#20 did not resolve the compilation warning under Swift 4.2: https://travis-ci.org/IBM-Swift/TypeDecoder/jobs/495496619#L1496

The reason, I think, is as follows:

There is a difference between the compiler and language version: #if swift is talking about the language version, but that may be lower than the compiler version (in our case, 5.0). The language version is directed by the top line of the Package.swift (swift-tools-version).

For TypeDecoder this is 4.0. Swift Evolution SE-0212 suggests that this combination will yield a swift language version of 4.1, and hence the #if swift(>=4.2) test is failing.

The solution should be to update Package.swift to tools-version 4.2 (just changing the version in the first line should be sufficient), but we'll also need to duplicate it to Package@swift-4.0.swift and Package@swift-4.1.swift as well (retaining tools-version 4.0).