Comment by Garo Hussenjian (JIRA)

Here are some details from our crash report..

Fatal Exception: NSInvalidArgumentException
-[_SwiftValue _fastCStringContents:]: unrecognized selector sent to instance 0x280bfc5c0

Fatal Exception: NSInvalidArgumentException
0 CoreFoundation 0x1eacd3ef8 __exceptionPreprocess
1 libobjc.A.dylib 0x1e9ea1a40 objc_exception_throw
2 CoreFoundation 0x1eabeb154 -[NSOrderedSet initWithSet:copyItems:]
3 CoreFoundation 0x1eacd9810 _forwarding_
4 CoreFoundation 0x1eacdb4bc _CF_forwarding_prep_0
5 libswiftCore.dylib 0x104130314 *makeCocoaStringGuts(*🙂
6 libswiftCore.dylib 0x104130030 String.init(_cocoaString🙂
...

I suspect that what's happening is that `reason` is causing the entire Dictionary to be inferred with `StaticString` keys, since `StaticString` is ExpressibleByStringLiteral and the type checker favors forming a dictionary with one value type over one with `Any` or some other polymorphic type. The `localizedDescription` property implementation expects the value of NSLocalizedDescriptionKey to be a `String`. This is all working "correctly" as intended, though the combination of behaviors here is definitely surprising. You could do this to work around the issue:

```
import Foundation

let reason: StaticString = "Static String"
let userInfo: [String: Any] = [
NSLocalizedDescriptionKey: "Localized Description",
"Reason": reason
]
let error = NSError(domain: "Some Error", code: 0, userInfo: userInfo)
print(error.localizedDescription)
```

By explicitly giving the dictionary `Any` as its value type, "Localized Description" is allowed to default to String, even though reason is a StaticString.

Comment by Garo Hussenjian (JIRA)

Fascinating, thanks @jckarter