[DNM] Protocol extensions with conformances?

[johnno1962]

Hi Apple,

Another speculative PR, this time exploring the feasibility of allowing conformances when declaring a protocol extension. The implementation is functional but at the stage where it could do with some guidance from someone who knows what they are doing. The new feature can be used to give behaviour to all nominals conforming to the protocol being extended. An example would be the following:

extension FixedWidthInteger: ExpressibleByUnicodeScalarLiteral {
    public init(unicodeScalarLiteral value: Unicode.Scalar) {
      self = Self(value.value)
    }
}

Thereafter strings could be converted to fixed width integers.

Most of the compiler mechanics required is already in place in the existing implementation of protocol extensions and this PR is largely based around a relatively minor change adapting the function ConformanceLookupTable::addInheritedProtocols which looks after implying the extension's conformance to any nationals conforming to the protocol being extended and tracking additional witness tables that need to be emitted. There is one limitation in that usage of nominals acquiring the new conformance must be in the same module as an extension of the protocol being extended. A test has been added to demonstrate what is possible. As this is additive all tests run fine.

A toolchain is available for evaluation here

Resolves SR-11013.

[johnno1962]

There are seemingly irresolvable issues with this idea if you use it across files. See the JIRA