Print range computed by expression range analysis in error messages

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2013-06-23T13:11:24Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=10455

Description

A low-priority enhancement request.


For this code:

void main() {
    ubyte x;
    ubyte y = x << 1;
}


The range analysis determines that it's conceivable to the result of that expression to not fit in y, so the D compiler 2.064alpha gives:


temp.d(3): Error: cannot implicitly convert expression (cast(int)x << 1) of type int to ubyte


To help the programmer understand faster the mistake in his/her code when expressions become more complex I think it's also useful to print the range resulting from the analysis, like:


temp.d(3): Error: cannot implicitly convert expression (cast(int)x << 1) in interval [0 ... 510] of type int to ubyte


It uses 3 dots because it's an interval that includes the right end. Otherwise if you print an interval open on the right in a case like this you have to print a ulong.max+1 value:


void main() {
    ulong x;
    int y = x;
}


One downside: those error messages become a little longer (and a bit more noisy).

[dlangBugzillaToGithub]

bearophile_hugs commented on 2013-06-23T14:08:52Z

> temp.d(3): Error: cannot implicitly convert expression (cast(int)x << 1) in
> interval [0 ... 510] of type int to ubyte

Jonathan M Davis suggests to use a more mathematical syntax to denote an interval closed on the right, like:

temp.d(3): Error: cannot implicitly convert expression (cast(int)x << 1) in interval [0, 510] of type int to ubyte