This is with Vala 0.7.7:

delegate void Delegate();

void call_delegate(Delegate d) {
    d();
}

void heads() {
    debug("heads");
}

void tails() {
    debug("tails");
}

void main() {
    Rand prng = new Rand();
    
    Delegate d = prng.boolean() ? heads : tails;
    
    call_delegate(d);
}

Produces this error output:

test.vala:19.35-19.39: error: Assignment: Cannot convert from `heads' to `heads'
    Delegate d = prng.boolean() ? heads : tails;
                                  ^^^^^
test.vala:19.43-19.47: error: Assignment: Cannot convert from `tails' to `tails'
    Delegate d = prng.boolean() ? heads : tails;
                                          ^^^^^
test.vala:19.18-19.31: error: Incompatible expressions
    Delegate d = prng.boolean() ? heads : tails;
                 ^^^^^^^^^^^^^^
test.vala:19.14-19.47: error: expression type not allowed as initializer
    Delegate d = prng.boolean() ? heads : tails;
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
test.vala:21.19-21.19: error: The name `d' does not exist in the context of `main'
    call_delegate(d);
                  ^
Compilation failed: 5 error(s), 0 warning(s)
make: *** [test] Error 1

However, if I replace the ternary operator with a traditional if-else statement:

    Delegate d = null;
    if (prng.boolean())
        d = heads;
    else
        d = tails;

Compiles and runs fine.