are abstract / base specifiers permitted or required on class forward declarations?

[zygoloid]

Summary of issue:

Given this class:

base class C {}

how does one write a forward declaration? Is

base class C;

valid? Is

class C;

valid?

Details:

There doesn't seem to be a reason to require the abstract or base specifier to be present on a forward declaration. But usually our forward declaration syntax for classes is a prefix of the definition, and there also doesn't seem to be any particular hardship to developers in requiring the forward declaration to repeat the specifier.

In addition to the three answers suggested by the question (specifier is disallowed, specifier is allowed but not required, specifier is required), we could change the class definition syntax to put the specifier in the class body, as we did for inheritance:

class C {
  base class;
  extend base: B;
}

Any other information that you want to share?

No response

[chandlerc]

@justzh - Please keep comments on PRs and issues constructive and on-topic. I'm hiding yours, but you've had warnings now and so I will block if this continues.

[josh11b]

This question was discussed at the end of open discussion on 2024-05-09. I believe the conclusions were:

• modifiers on forward declarations should match those on extern declarations (except for the extern modifier)
• otherwise prefer not repeating modifiers
• prefer putting modifiers on definitions over forward declarations

[zygoloid]

This question was addressed by #3762 after consideration of alternatives:

• Other class, impl, and interface modifiers (abstract, base, final) exist only on the definition, not on the forward declaration.