Issue Description:

I'm aware and very happy that this was fixed on master and will be fixed in Swift 5.3: #2713 ( https://bugs.swift.org/browse/SR-12275 https://bugs.swift.org/browse/SR-6163 ).

Things are not in sync between corelibs foundation and foundation on 5.2.4 though, which causes us a bunch of painful workaround and headaches - is there a chance that the patch will also apply to any 5.2.x if/when there's going to be one?

macOS, swift 5.2.4, good:

Welcome to Apple Swift version 5.2.4 (swift-5.2.4-RELEASE).
 1> import Foundation
 3> JSONDecoder().decode(Int.self, from: "23".data(using: .utf8)!)
$R1: Int = 23

ubuntu, swift 5.2.4, bad:

Welcome to Swift version 5.2.4 (swift-5.2.4-RELEASE).
1> import Foundation
3> let s = JSONDecoder().decode(Int.self, from: "42".data(using: .utf8)!)
s: Int = 0 // oh oh...

–

Things are correct on Swift 5.3, however we're not really ready to require users to bump to that version, so we're stuck with building manually looking ahead at the bytes if it maybe it a top level value and trying to handle it "manually" today...