Hi @LebJe,

I have found a reproducible fatal crash in TOMLKit. Here's the minimum code that I have been able to reproduce the crash with:

import Foundation
import TOMLKit

let string = "[[apps.test]]"

struct Child: Codable {
    var value: String // if Child is an empty struct it doesn't crash
}

struct Parent: Codable {
    var apps: [String: Child] // if this is just `[String: String]` it doesn't crash
}

try? TOMLDecoder().decode(Parent.self, from: string)

The realworld use case I have that runs into this fatal crash (with incorrect TOML input) is decoding this Configuration struct from this string:

[[apps.HelloWorld]]
product = "HelloWorld"
version = "0.1.0"

The TOML is invalid because it's not meant to have double square brackets, however fatal crashes shouldn't occur even with invalid input.

I have done a bit of poking around and I think I have found the issue. TOMLKit thinks that the array at coding path apps.HelloWorld is both a table and an array (these values are from inside TOMLTable.keys):

(lldb) po tomlValue.array
▿ Optional<TOMLArray>
  ▿ some : [ { minimum_macos_version = '11', product = 'HelloWorld', version = '0.1.0' } ]

(lldb) po tomlValue.table
▿ Optional<TOMLTable>
  ▿ some : [ { minimum_macos_version = '11', product = 'HelloWorld', version = '0.1.0' } ]

For some reason casting the value to both an array and a table succeeds.

I have found a way to fix the crash: checking the type of the node before attempting to cast it so that these invalid type casts can't occur anymore.

I will submit a PR soon with the required changes (once I have run the tests).