Constructor ignores visibility modifier

[axic]

For any of the visibilities (internal, external, private, public):

contract Test {
    function Test() internal {
    }
}

the following ABI is generated: [{"inputs":[],"type":"constructor"}], and contract deployment works without hiccups.

I think it would make sense rejecting any visibility modifier on the constructor. Are there any cases currently where a non-public constructor is useful? We do not have static methods, so I don't see a way a non-public constructor could be executed.

[axic]

Fixing this will also "fix" #422.

[chriseth]

A non-public constructor is useful for base classes. We should enforce visibility and make contracts with non-public constructors abstract.

[chriseth]

An external constructor does not make much sense. The external keyword more or less forces all parameters to be in calldata, although constructor arguments are either in code or in memory. Furthermore, the other purpose of external is that such a function cannot be called internally. As constructors cannot really be "called" anyway, I would say it is fine to disallow external for constructors.

Therefore, I would say we should only allow internal and public for constructors.