Full Handbook Page on Soundness

[karlhorky]

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!

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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

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This has been prompted by the high-quality criticisms as of late on Twitter by @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

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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."

[orta]

I've included an example on soundness in microsoft/TypeScript-Website#43 which I think is probably enough, it doesn't aim to be comprehensive but it explains what soundness is, why TS isn't sound, provides examples and then links to the two main places where it is documented.

[karlhorky]

Ah cool, thanks @orta!

I think I would still suggest documenting and publicizing this more openly than an example - feels like having this in the top-level handbook documentation would help clear up a lot of things for beginners and experienced developers alike.