Allow substituting types

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateSchema.swift

@@ -87,6 +87,21 @@ extension TypesFileTranslator {
                 )
             )
         }
+        if let substituteTypeName = schema.vendorExtensions["x-swift-open-api-substitute-type"]?.value
+            as? String
+        {
+            try diagnostics.emit(.note(message: "Substituting type \(typeName) with \(substituteTypeName)"))
+            let substitutedType = TypeName(swiftKeyPath: substituteTypeName.components(separatedBy: ".")).asUsage
+
+            let typealiasDecl = try translateTypealias(
+                named: typeName,
+                userDescription: overrides.userDescription ?? schema.description,
+                to: substitutedType.withOptional(
+                    overrides.isOptional ?? typeMatcher.isOptional(schema, components: components)
+                )
+            )
+            return [typealiasDecl]
+        }
 
         // If this type maps to a referenceable schema, define a typealias
         if let builtinType = try typeMatcher.tryMatchReferenceableType(for: schema, components: components) {

Tests/OpenAPIGeneratorCoreTests/Translator/TypesTranslator/Test_translateSchemas.swift

@@ -47,4 +47,30 @@ class Test_translateSchemas: Test_Core {
             XCTAssertEqual(collector.diagnostics.map(\.description), diagnosticDescriptions)
         }
     }
+
+    func testSchemaTypeSubstitution() throws {
+        let typeName = TypeName(swiftKeyPath: ["Foo"])
+
+        let schema = try loadSchemaFromYAML(
+            #"""
+            type: string
+            x-swift-open-api-substitute-type: MyLibrary.MyCustomType
+            """#
+        )
+        let collector = AccumulatingDiagnosticCollector()
+        let translator = makeTranslator(diagnostics: collector)
+        let translated = try translator.translateSchema(typeName: typeName, schema: schema, overrides: .none)
+
+        XCTAssertEqual(
+            collector.diagnostics.map(\.description),
+            ["note: Substituting type Foo with MyLibrary.MyCustomType"]
+        )
+        XCTAssertTrue(translated.count == 1, "Should have one translated schema")
+        guard case let .typealias(typeAliasDescription) = translated.first?.strippingTopComment else {
+            XCTFail("Expected typealias description got")
+            return
+        }
+        XCTAssertEqual(typeAliasDescription.name, "Foo")
+        XCTAssertEqual(typeAliasDescription.existingType, .member(["MyLibrary", "MyCustomType"]))
+    }
 }