fix issue 18415 - Typedef ignores @disabled default constructor

[ghost]

The problem is only fixable for struct and union and not class, since Typedef is a struct.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18415	normal	Typedef ignores @disabled default constructor

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 + phobos#6514"

[MetaLang]

I don't quite understand; why can't Typedef "forward" the @disable'd default constructor if T is a class?

[ghost]

For a class that has its default this disabled you can still declare a variable without initializer.
For a struct this is not the case.
Since the Typedef of a class is a struct this would end up with this scenario:

class A{@disable this();}
alias TA  = Typedef!A;

A a; // ok;
TA ta; // compile error, because Typedef wraps A in a struct with default construction disabled.

[MetaLang]

Ah, makes perfect sense. Thanks @bbasile.