Fix Swift OpenAPI ClientMiddleware protocol conformance error

Problem

When creating a middleware class conforming to ClientMiddleware protocol from swift-openapi-runtime, Xcode reports the class doesn't conform even though it implements the required intercept function.

extension AuthenticationMiddleware: ClientMiddleware {
    func intercept(
        _ request: HTTPRequest,
        body: HTTPBody?,
        baseURL: URL,
        operationID: String,
        next: @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
    ) async throws -> (HTTPResponse, HTTPBody?) {
        // Implementation
    }
}

Solution 1: Disable Approachable Concurrency (Recommended)

Change the "Approachable Concurrency" build setting from Yes to No:

• In Xcode: Build Settings → Search "Approachable Concurrency" → Set to No
• Or in project.pbxproj: Change SWIFT_APPROACHABLE_CONCURRENCY = YES; to NO

Solution 2: Add @concurrent Attribute (Keep Approachable Concurrency)

If you want to keep the approachable concurrency feature enabled, add @concurrent to the next parameter:

func intercept(
    _ request: HTTPRequest,
    body: HTTPBody?,
    baseURL: URL,
    operationID: String,
    next: @concurrent @Sendable (HTTPRequest, HTTPBody?, URL) async throws -> (HTTPResponse, HTTPBody?)
) async throws -> (HTTPResponse, HTTPBody?) {
    // Implementation
}

This undoes the default nonisolated(nonsending) behavior and allows next to run on the global executor.

Alternative Solutions

• Make AuthenticationMiddleware an actor instead of a struct/class
• Move middleware to a separate Swift package and import it

Source: https://stackoverflow.com/questions/79790161