Why a deconstruction cannot mix declarations and expressions on the left

Question

I have this code:

public static (int a, int b) f12()
{
    return (1, 2);
}

public static void test()
{
    int a;
    (a, int b) = f12(); //here is the error
}

When I try to compile it I get an error:

A deconstruction cannot mix declarations and expressions on the left

I don't understand why. Any suggestions?

Answer 1

This is a more precise answer to my question:

May lead to occasional confusion, as in M((int x, y) = e) (declaring y)? [...]

Taken from Mixing fresh and existing variables in deconstruction mentioned by Brian's answer.