Why is (int[int] s = int[int].init) not allowed

[Andre Pany via Digitalmars-d-learn]

Hi,

I am really confused, why is this valid:
void sample(string[string] s = string[string].init){}

while this causes syntax errors?

void sample_invalid1(double[string] s = double[string].init){}
void sample_invalid2(int[int] s = int[int].init){}

Kind regards
André