Fix JSONDecoder.decode(_,from:) behavior

[ypli-chn]

JSONDecoder.decode(_,from:) accepts a generic parameter, but it does not use any concrete type in practice.
This PR fixes that use parameter type instead of T.self.

This PR solves #4806

Example Code

import Foundation

class Base: Codable { }
class Child: Base { }

func dynamicType() -> Base.Type {
    return Child.self
}

let data = "{}".data(using: .utf8)!

let classType = dynamicType()
let instance = try JSONDecoder().decode(classType, from: data)
print("type: \(instance) => \(type(of: instance))")

on macOS:

type: JSONDecoderExample.Child => Child

on Linux:

type: JSONDecoderExample.Base => Base

[lxbndr]

Would you mind adding a test case to the TestJSONEncoder? This is a severe regression, and we would like to catch similar issues in future.

[ypli-chn]

Sure. I added a method named test_childTypeDecoder to test this issue.

[glessard]

Thank you for the contribution!

[glessard]

@swift-ci please test

[glessard]

The discrepancy you describe in #4806 has probably been present for a long time. It appears that it is fixed already in https://github.com/apple/swift-foundation, which will become the Foundation implementation for the Linux and Windows toolchains at a future point. I will make sure that a test like this exists in swift-foundation.

[glessard]

@swift-ci please test windows platform

[glessard]

The macOS test failure is a known issue.

[glessard]

Circling back, the behaviour fixed here is tested for in the following test of swift-foundation:
https://github.com/apple/swift-foundation/blob/62500a5d7921cc61703c2a1b4c961adb12dc80c3/Tests/FoundationEssentialsTests/JSONEncoderTests.swift#L680