Unexpected safety check error in call expression

[tsandall]

Given a trivial rule:

g(x)

The following rule is considered unsafe:

> f(x) { g(x[_]) }
1 error occurred: 1:1: rego_unsafe_var_error: var _ is unsafe

But the others are not:

> f(x) { g(true, x[_]) }
> f(x) { g(x[_], true) }
> f(x) { g(x[_]) = y }