Fix Issue 3332 - Mixin a constructor with a construct already present fails

[RazvanN7]

Previously, you were not allowed to add a constructor to an overload set because __ctor is specifically forbidden to be used for defining a symbol. However, this makes it impossible to add constructors defined in mixin templates to a constructor overload set. I see no harm in allowing this.

[dlang-bot]

Thanks for your pull request and interest in making D better, @RazvanN7! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	3332	normal	Mixin a constructor with a construct already present fails

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#14644"

[dkorpel]

What happens if you now alias a regular function to __ctor?

[RazvanN7]

Good question. It seems that it fails some asserts (form a first glance can easily be fixed), depending on the test case. However, it should not be possible to add normal functions to a constructor overload set.

It seems that this should be possible only in the case of mixing in a constructor. I wonder if we should allow that case.

[dkorpel]

It seems that this should be possible only in the case of mixing in a constructor. I wonder if we should allow that case.

Given the many bug reports, I think there should be something to facilitate it. I'm not a fan of exposing __ctor in user code however, and adding new special syntax for this doesn't sound great either (it doesn't help alias this already means something else).

[RazvanN7]

Well, technically, there is a workaround. Just add the alias after the constructor declaration (as pointed out in the bug report), but it's not a nice one.