Cannot use dictionary in function head

[jc-m]

in this use case:
x {
"a": 1,
"b": 2
}

w = x.a {true}
w returns 1

r(obj) = x.a {true}
fails with:
1 error occurred: 1:1: rego_unsafe_var_error: var data is unsafe

the following form works:
r(obj) = num {true; num = x.a}