Full Handbook Page on Soundness #1059

From my tweet, where I suggested that it would be beneficial to have a Handbook page on Soundness:

https://twitter.com/karlhorky/status/1146687994142842880

My initial top-of-my-head suggestions in the tweet were to include at least the following information:

1. What is soundness / type safety and why is it important?
2. Maybe: what is completeness?
3. An overview of the general state of soundness in TypeScript (maybe from Type Compatibility)
4. What common structures or patterns can be expected to be unsound

Happy to add to the list, if anyone thinks of anything!

Unsound Behaviors

For 2. above, starting a list of unsound behaviors here:

No excess property checks in object types with spread (Playground)

• No excess property error for spread properties TypeScript#26798
• Object spread allows unknown properties as function return values TypeScript#17422
• potential solution: Exact Types

Incorrect definition of spread and Object.assign (Playground)

• Object.assign return assigned type instead merged TypeScript#28553
• Generic spread expressions in object literals TypeScript#28234
• Bug Report + Proposal: Object.assign() TypeScript#31982

Trade-offs in Control Flow Analysis

• Trade-offs in Control Flow Analysis TypeScript#9998

This has been prompted by the high-quality criticisms as of late on Twitter by Sebastian (@sebmck):

• https://twitter.com/sebmck/status/1146524039227174914
• https://twitter.com/sebmck/status/1142601004241657856

This has been going on since ~ June 18:

• https://twitter.com/sebmck/status/1141052364331184128

Articles / discussion on the topic:

• Surviving the TypeScript Ecosystem — Part 3: Interfaces and Structural Typing
• Are unsound type systems wrong?
• Type-checking unsoundness: standardize treatment of such issues among TypeScript team/community?
• TypeScript Design Goals - Non-Goals: "Non-goal 3: Apply a sound or "provably correct" type system. Instead, strike a balance between correctness and productivity."