Fix Bugzilla Issue 24818 Sumtype with single-type wastes space

[Inkrementator]

Github issue #10563

I tried doing this with a template mixin that takes the new tag as an alias parameter to avoid having to deplicate the static check all over the place, but that gave me deprecation warnings about assigning a variable to itself and also failed the unittest in line 936 where one postblit destructor didn't execute correctly ...

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	24818	normal	Tag of a single-type SumType wastes space

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#10642"

[pbackus]

Please rewrite this to use getter and setter functions for tag, so that we don't have to add checks for Types.length > 1 in a bunch of different places.

[Inkrementator]

Please rewrite this to use getter and setter functions for tag, so that we don't have to add checks for Types.length > 1 in a bunch of different places.

Some constructors are immutable, const and inout, which will not allow calling other functions that mutate the object (as far as I know).

I can do it for all other constructors if you prefer it.

I tried this first:

mixin template setTag(alias newTag)
{
    auto _ = (this.tag = newTag);
}

But this caused some unit tests to fail. I might eventually create a bug report for it, but I don't understand enough yet to isolate the error.

[Inkrementator]

https://github.com/Inkrementator/phobos/tree/sumtype_showerror

see this branch std/sumtype.d line 510 to see how this would fail with template mixin

[pbackus]

Some constructors are immutable, const and inout, which will not allow calling other functions that mutate the object (as far as I know).

I guess there's no better way to do it, then. Oh well.