[Issue 5906] Just pre-conditions at compile-time when arguments are static

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5906



--- Comment #4 from bearophile_hugs at eml.cc 2012-12-25 03:42:02 PST ---
If I write:

void main() {
    byte x = 200;
}


The compiler refuses that code statically, it means it performs a run-time test
of the value:
test.d(2): Error: cannot implicitly convert expression (200) of type int to
byte


But if I write code like this:

void main(string[] args) {
    byte x = args.length;
}


It does not perform that test at compile-time, because it can't, the value is
known only at run-time.


What I'm trying to archive here is a way to emulate that built-in behavour in
user-defined types. This means if I define a MyByte type (that is an integral
struct value with the same admissible values interval as a signed byte), I'd
like a way to define MyByte contracts so this code produces a compile-time
error in the first assignment and a run-time error in the second assignment:


void main(string[] args) {
    MyByte x = 200; // compile-time error here
    MyByte x = 200 + args.length; // run-time error here
}


That's why I have suggested a "static in". But according to your answer my
solution is not good enough.

Other better ideas are welcome.

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