[Issue 10751] New: Propagate some value ranges from contracts

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10751

           Summary: Propagate some value ranges from contracts
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2013-08-03 04:28:59 PDT ---
I suggest to take a bit more seriously D contract programming, to increase a
little its usefulness.

So I think the following code should compile:


char foo(immutable uint digit)
in {
    assert(digit < 10);
} body {
    return digit + '0'; // Error.
}
uint bar()
out(result) {
    assert(result < 100);
} body {
    return 99;
}
void main() {
    ubyte x = bar(); // Error.
}


DMD 2.064alpha gives:

test.d(5): Error: cannot implicitly convert expression (digit + 48u) of type
uint to char
test.d(14): Error: cannot implicitly convert expression (bar()) of type uint to
ubyte


In the first case 'digit' is immutable and the pre-condition asserts it can't
be bigger than 9, so the result should be cast-able to char implicitly.

In the second case the post-condition tells that the result of 'bar' is an uint
that fits in the range of an ubyte.

(Probably if D contracts become a bit more useful their usage will increase a
little. Every useless cast removed from the code reduces the chance of bugs.)

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