Variant concat of char and string doesn't work

[dlangBugzillaToGithub]

simen.kjaras reported this on 2019-07-05T11:25:16Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=20030

CC List

• bugzilla (@WalterBright)
• shove

Description

The following code throws a VariantException complaining about different types (char and immutable(char)[]):

unittest {
    import std.variant;
    Variant v = "";
    v ~= 'a';
}

It's correct in some ways, but clearly a more useful behavior would be to concat the string and char and move on.

[dlangBugzillaToGithub]

shove commented on 2019-07-08T02:10:01Z

(In reply to Simen Kjaeraas from comment #0)
> The following code throws a VariantException complaining about different
> types (char and immutable(char)[]):
> 
> unittest {
>     import std.variant;
>     Variant v = "";
>     v ~= 'a';
> }
> 
> It's correct in some ways, but clearly a more useful behavior would be to
> concat the string and char and move on.

This shouldn't be a bug. Variant's design has strict restrictions on type qualifiers, because string = immutable (char) [], so the ~ operator restricts char. v ~= cast (immutable char)'a'; is possible because its qualifier is consistent.

For these limitations, you can see the unittest:
https://github.com/dlang/phobos/blob/master/std/variant.d#L2655

There are many anomaly collection behaviors for qualifier inconsistencies.

I don't know whether it was intentionally designed to do this, or because it was temporarily done for the sake of simplicity. I think, if there are no other factors, it would be better to fix the variant implementation and remove these restrictions.