fix Issue 21546 - covariant return checks for functions wrong if retu…

[WalterBright]

…rning by ref

This is an ugly problem. How did it go undetected this long?

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21546	critical	covariant return checks for functions wrong if returning by ref

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

To target stable perform these two steps:

1. Rebase your branch to upstream/stable:

git rebase --onto upstream/stable upstream/master

2. Change the base branch of your PR to stable

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 + dmd#13722"

[WalterBright]

This fix may break existing code, so I'm not going to target stable with it.