Fix issue 19738 -- std.range.choose assignment breaks @safe-ty

[schveiguy]

This was more tricky than I originally thought, mostly due to the existing requirement that chooseAmong works with strings during CTFE.

Essentially, we only trust the fetching of the pointer to the chosen range, everything else should be inferred.

I'm not sure why I had to remove return scope, but that doesn't seem to work, I let the attributes get inferred. The error messages are totally unrelated to this code, it's just not correctly inferring safety with those attributes. But I don't really understand what the return scope did in the first place.

Should obsolete #6933

@thewilsonator @dnadlinger

[dlang-bot]

Thanks for your pull request, @schveiguy!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19738	regression	std.range.choose assignment breaks @safe-ty

Testing this PR locally

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

dub fetch digger
dub run digger -- build "stable + phobos#7348"

[thewilsonator]

Doc tester timed out please force push to restart.

[schveiguy]

I don't have much confidence that the macair is going to succeed on a third run. So I'll force push. Meanwhile, @braddr you have any ideas? It seems to time out quite a bit.