Code does not build when `associatedtype` inherits from Error

[nermolenko-fhl]

Hey! Faced a strange problem. The following code fails to compile:

protocol TestProtocol: AnyObject {
    associatedtype Err: Error
}

private extension TestProtocol {
    @AssociatedObject(.retain(.nonatomic))
    var test: Bool = false
}

The compilation error occurs when associatedtype Err: Error is present. If I remove : Error (so it becomes just associatedtype Err), the code builds successfully.

Sadly, but the error message is not helpful:

Command EmitSwiftModule failed with a nonzero exit code

[p-x9]

We were actually able to reproduce this error and obtained the following log

Redundant conformance requirements in signature <Self where Self : TestProtocol>:
Ours:
- Copyable
- Error
- Escapable
Theirs:
- Error
All requirements:
(requirement "Self" has_layout "AnyObject")
(requirement "Self" conforms_to "Copyable")
(requirement "Self" conforms_to "Escapable")
(requirement "Self.[TestProtocol]Err" conforms_to "Copyable")
(requirement "Self.[TestProtocol]Err" conforms_to "Error")
(requirement "Self.[TestProtocol]Err" conforms_to "Escapable")
Requirement machine for <τ_0_0 where τ_0_0 : TestProtocol>
Rewrite system: {
- [TestProtocol].[TestProtocol] => [TestProtocol] [permanent]
- [TestProtocol].Err => [TestProtocol:Err] [permanent]
- [TestProtocol].[Copyable] => [TestProtocol] [explicit]
- [TestProtocol].[Escapable] => [TestProtocol] [explicit]
- [TestProtocol:Err].[Copyable] => [TestProtocol:Err] [explicit]
- [TestProtocol:Err].[Escapable] => [TestProtocol:Err] [explicit]
- [TestProtocol].[TestProtocol:Err] => [TestProtocol:Err]
- τ_0_0.[TestProtocol] => τ_0_0 [explicit]
- τ_0_0.Err => τ_0_0.[TestProtocol:Err]
- τ_0_0.[Copyable] => τ_0_0
- τ_0_0.[Escapable] => τ_0_0
}
Property map: {
  [TestProtocol] => { conforms_to: [TestProtocol Copyable Escapable] }
  [TestProtocol:Err] => { conforms_to: [Copyable Escapable] }
  τ_0_0 => { conforms_to: [TestProtocol Copyable Escapable] }
}
Conformance paths: {
}
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.

[p-x9]

And I found that I could build the code directly after the expansion.
This may be a compiler-related problem.

protocol TestProtocol: AnyObject {
    associatedtype Err: Error
}

private extension TestProtocol {
    var test: Bool {
        get {
            if let value = getAssociatedObject(
                self,
                Self.__associated_testKey
            ) as? Bool {
                return value
            } else {
                let value: Bool = false
                setAssociatedObject(
                    self,
                    Self.__associated_testKey,
                    value,
                    .retain(.nonatomic)
                )
                return value
            }
        }
        set {
            setAssociatedObject(
                self,
                Self.__associated_testKey,
                newValue,
                .retain(.nonatomic)
            )
        }
    }

    @inline(never) static var __associated_testKey: UnsafeRawPointer {
        let f: @convention(c) () -> Void = {
        }
        return unsafeBitCast(f, to: UnsafeRawPointer.self)
    }
}