Fix Issue 17970 - shared struct destructor doesn't compile anymore

[RazvanN7]

The problem here is that if a struct is declared as shared the dtor is automatically considered to be shared, but when the struct is instantiated the instance is no longer considered to be shared when the function call matching is done. The fix makes it so that if a struct declaration is shared, when the destructor is called, the instantiated struct is also considered shared.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	17970	regression	shared struct destructor doesn't compile anymore

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

[wilzbach]

See also: #7566 (for disallowing shared destructors)

[wilzbach]

if (auto dt = fd.isDtorDeclaration())

[wilzbach]

What's about the case of shared_this && shared_dtor?

[RazvanN7]

If either the struct/class declaration is shared or the destructor is shared, it makes sure that when the matching is done, both are considered shared.

[wilzbach]

(I asked because such motivation belongs into the source code - it will help people in the future and also the review of this PR)

[wilzbach]

The problem here is that if a struct is declared as shared the dtor is automatically considered to be shared, but when the struct is instantiated the instance is no longer considered to be shared when the function call matching is done. The fix makes it so that if a struct declaration is shared, when the destructor is called, the instantiated struct is also considered shared.

This explanation should be in the source code ;-)

[andralex]

I'll approve this because it doesn't compete with #7566 and unblocks other students' work.

[jacob-carlborg]

This explanation should be in the source code ;-)

Putting it in the commit message is also good. When a PR only consists of one commit GitHub will automatically use the commit messages as the title and description for the PR.