In this program the assignment to x1 is accepted, but the assignments to x2 and x3 are refused:

// Program #1.
void main() {
    int[200] data1;
    ubyte x1 = data1.length;
    int[300] data2;
    ubyte x2 = data2.length;
    auto data3 = new int[300];
    ubyte x3 = data3.length;
}


It gives (dmd 2.066alpha):

test.d(5,16): Error: cannot implicitly convert expression (300u) of type uint to ubyte
test.d(7,16): Error: cannot implicitly convert expression (data3.length) of type uint to ubyte


Likewise the x2 case, this code is refused statically:

// Program #2.
void main() {
    int[300] data;
    foreach (ubyte i, x; data)   {} // Error
}


test.d(3,5): Error: index type 'ubyte' cannot cover index range 0..300



But this code compiles and runs (and it seems to go in infinite loop):

// Program #3.
void main() {
    auto data1 = new int[300];
    foreach (ubyte i, x; data1) {}
    int[300] data2;
    foreach (ubyte i, x; data2[]) {}
}


I think accepting silently the ubyte <- size_t assignment is bad and it goes against the behavour we expect from D as shown in the x2 and x3 cases.

So I suggest to:

1) Give a warning in situations like program #3. (If you want you can also insert a run-time assert in non-release mode, that gards against the infinite loop with a clean run time error message. Perhaps Steven Schveighoffer has suggested something like this).

2) Turn the warning into a deprecation (and keep the run-time test, if already implemented).

3) Turn the deprecation into an error.

If you want you can skip the first step, and introduce a deprecation already.

See also Issue 9570