Conformance to RawRepresentable prevents static members from being initialized

[lufinkey]

Description

I have a short piece of code here that works perfectly when it doesn't conform to RawRepresentable. However, if it conforms to RawRepresentable, it crashes with EXC_BREAKPOINT when accessing the static member dictionary (when .rawValue is called). Basically seems like the static member is never getting initialized.

I tested this in the swift playground.

Reproduction

enum NormalEnum: RawRepresentable {
	case blue
	
	private static let mapping: [Self: String] = [
		.blue: "helloworld"
	]
	
	var rawValue: String {
		return Self.mapping[self] ?? "what" // fallback here just to be sure it isn't crashing from nil unwrap
	}
	
	init(rawValue: String) {
		if rawValue == "helloworld" {
			self = .blue
		} else {
			fatalError("wrong rawValue")
		}
	}
}

print("blue = \(NormalEnum.blue.rawValue)")

Expected behavior

The program should print blue = helloworld and not crash

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Target: arm64-apple-macosx15.0

Additional information

No response

[lufinkey]

I just checked, and this also happens in a normal NSApplication. I added the print line in my applicationDidFinishLaunching function to test it.

[tbkka]

If you run this under LLDB and look at the backtrace, you'll see something interesting:

  * frame #0: 0x00000001825a8c28 libdispatch.dylib`_dispatch_once_wait.cold.1 + 200
    frame #1: 0x0000000182575c48 libdispatch.dylib`_dispatch_once_wait + 60
    frame #2: 0x0000000100000d68 enum_raw`NormalEnum.mapping.unsafeMutableAddressor at enum_raw.swift:6:21
    frame #3: 0x0000000100000ab4 enum_raw`NormalEnum.rawValue.getter() at enum_raw.swift:11:15
    frame #5: 0x0000000195c08f30 libswiftCore.dylib`Swift.RawRepresentable< where τ_0_0: Swift.Hashable, τ_0_0.Swift.RawRepresentable.RawValue: Swift.Hashable>.hash(into: inout Swift.Hasher) -> () + 212
    frame #7: 0x0000000195bd3b1c libswiftCore.dylib`Swift.RawRepresentable< where τ_0_0: Swift.Hashable, τ_0_0.Swift.RawRepresentable.RawValue: Swift.Hashable>._rawHashValue(seed: Swift.Int) -> Swift.Int + 160
    frame #9: 0x0000000195c475ac libswiftCore.dylib`Swift.__RawDictionaryStorage.find<τ_0_0 where τ_0_0: Swift.Hashable>(τ_0_0) -> (bucket: Swift._HashTable.Bucket, found: Swift.Bool) + 60
    frame #10: 0x0000000195bd408c libswiftCore.dylib`Swift.Dictionary.init(dictionaryLiteral: (τ_0_0, τ_0_1)...) -> Swift.Dictionary<τ_0_0, τ_0_1> + 772
    frame #11: 0x0000000100000d08 enum_raw`one-time initialization function for mapping at enum_raw.swift:6:47
    frame #12: 0x000000018258da48 libdispatch.dylib`_dispatch_client_callout + 16
    frame #13: 0x0000000182576a60 libdispatch.dylib`_dispatch_once_callout + 32
    frame #14: 0x0000000100000d68 enum_raw`NormalEnum.mapping.unsafeMutableAddressor at enum_raw.swift:6:21
    frame #15: 0x0000000100000ab4 enum_raw`NormalEnum.rawValue.getter() at enum_raw.swift:11:15
    frame #16: 0x0000000100000990 enum_raw`main at enum_raw.swift:23:33
    frame #17: 0x0000000182379b5c dyld`start + 6904

Reading from the bottom, you see that someone calls rawValue.getter() to get the raw value, which triggers the construction of the static property. But the static property needs to hash the value .blue in order to build the dictionary, which requires getting the raw value. This crashes because it's impossible: You can't have the dictionary depend on the raw value if the raw value calculation requires a dictionary.

There are a few ways you might rework this to use the same basic idea but without the recursive deadlock.

But no, this does not appear to be a bug in Swift.