Swift: fix compilation errors for extension accessors

wire-swift-generator/build.gradle.kts

@@ -9,4 +9,7 @@ plugins {
 dependencies {
   api(libs.swiftpoet)
   api(projects.wireSchema)
+  testImplementation(projects.wireTestUtils)
+  testImplementation(libs.kotlin.test.junit)
+  testImplementation(libs.assertk)
 }

wire-swift-generator/src/main/java/com/squareup/wire/swift/SwiftGenerator.kt

@@ -149,6 +149,8 @@ class SwiftGenerator private constructor(
     get() = type!!.keyType!!
   internal val Field.valueType: ProtoType
     get() = type!!.valueType!!
+  private val Field.extensionAccessorType: TypeName
+    get() = if (isMap || isRepeated) typeName else typeName.makeOptional()
 
   private val Field.isRequiredParameter: Boolean
     get() = !isOptional && !isRepeated && !isMap
@@ -1195,7 +1197,7 @@ class SwiftGenerator private constructor(
         .addDoc("Extensions of %T\n", structType)
         .apply {
           type.extensionFields.forEach { field ->
-            val property = PropertySpec.varBuilder(field.safeName, field.typeName, PUBLIC)
+            val property = PropertySpec.varBuilder(field.safeName, field.extensionAccessorType, PUBLIC)
               .apply {
                 if (field.documentation.isNotBlank()) {
                   addDoc("\n%L\n", field.documentation.sanitizeDoc())
@@ -1260,7 +1262,7 @@ class SwiftGenerator private constructor(
             addDoc("Extensions of %T\n", structType)
           }
           type.extensionFields.forEach { field ->
-            val property = PropertySpec.varBuilder(field.safeName, field.typeName, PUBLIC)
+            val property = PropertySpec.varBuilder(field.safeName, field.extensionAccessorType, PUBLIC)
               .apply {
                 if (!forStorageType && field.documentation.isNotBlank()) {
                   addDoc("\n%L\n", field.documentation.sanitizeDoc())

wire-swift-generator/src/test/java/com/squareup/wire/swift/SwiftGeneratorTest.kt

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2026 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.wire.swift
+
+import assertk.assertThat
+import assertk.assertions.contains
+import assertk.assertions.doesNotContain
+import com.squareup.wire.buildSchema
+import com.squareup.wire.schema.Schema
+import io.outfoxx.swiftpoet.FileSpec
+import kotlin.test.Test
+import okio.Path.Companion.toPath
+
+class SwiftGeneratorTest {
+  @Test fun singularProto3ExtensionAccessorsAreOptional() {
+    val schema = buildSchema {
+      add(
+        "custom_options.proto".toPath(),
+        """
+        |syntax = "proto3";
+        |
+        |package squareup.protos3.kotlin.custom_options;
+        |
+        |import "google/protobuf/descriptor.proto";
+        |
+        |enum Proto3ExtensionEnum {
+        |  UNKNOWN = 0;
+        |  ENABLED = 1;
+        |}
+        |
+        |extend google.protobuf.MessageOptions {
+        |  string implicit_scalar = 50001;
+        |  Proto3ExtensionEnum implicit_enum = 50002;
+        |  repeated string repeated_scalar = 50003;
+        |}
+        """.trimMargin(),
+      )
+    }
+
+    val code = schema.generateSwift("google.protobuf.MessageOptions")
+
+    assertThat(code).contains("public var implicit_scalar: String? {")
+    assertThat(code).contains("self.parseUnknownField(fieldNumber: 50001, type: String.self)")
+    assertThat(code).doesNotContain("public var implicit_scalar: String {")
+
+    assertThat(code).contains("public var implicit_enum: Proto3ExtensionEnum? {")
+    assertThat(code).contains("self.parseUnknownField(fieldNumber: 50002, type: Proto3ExtensionEnum.self)")
+    assertThat(code).doesNotContain("public var implicit_enum: Proto3ExtensionEnum {")
+
+    assertThat(code).contains("public var repeated_scalar: [String] {")
+    assertThat(code).contains("self.parseUnknownField(fieldNumber: 50003)")
+  }
+
+  private fun Schema.generateSwift(typeName: String): String {
+    val swiftGenerator = SwiftGenerator(this)
+    val type = requireNotNull(getType(typeName))
+    val generatedTypeName = swiftGenerator.generatedTypeName(type)
+    return FileSpec.builder(generatedTypeName.moduleName, generatedTypeName.simpleName)
+      .indent("    ")
+      .apply {
+        swiftGenerator.generateTypeTo(type, this)
+      }
+      .build()
+      .toString()
+  }
+}