Fix Issue 21543 - dmd infinite loop on alias this and std.typecons.Nu…

[BorisCarvajal]

…llable

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21543	normal	dmd infinite loop on alias this and std.typecons.Nullable

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#12157"

[RazvanN7]

@BorisCarvajal Can you please briefly explain what the issue is and how does this patch fix it?

[BorisCarvajal]

There is a infinite loop, the second condition handles an alias this of the same type as its parent (like: T1 {alias T1 this}), but doesn't check the case of a cyclic alias this, something like T1 {alias T2 this}, T2 { alias T1 this }.

[BorisCarvajal]

@RazvanN7, What's the deal with cirrus-ci coverage failing? My other pulls have the same problem.

->  Pinging Codecov
https://codecov.io/upload/v4?package=bash-20210115-cec3c92&token=secret&branch=pull%2F12157&commit=1b7909da1809af37d2123f0cc33ad8f3b6fe679f&build=5058485912076288&build_url=&name=&tag=&slug=dlang%2Fdmd&service=cirrus-ci&flags=&pr=12157&job=macOS10.15x64,DMD(coverage)&cmd_args=p,Z
HTTP 400
Please provide the repository token to upload reports via `-t :repository-token`
400

[MoonlightSentinel]

@BorisCarvajal Should be fixed now. Restarted your builds.

[BorisCarvajal]

@MoonlightSentinel Thanks!