Readers of Effective TypeScript and followers of this blog will know that testing types is a long-standing interest of mine: typings-checker (2017) implemented $ExpectType and $ExpectError directives and helped to influence dtslint, which is used to test types on DefinitelyTyped. I gave a talk at TSConf 2019 entitled Testing Types: An Introduction to dtslint. I included Item 52: Be Aware of the Pi