Symbol description property should not forget its value.

[rotu]

🔎 Search Terms

symbol, description

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?esModuleInterop=false&declaration=false&target=9&jsx=0&module=7&stripInternal=false&noErrorTruncation=true&ts=5.4.0-dev.20231124&ssl=10&ssc=48&pln=1&pc=1#code/MYewdgzgLgBBCeBbGBeGBlJAjEAbAFAOQBmIIhAlAFBUD0tMUAFgJYQwCGEELA5mIgCmYWBCYgArrgAmMAO4gATgGsqoSLAAeALgDe0wRGCKWABygtw2kmUIBfVHCQ16jVuwUqOiyWFnSQQxgwEFhPVXVoGHhHBGQuGHwJMBYARwlBJ0QcXBgAMhh9Q2MzCysbcjtqOgZmNnklZXYsCVhBRS4guJyYZLSMsEMINXAogC9YpE52fG68fMKDIxNzSzBrUkqKIA

💻 Code

const sym = Symbol('foo')

// this assignment should work
const x:{description:'foo'} = sym

// this workaround does not work
const y = sym as (unique symbol & {description:'foo'})

// this works but erases symbol uniqueness
const z = sym as (symbol & {description:'foo'})

🙁 Actual behavior

The assignment fails with

Type 'typeof sym' is not assignable to type '{ description: "foo"; }'.

Typescript infers sym as a unique symbol with a readonly description property of type string | undefined.

Note that you can't correct this with an intersection type (as also illustrated in the example)

🙂 Expected behavior

Typescript should infer that description is of type "foo" and permit the assignment.

Additional information about the issue

No response

[andrewbranch]

You’re looking for

- const y = sym as (unique symbol & {description:'foo'})
+ const y = sym as (typeof sym & {description:'foo'})

[rotu]

@andrewbranch You're right! Adding a wrapper type works.

It seems an assertion does as well:

const sym = Symbol('foo')
assert(sym.description === 'foo')

[typescript-bot]

This issue has been marked as "Question" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[rotu]

@andrewbranch why did you mark this as a question? Is there a design reason the auxiliary type or the assertion should be needed at all?

[andrewbranch]

Yes, this is working as intended; I sometimes use "Question" to indicate that the OP was trying to solve a problem and found a solution, even if they went in thinking they were seeing a bug.