Support optional return types

Summary:
This fixes an issue in the C++ TurboModule codegen where optional return types would result in a compiler error because they were not being defaulted to `null` when converting to `jsi::Value`.

Changelog:
Internal

Reviewed By: javache

Differential Revision: D36989312

fbshipit-source-id: 525f9ce7a5638ba5a655fa69ba9647978030ab0b

packages/react-native-codegen/src/generators/modules/GenerateModuleCpp.js

@@ -17,6 +17,7 @@ import type {
   NativeModulePropertyShape,
   NativeModuleFunctionTypeAnnotation,
   NativeModuleParamTypeAnnotation,
+  NativeModuleTypeAnnotation,
 } from '../../CodegenSchema';
 
 import type {AliasResolver} from './Utils';
@@ -28,21 +29,33 @@ type FilesOutput = Map<string, string>;
 const HostFunctionTemplate = ({
   hasteModuleName,
   methodName,
-  isVoid,
+  returnTypeAnnotation,
   args,
 }: $ReadOnly<{
   hasteModuleName: string,
   methodName: string,
-  isVoid: boolean,
+  returnTypeAnnotation: Nullable<NativeModuleTypeAnnotation>,
   args: Array<string>,
 }>) => {
+  const isNullable = returnTypeAnnotation.type === 'NullableTypeAnnotation';
+  const isVoid = returnTypeAnnotation.type === 'VoidTypeAnnotation';
   const methodCallArgs = ['rt', ...args].join(', ');
-  const methodCall = `static_cast<${hasteModuleName}CxxSpecJSI *>(&turboModule)->${methodName}(${methodCallArgs});`;
+  const methodCall = `static_cast<${hasteModuleName}CxxSpecJSI *>(&turboModule)->${methodName}(${methodCallArgs})`;
 
   return `static jsi::Value __hostFunction_${hasteModuleName}CxxSpecJSI_${methodName}(jsi::Runtime &rt, TurboModule &turboModule, const jsi::Value* args, size_t count) {${
-    isVoid ? `\n  ${methodCall}` : ''
+    isVoid
+      ? `\n  ${methodCall};`
+      : isNullable
+      ? `\n  auto result = ${methodCall};`
+      : ''
   }
-  return ${isVoid ? 'jsi::Value::undefined();' : methodCall}
+  return ${
+    isVoid
+      ? 'jsi::Value::undefined()'
+      : isNullable
+      ? 'result ? jsi::Value(std::move(*result)) : jsi::Value::null()'
+      : methodCall
+  };
 }`;
 };
 
@@ -173,13 +186,11 @@ function serializePropertyIntoHostFunction(
 ): string {
   const [propertyTypeAnnotation] =
     unwrapNullable<NativeModuleFunctionTypeAnnotation>(property.typeAnnotation);
-  const isVoid =
-    propertyTypeAnnotation.returnTypeAnnotation.type === 'VoidTypeAnnotation';
 
   return HostFunctionTemplate({
     hasteModuleName,
     methodName: property.name,
-    isVoid,
+    returnTypeAnnotation: propertyTypeAnnotation.returnTypeAnnotation,
     args: propertyTypeAnnotation.params.map((p, i) =>
       serializeArg(p, i, resolveAlias),
     ),