Interfaces are useless, scrap them.

[fulldecent]

Interfaces have extremely limited utility and only academic use in real-world scenarios. While working on standardizing an interface, several shortcomings are apparent:

• Interfaces handle implicit getters differently than contracts Allow public constants in interfaces #3411
• Interfaces do not reason well about state mutability Inheritance: allow stricter mutability #3412
• Interfaces can't inherit Interfaces can't inherit #3419
• Interfaces don't allow default implementations Allow public constants in interfaces #3411

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To recognize the problem, see that none of the draft ERCs https://github.com/ethereum/solidity/issues/ are using interface to specify their standards. Instead, they resort to using functions inlined in markdown. And the functions are not even correct (missing public or other warnings). Clearly interface is not filling the needs of today's ERC writers.

Solution

Completely scrap the way interface keyword is understood. Here is the new definition of interface:

An interface is identical to a contract in all ways except that a) an interface cannot inherit from a contract and b) you cannot compile an interface.

Such a definition would be immediately useful and simpler to explain than the current mechanism.

Additional work:

• Create metadata for abstract contracts: Metadata is not created for abstract contracts #3437

[fulldecent]

I no longer think this is the correct solution to the interface problem. But I'll keep this issue open until the language redesign is complete as a reminder of the current inconsistencies.

[fulldecent]

Closing here. Those others issues that need to be open are open.

And there is an Inheritance Manifest #3729 (comment) which is a better collection of issues.