Passing field alias to alias predicate doesn't work, predicate needs explicit instantiation

[dlangBugzillaToGithub]

Vladimir Panteleev (@CyberShadow) reported this on 2019-10-13T17:25:04Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20298

CC List

• RazvanN

Description

///////////// test.d /////////////
struct S
{
    int i;
    alias field = i;

    int fun(alias pred)() const
    {
        // Doesn't work:
        //return pred(field);

        // Works:
        alias ipred = pred!int;
        return ipred(field);
    }

    alias fun1 = fun!(i => i + 1);
}
//////////////////////////////////

Error is:

test.d(9,14): Error: template `test.S.__lambda5` cannot deduce function from argument types `!()(const(int)) const`, candidates are:
test.d(16,20):        `__lambda5`
test.d(16,15): Error: template instance `test.S.fun!((i) => i + 1)` error instantiating

[dlangBugzillaToGithub]

razvan.nitu1305 commented on 2019-10-15T12:37:10Z

Isn't the real bug here the fact that calling ipred passes compilation? As far as I could tell the lambda template instantion is inside the scope of the S struct => you cannot call it from fun, since fun may call only const functions and pred is mutable.

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2019-10-15T12:59:51Z

Thanks for looking at this!

(In reply to RazvanN from comment #1)
> As 
> far as I could tell the lambda template instantion is inside the scope of
> the S struct

Hmm! But, shouldn't it infer that since it doesn't access anything other than its arguments, it should not have a context?

I see that ipred is typed as a function not delegate.

> you cannot call it from fun, since fun may call only const
> functions and pred is mutable.

Thanks, I actually didn't realize that fun being const was a crucial part of this. I guess the error message could be better.