@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
}