Fix issue 21253 - Can't compile Variant.visit!(...) with generic func…

[Biotronic]

…tion

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21253	normal	Can't compile Variant.visit!(...) with generic function

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7637"

[Biotronic]

Turns out, the tests on lines 2433-2437 probably should have referred to maybenumber all along, and were testing for the wrong thing.

[pbackus]

The nonmatch check is a breaking change, because visit currently ignores handlers that don't match any types. Here's a unit test that passes on master but breaks with this PR:

@system unittest
{
    Algebraic!(int, string) x = 123;
    assert(__traits(compiles,
        x.visit!(
            (int n) => "got int",
            (string s) => "got string",
            (char c) => "got char"
        )
    ));
}

[myCrack]

breaking change, because visit currently ignores handlers that don't match any types.

https://dlang.org/phobos/std_variant.html#.visit:

It is statically ensured that all held types of variant are handled across all handlers.

So, behavior from the master branch must be treated as a bug.

[pbackus]

I'm pretty sure that just means that you have to handle every type (which, indeed, is statically ensured on master), but the wording is certainly not as clear as it could be.