fix(eslint-plugin): [return-await] properly handle fixes for TSAsExpression

packages/eslint-plugin/src/rules/return-await.ts

@@ -147,8 +147,15 @@ export default util.createRule({
     function insertAwait(
       fixer: TSESLint.RuleFixer,
       node: TSESTree.Expression,
-    ): TSESLint.RuleFix | null {
-      return fixer.insertTextBefore(node, 'await ');
+    ): TSESLint.RuleFix | TSESLint.RuleFix[] {
+      if (node.type !== AST_NODE_TYPES.TSAsExpression) {
+        return fixer.insertTextBefore(node, 'await ');
+      }
+
+      return [
+        fixer.insertTextBefore(node, 'await ('),
+        fixer.insertTextAfter(node, ')'),
+      ];
     }
 
     function test(node: TSESTree.Expression, expression: ts.Node): void {

packages/eslint-plugin/tests/rules/return-await.test.ts

@@ -827,5 +827,34 @@ const buzz = async () => ((await foo()) ? 1 : await bar());
         },
       ],
     },
+    {
+      // https://github.com/typescript-eslint/typescript-eslint/issues/2109
+      code: `
+async function test<T>(): Promise<T> {
+  const res = await fetch('...');
+  try {
+    return res.json() as Promise<T>;
+  } catch (err) {
+    throw Error('Request Failed.');
+  }
+}
+      `,
+      output: `
+async function test<T>(): Promise<T> {
+  const res = await fetch('...');
+  try {
+    return await (res.json() as Promise<T>);
+  } catch (err) {
+    throw Error('Request Failed.');
+  }
+}
+      `,
+      errors: [
+        {
+          line: 5,
+          messageId: 'requiredPromiseAwait',
+        },
+      ],
+    },
   ],
 });