Add validation for fieldmerging w/selection set init

Tests/ApolloCodegenInternalTestHelpers/MockApolloCodegenConfiguration.swift

@@ -30,6 +30,7 @@ extension ApolloCodegenConfiguration {
   public static func mock(
     _ moduleType: ApolloCodegenConfiguration.SchemaTypesFileOutput.ModuleType,
     options: ApolloCodegenConfiguration.OutputOptions = .init(),
+    experimentalFeatures: ExperimentalFeatures = .init(),
     schemaNamespace: String = "TestSchema",
     to path: String = "MockModulePath"
   ) -> Self {
@@ -42,7 +43,8 @@ extension ApolloCodegenConfiguration {
       output: .init(
         schemaTypes: .init(path: path, moduleType: moduleType)
       ),
-      options: options
+      options: options,
+      experimentalFeatures: experimentalFeatures
     )
   }
 }

Tests/ApolloCodegenTests/ApolloCodegenTests.swift

@@ -2374,6 +2374,46 @@ class ApolloCodegenTests: XCTestCase {
     })
   }
 
+  func test__validation__givenFieldMerging_notAll_andSelectionSetInitializers_enabled_shouldThrowError() throws {
+    // given
+    let fieldMergingOptions: [ApolloCodegenConfiguration.FieldMerging] = [
+      .none,
+      .ancestors,
+      .namedFragments,
+      .siblings,
+      [.ancestors, .namedFragments],
+      [.siblings, .ancestors],
+      [.siblings, .namedFragments]
+    ]
+    let initializerOptions: [ApolloCodegenConfiguration.SelectionSetInitializers] = [
+      .all,
+      .localCacheMutations,
+      .operations,
+      .namedFragments,
+      .fragment(named: "TestFragment"),
+      [.operations, .localCacheMutations]
+    ]
+
+    for fieldMergingOption in fieldMergingOptions {
+      for initializerOption in initializerOptions {
+
+        let config = ApolloCodegenConfiguration.mock(
+          .other,
+          options: .init(
+            selectionSetInitializers: initializerOption
+          ),
+          experimentalFeatures: .init(
+            fieldMerging: fieldMergingOption
+          )
+        )
+
+        // then
+        expect(try ApolloCodegen._validate(config: config))
+          .to(throwError(ApolloCodegen.Error.fieldMergingIncompatibility))
+      }
+    }
+  }
+
   // MARK: Path Match Exclusion Tests
 
   func test__match__givenFilesInSpecialExcludedPaths_shouldNotReturnExcludedPaths() throws {

apollo-ios-codegen/Sources/ApolloCodegenLib/ApolloCodegen+Errors.swift

@@ -17,6 +17,7 @@ extension ApolloCodegen {
     case invalidConfiguration(message: String)
     case invalidSchemaName(_ name: String, message: String)
     case targetNameConflict(name: String)
+    case fieldMergingIncompatibility
 
     public var errorDescription: String? {
       switch self {
@@ -52,6 +53,13 @@ extension ApolloCodegen {
           Target name '\(name)' conflicts with a reserved library name. Please choose a different \
           target name.
           """
+      case .fieldMergingIncompatibility:
+        return """
+          Options for disabling 'fieldMerging' and enabling 'selectionSetInitializers' are
+          incompatible.
+
+          Please set either 'fieldMerging' to 'all' or 'selectionSetInitializers' to be empty.
+          """
       }
     }
   }

apollo-ios-codegen/Sources/ApolloCodegenLib/ConfigurationValidation.swift

@@ -21,6 +21,12 @@ extension ApolloCodegen.ConfigurationContext {
         """)
     }
 
+    guard self.experimentalFeatures.fieldMerging == .all ||
+            self.options.selectionSetInitializers == []
+    else {
+      throw ApolloCodegen.Error.fieldMergingIncompatibility
+    }
+
     guard
       !SwiftKeywords.DisallowedSchemaNamespaceNames.contains(self.schemaNamespace.lowercased())
     else {