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[SR-15289] Empty dictionary unable to decode itself #57611

swift-ci opened this issue Oct 7, 2021 · 2 comments

[SR-15289] Empty dictionary unable to decode itself #57611

swift-ci opened this issue Oct 7, 2021 · 2 comments
bug A deviation from expected or documented behavior


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swift-ci commented Oct 7, 2021

Previous ID SR-15289
Radar None
Original Reporter SDGGiesbrecht (JIRA User)
Type Bug

New to Swift 5.5, attested on Ubuntu, CentOS and Amazon Linux (and proven not present on macOS)

Additional Detail from JIRA
Votes 1
Labels Bug
Assignee None
Priority Medium

md5: 0bbfd99264529883580e23a5166acfac

Issue Description:

For a key type that normally makes the dictionary encode as an array, empty instances end up encoded as empty dictionaries instead of empty arrays, leading to a type‐mismatch error when decoding.

struct Key: Codable, Hashable {
  var string: String
struct Value: Codable {
  var string: String
struct Context: Codable {
  var dictionary: [Key: Value] = [:]

let encoded = try JSONEncoder().encode([Context(dictionary: [Key(string: "..."): Value(string: "...")])])
print(String(data: encoded, encoding: .utf8))
// Optional("[{\"dictionary\":[{\"string\":\"...\"},{\"string\":\"...\"}]}]")
let decoded = try JSONDecoder().decode([Context].self, from: encoded)
// ✓ Works just fine.

let emptyEncoded = try JSONEncoder().encode([Context()])
print(String(data: emptyEncoded, encoding: .utf8))
// Optional("[{\"dictionary\":{}}]")
let emptyDecoded = try JSONDecoder().decode([Context].self, from: emptyEncoded)
// ✗ Throws: “typeMismatch(Swift.Array<Foundation.JSONValue>, Swift.DecodingError.Context(codingPath: [_JSONKey(stringValue: "Index 0", intValue: 0), CodingKeys(stringValue: "dictionary", intValue: nil)], debugDescription: "Expected to decode Array<JSONValue> but found a dictionary instead.", underlyingError: nil))”

I haven’t figured out yet if the problem comes from the Codable conformance of Dictionary in the standard library, the Codable synthesis in the compiler, or just JSONEncoder in Foundation.

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swift-ci commented Oct 18, 2021

Comment by Jeremy David Giesbrecht (JIRA)

I just tested the above code on macOS and it does not suffer from the problem; only the Linux flavours exhibit the issue.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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SDGGiesbrecht commented Apr 30, 2022

Gone as of 5.6. (Also duplicate of apple/swift-corelibs-foundation#3180.)

Can be closed by whomever has the relevant permissions.

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bug A deviation from expected or documented behavior
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3 participants