Fix Issue 20842 - Structs with disabled default/copy ctors can't be i…

[pbackus]

…nitialized

A struct must now have at least one non-@disabled constructor to be
ineligible for brace initialization.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20842	normal	Structs with disabled default/copy ctors can't be initialized

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#11159"

[thewilsonator]

Use

scope v = new HasNonDisabledCtorVisitor();

[thewilsonator]

if (!ctor)
     return false;

static extern (C++) class HasNonDisabledCtorVisitor : Visitor
{
    ...
}
scope v = new HasNonDisabledCtorVisitor();
ctor.accept(v);
return v.result;

[Geod24]

Thanks a lot! This bug is indeed very annoying.
Two things:

• Can you use scope instead of auto for the visitor ? auto leads to GC allocation, while scope means it will be class allocated, so it matters quite a bit.
• Can you add similar tests for union ?

The other comments are purely stylistic.

[Geod24]

It's the default value, no need to initialize it

[Geod24]

Suggested change

	if (ctor)
	{
	auto v = new HasNonDisabledCtorVisitor();
	ctor.accept(v);
	return v.result;
	}
	else
	{
	return false;
	}
	if (!ctor)
	return false;
	scope v = new HasNonDisabledCtorVisitor();
	ctor.accept(v);
	return v.result;

In general, we prefer to use early returns like this, to get the obvious case "out of the way" and simplify the flow, as well as reduce the amount of unneeded indentation.

[Geod24]

Here and in the OverloadSet overload, there is a 100% overhead in vertical space because of the optional {}.
Personally not found of it, but if you feel strongly about keeping it, it's fine.

[pbackus]

@Geod24 @thewilsonator Thanks for the feedback; I've made the changes you suggested.

[Geod24]

Nice, LGTM

[pbackus]

CI failures appear to be spurious (timeouts).

[RazvanN7]

This PR has introduced a regression: https://issues.dlang.org/show_bug.cgi?id=21547 .

I have looked at this PR and the problem is that the visitor does not check for all overloads. It simply looks at the first ctor declaration.