Fix Bugzilla 14128 - AliasDeclaration allows expressions, causing false …

[ntrel]

…code for ThisExp

Deprecate aliasing a field of a variable.

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[RazvanN7]

ping @ntrel

[pbackus]

Do we also want to deprecate binding var.member to a template alias parameter, since it has the same behavior?

[WalterBright]

https://issues.dlang.org/show_bug.cgi?id=14128

[WalterBright]

We weren't going to add deprecations anymore, because we don't want to break existing code. I suggest making this an error, but put it behind a -preview switch. In the future, we can roll it into the "editions" feature.

[ntrel]

@WalterBright OK. I wanted to make it an obsolete feature instead of deprecated, but then -wo got neutered (https://github.com/dlang/dmd/pull/15612). I don't understand why that was done. So do we need a separate preview switch for each obsolete feature?

[WalterBright]

Until we get editions, yes, I don't see a better way.

[ntrel]

I think generally people would not really opt-in to a preview switch which only detects this. Given that we no longer have a switch to detect all obsoleted/bug-prone code, I think this should wait until we have editions. Is there a github label for PRs that need editions? Perhaps that could be added to this one.

[WalterBright]

Let's wait and see what the editions is going to be.

[ntrel]

Updated to use __edition_latest_do_not_use instead of deprecation. See also comment about changelog.