redefinition of '...' as different kind of symbol when trying to use an interface more than once

[joergbrech]

This example on cppx.godbolt introduces an interface that injects code into a class. If I try to use the interface twice, e.g. by defining a second class Foo after Shape like this:

class Foo(interface) {};

I get a compilation error:

error: redefinition of 'interface' as different kind of symbol

https://cppx.godbolt.org/z/MMx1jxoqf

At first sight this looks like a parsing issue to me (class interface instead of class(interface)), but then I wonder why it works for the first use....

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Use Case: I tried to implement an interface equality_comparable that injects a bool operator==()(X const& other) method into a class, similar to the #[derive(PartialEq)] attribute in rust or the defaulted comparison operators (as of C++20).

Injecting the code into the classes directly works: https://cppx.godbolt.org/z/W5hzojM6o, injecting via interface fails with the above error: https://cppx.godbolt.org/z/nor384hWb

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PS: This is an awesome powerful proposal. I would love to see it happen!