enum FOO = 2;

void main() {
    for(uint i = 0; i < 2; i += FOO) {}
}

Results in a compile time error:

test.d(4): Error: variable FOO used before set

Marking as minor because it appears to be an extremely weird corner case bug.  The only place I could reproduce it was when the enum is used in a for loop like the above.

Also, changing FOO from enum to immutable resolves this.