Stop using opaque return types to resolve build error

CONTRIBUTORS.txt

@@ -13,8 +13,10 @@ needs to be listed here.
 
 - Franz Busch <[email protected]>
 - Guoye Zhang <[email protected]>
+- Gwynne Raskind <[email protected]>
 - Jager-yoo <[email protected]>
 - Sergey Dmitriev <[email protected]>
+- Tim Condon <[email protected]>
 - Tomohiro Kumagai <[email protected]>
 
 **Updating this list**

Sources/HTTPTypes/HTTPFields.swift

@@ -216,30 +216,30 @@ public struct HTTPFields: Sendable, Hashable {
         }
     }
 
-    private func fields(for name: HTTPField.Name) -> some Sequence<HTTPField> {
-        struct HTTPFieldSequence: Sequence {
-            let fields: [(field: HTTPField, next: UInt16)]
-            let index: UInt16
+    private struct HTTPFieldSequence: Sequence {
+        let fields: [(field: HTTPField, next: UInt16)]
+        let index: UInt16
 
-            struct Iterator: IteratorProtocol {
-                let fields: [(field: HTTPField, next: UInt16)]
-                var index: UInt16
+        struct Iterator: IteratorProtocol {
+            let fields: [(field: HTTPField, next: UInt16)]
+            var index: UInt16
 
-                mutating func next() -> HTTPField? {
-                    if self.index == .max {
-                        return nil
-                    }
-                    let (field, next) = self.fields[Int(self.index)]
-                    self.index = next
-                    return field
+            mutating func next() -> HTTPField? {
+                if self.index == .max {
+                    return nil
                 }
+                let (field, next) = self.fields[Int(self.index)]
+                self.index = next
+                return field
             }
+        }
 
-            func makeIterator() -> Iterator {
-                Iterator(fields: self.fields, index: self.index)
-            }
+        func makeIterator() -> Iterator {
+            Iterator(fields: self.fields, index: self.index)
         }
+    }
 
+    private func fields(for name: HTTPField.Name) -> HTTPFieldSequence {
         let index = self._storage.ensureIndex[name.canonicalName]?.first ?? .max
         return HTTPFieldSequence(fields: self._storage.fields, index: index)
     }