[Issue 21298] Missing error when overriding interface method without in contract with class method with contract

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21298

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
Changing I to a class still does not cause the error. A function body has to be
provided to I.foo() to trigger the error.

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