Found while looking into issue 20047.

Should compile:

----
int g = 0;
void impure_function_1() { ++g; }
void function() get_impure_function() pure
{
    static void impure_function_2()
    {
        ++g; /* Error: pure function test.get_impure_function cannot access mutable static data g */
        impure_function_1(); /* Error: pure function test.get_impure_function cannot call impure function test.impure_function_1 */
    }
    return &impure_function_2;
}
void main()
{
    auto f = get_impure_function();
    f();
    assert(g == 2);
}
----

Just because `get_impure_function` is pure doesn't mean that `impure_function_2` also has to be pure.