Repository: firebase/firebase-ios-sdk (language: cpp)

Issue:
FirebaseFirestoreSwift-beta fails to build on Xcode 12.4
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### [REQUIRED] Step 1: Describe your environment

  * Xcode version: 12.4
  * Firebase SDK version: 8.13
  * Installation method: `Swift Package Manager`
  * Firebase Component: Firestore (Swift)
  * Target platform(s): `All`

### [REQUIRED] Step 2: Describe the problem

Compilation fails on line 39 of `DocumentSnapshot+ReadDecodable.swift` with the error:
"Variable used within its own initial value"

Reported on Stack Overflow here: https://stackoverflow.com/questions/71417503/error-after-adding-firebasefirestoreswift-beta-with-swift-package-manager

#### Steps to reproduce:

Build the component with Xcode 12.4

#### Relevant Code:

No code needed to reproduce, just build the library.


Discussion / hints:

Fix patch:
diff --git a/Firestore/Swift/Source/Codable/DocumentSnapshot+ReadDecodable.swift b/Firestore/Swift/Source/Codable/DocumentSnapshot+ReadDecodable.swift
index 12a3d551bd0..ad9a10bd92a 100644
--- a/Firestore/Swift/Source/Codable/DocumentSnapshot+ReadDecodable.swift
+++ b/Firestore/Swift/Source/Codable/DocumentSnapshot+ReadDecodable.swift
@@ -36,7 +36,7 @@ public extension DocumentSnapshot {
   func data<T: Decodable>(as type: T.Type,
                           with serverTimestampBehavior: ServerTimestampBehavior = .none,
                           decoder: Firestore.Decoder = .init()) throws -> T {
-    let data: Any = data(with: serverTimestampBehavior) ?? NSNull()
-    return try decoder.decode(T.self, from: data, in: reference)
+    let d: Any = data(with: serverTimestampBehavior) ?? NSNull()
+    return try decoder.decode(T.self, from: d, in: reference)
   }
 }


Test patch: