Make sure you turn on the native TypeScript Language Service feature, otherwise you'll be getting your IDE's parser, which is always behind the standard and doesn't work very well:

> Select the Use TypeScript Service check box to get native support from the TypeScript Language Service according to the up-to-date specifications. In this case syntax and error highlighting is performed based on the annotations retrieved from the TypeScript Language Service while code completion lists contain both suggestions from the TypeScript Language Service and suggestions calculated by IntelliJ IDEA itself.

https://www.jetbrains.com/help/idea/2016.2/typescript-suppor...