[SR-10568] Errors thrown by JSONDecoder don't have meaningful messages shown to Xcode

[NSExceptional]

Previous ID	SR-10568
Radar	None
Original Reporter	@NSExceptional
Type	Improvement

Attachment: Download

Additional Detail from JIRA

| | | |------------------|-----------------| |Votes | 0 | |Component/s | Foundation, Standard Library | |Labels | Improvement, Codable | |Assignee | None | |Priority | Medium | md5: 5be1df8769f1d742c91f8fdc5a7e5aec

Issue Description:

What Xcode shows you:

The data couldn’t be read because it is missing.

What the actual underlying error, which is actually useful:

No value associated with key CodingKeys(stringValue: \"age\", intValue: nil) (\"age\").

To see this error in detail, you have to jump through some hoops and print it in the debugger, when the Xcode test suite should just show you the more descriptive error in the first place:

do {
    let t: T = decoder.decode(T.self, ...)
} catch {
    let _ = error // breakpoint here, po error
}

I assume this is as simple as making the `.failureReason` more descriptive. I couldn't find where `DecodingError` was defined in `apple/swift-corelibs-foundation` or I would have checked.

[belkadan]

Can you attach a self-contained project or playground that demonstrates the issue? I'm not sure how to set up the test case such that you'd get this behavior.

[NSExceptional]

Yes, I've attached a reduced test case. Trying to build it has made me realize it only seems to happen in a test environment when failed tests are reported; other environments like Playgrounds or regular debugging do show the appropriate error. My apologies. I've just seen this error before and it never occurred to me that I might have only seen it while testing.

JSONDecoderErrorsTest.zip