@nogc with assoc array

[Jonathan Marler via Digitalmars-d-learn]

Why is the 'in' operator nogc but the index operator is not?

void main() @nogc
{
    int[int] a;
    auto v = 0 in a; // OK
    auto w = a[0];   // Error: indexing an associative
                     // array in @nogc function main may
                     // cause GC allocation
}