Parse error from string escapes in `elm.json`

[LiberalArtist]

Quick Summary: The \/ JSON string escape is not parsed correctly in elm.json files.

SSCCE

In elm.json:

{
    "type": "application",
    "source-directories": [
        "src"
    ],
    "elm-version": "0.19.1",
    "dependencies": {
        "direct": {
            "elm\/core": "1.0.5",
            "elm\/json": "1.1.3"
        },
        "indirect": {
        }
    },
    "test-dependencies": {
        "direct": {},
        "indirect": {}
    }
}

In src/Main.elm (though it doesn't really matter):

module Main exposing (..)
main : Program () () ()
main = Platform.worker
 { init = \_ -> ( (), Cmd.none )
 , update = \_ -> \_ -> ( (), Cmd.none )
 , subscriptions = \_ -> Sub.none }

Output of elm make src/Main.elm:

Dependencies ready!         
-- PROBLEM WITH DEPENDENCY NAME --------------------------------------- elm.json

I got stuck while reading your elm.json file. There is something wrong with this
dependency name:

9|             "elm\/core": "1.0.5",
                   ^
Package names always include the name of the author, so I am expecting to see
dependencies like "mdgriffith/elm-ui" and "Microsoft/elm-json-tree-view".

I generally recommend finding the package you want on the package website, and
installing it with the elm install command!

Changing:

• "elm\/core" -> "elm/core"
• "elm\/json" -> "elm/json"

produces instead:

Dependencies ready!         
Success! Compiled 1 module.

    Main ───> index.html

• Elm: 0.19.1
• Browser: n/a
• Operating System: Tested on Debian Bullseye, Kubuntu 21.10, and Guix

Additional Details

According to RFC 7159 §7, the syntax for JSON strings is:

      string = quotation-mark *char quotation-mark

      char = unescaped /
          escape (
              %x22 /          ; "    quotation mark  U+0022
              %x5C /          ; \    reverse solidus U+005C
              %x2F /          ; /    solidus         U+002F
              %x62 /          ; b    backspace       U+0008
              %x66 /          ; f    form feed       U+000C
              %x6E /          ; n    line feed       U+000A
              %x72 /          ; r    carriage return U+000D
              %x74 /          ; t    tab             U+0009
              %x75 4HEXDIG )  ; uXXXX                U+XXXX

      escape = %x5C              ; \

      quotation-mark = %x22      ; "

      unescaped = %x20-21 / %x23-5B / %x5D-10FFFF

so the concrete syntax "\/" should parse the same as "/".

I encountered this issue while experimenting with improvements to the support for Elm in Guix (a "purely functional" package manager à la Nix). The (guix build json) library chooses to always escape / using the \/ sequence (even though it is superfluous).

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