Swift-nio failing to build with Swift version 4.2

[justice-adams-apple]

Expected behavior

Expected the project to build successfully with Swift version 4.2

Actual behavior

When building with 4.2, the compilation fails with

swift-nio/Sources/NIOPosix/ServerSocket.swift:132:64: error: value of optional type 'String?' must be unwrapped to a value of type 'String'

Steps to reproduce

1. Git clone https://github.com/apple/swift-nio
2. Git checkout f5448fb
3. swift build --verbose --configuration release -Xswiftc -swift-version -Xswiftc 4.2

Note that the build succeeds when you use version 5. instead

SwiftNIO version/commit hash

f5448fbbc24495a6659ef64df4dd3c1fc4a0fb11

[justice-adams-apple]

This popped up when investigating swiftlang/swift-source-compat-suite#695

[dnadoba]

NIO 2.x.x only supports Swift 5.4 and newer. I think the right move will be to remove Swift 4.x support from the compatibility suite, at least for NIO 2.x

[justice-adams-apple]

@dnadoba Sure, I can do that. Thank you. Feel free to close this issue!