Fixes issues 16271, 18825, 8065, 5050 - Allow function literals return by reference

[BorisCarvajal]

respective change in spec dlang/dlang.org#2618

[dlang-bot]

Thanks for your pull request and interest in making D better, @BorisCarvajal! 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 annotated coverage diff directly on GitHub with CodeCov's browser extension
• 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
✓	5050	normal	No way to declare delegates with ref return
✓	8065	enhancement	No way to write function/delegate literals returning ref T
✓	16271	enhancement	Should be able to express that a lambda returns by reference
✓	18825	enhancement	No syntax for function literal returning a reference

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 "master + dmd#9558"

[thewilsonator]

Please take a look at the buildkite ocean failures

./src/ocean/transition.d(42): Error: basic type expected, not `static`
--
  | ./src/ocean/transition.d(42): Error: found `static` when expecting `)`
  | ./src/ocean/transition.d(42): Error: semicolon expected following auto declaration, not `immutable`
  | ./src/ocean/transition.d(42): Error: declaration expected, not `)`
  | ./src/ocean/meta/types/Qualifiers.d(73): Error: basic type expected, not `static`
  | ./src/ocean/meta/types/Qualifiers.d(73): Error: found `static` when expecting `)`
  | ./src/ocean/meta/types/Qualifiers.d(73): Error: found `immutable` when expecting `)`
  | ./src/ocean/meta/types/Qualifiers.d(73): Error: found `)` when expecting `;`
  | ./src/ocean/meta/types/Qualifiers.d(73): Error: found `)` instead of statement
  | ./src/ocean/meta/types/Qualifiers.d(169): Error: basic type expected, not `static`
  | ./src/ocean/meta/types/Qualifiers.d(169): Error: found `static` when expecting `)`
  | ./src/ocean/meta/types/Qualifiers.d(169): Error: found `immutable` when expecting `)`
  | ./src/ocean/meta/types/Qualifiers.d(169): Error: no identifier for declarator `U`
  | ./src/ocean/meta/types/Qualifiers.d(169): Error: declaration expected, not `)`
  | ./src/ocean/meta/types/Qualifiers.d(173): Error: declaration expected, not `else`
  | ./src/ocean/meta/types/Qualifiers.d(176): Error: unrecognized declaration
  | ./src/ocean/core/Test.d(230): Error: semicolon expected following function declaration
  | ./src/ocean/core/Test.d(232): Error: declaration expected, not `if`

[JinShil]

Please take a look at the buildkite ocean failures

This may have been caused by Ocean's recent 5.0.0 release.

[BorisCarvajal]

I checked and the error is the same with stable.

[PetarKirov]

Thanks for working on this @BorisCarvajal!

[RazvanN7]

Could you please add the text "Fix Issue" followed by the issue numbers in a commit so that the dlang bot automatically links the issues with this PR?

[thewilsonator]

Specifically the commit message should contain "Fixes issues 16271, 18825, 8065, 5050", i.e. make it the same as the PR title.

[BorisCarvajal]

done

[jacob-carlborg]

• It looks like all examples are using =>. Perhaps add some examples with using curly braces.
• What about declaring a variable of a delegate type that returns ref, is that already possible?

[BorisCarvajal]

• I just tested the refness of the returns, the syntax is fairly proved already.
  I was going to put some checks with those but I thought was overkill, I'll add a few anyways.
• Yes, you can do:

int x;
ref int dg() { return x; }
auto d = &dg; // or alias d = dg;
d() = 2;

[jacob-carlborg]

Make sure the commits are squashed when merging.

[thewilsonator]

@ZombineDev I had the spec PR on 72h -> merge, I don't think we need to wait for both.

[wilzbach]

@BorisCarvajal as this is definitely a new feature that DMD users should be made aware of (e.g. so that they can update their D grammar/parsers), it would be great if you could follow up with a small changelog entry:

https://github.com/dlang/dmd/tree/master/changelog

Thanks!

This also will help to expose your work to the wider D community.

[andralex]

Great work. Thanks @BorisCarvajal !