix has been implemented. Thanks a lot for reporting.