Interfaces: require external visibility

[fulldecent]

The Ethereum ABI specification does not make a distinction between external and public functions. Therefore the Interface keyword likewise should not so distinguish.

Currently private and internal are disallowed in interfacess.

For clarity, only external functions should be allowed in interfaces.

[chriseth]

@axic why did we not disallow public in interfaces?

[axic]

I thought it only supports external. I think it only should support external.

Removing the visibility specifier would be a bad idea because we require it everywhere else (starting 0.5.0) and that would create an exception, which could be confusing.

[fulldecent]

Understood, issue updated

[axic]

Actually it seems this is a duplicate of #2330.

[fulldecent]

This issue regards requiring external in Interface functions.

Issue #2330 regards subclassing ("implement"?, "overriding"?) interfaces with a public function.

[axic]

Right, it seems to be a mix, though there's this also: #2330 (comment)

[axic]

Actually #3038 has implemented this a while back for 0.5.0 mode.

We can however make public a warning now (that change allowed public in non-0.5.0 mode).