microsoft / microsoft/TypeScript
Inconsistent typing of endless generators between functions and lambdas
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Description
### 🔎 Search Terms
inconsistent generator lambda
inconsistent generator anonymous
inconsistent generator
wrong generator lambda
### 🕗 Version & Regression Information
This is the behavior in every version I tried, and I reviewed the FAQ for entries about generators
### ⏯ Playground Link
https://www.typescriptlang.org/play/?ts=6.0.3#code/GYVwdgxgLglg9mAVAAigUwM5QBQEpkDeAUMsgO4AWMANmstlAE4hr4CeMa1AJsgAwBuIgF8iRWlGQcu3DAHlIdALzJQkWAkR5CJKZx78ho8WknSeGAILUyAQzYZkKtdHhJtxUpRp0GzVnoyhiJiROayChDKqJhQQmH6stZ2Dk4xWPEA9JmoFHTUtljI1DBgdBgUcGSOaIyMcIxE2cgAKmwADnQA5NpKAHzIAOJoZYy2UA0APGAgALYARrUANMgAbnAw3CvgANZgVWB9XcgwjvuShRgwAOZgtvO0qHCoHd29A8Oj41MzC8vIZVW-12+zIhy6ADomjlkABhWzUajIK63cYgRh0DFQdFgF6dRxdT61b6MaZzRaMFbrTbbMB7A5HZC2MC8QkjYkTUm-CkrQHAumg8FMjEnSBwWbtcYwB5oKHNUitPJ4zDILE4tC8eZsZBdMoADxwEKNuGOthFpQg4slsBlyEWUDIaBGuUwdCgrwwcphCtarx1AEl0GNOQAlTAgahQMl-SlrDbcRmnAFwC4YFF3W0TZUBoMksMYCNR7n-PmMI5en0+tqdHMchph7GMMD5wuTakJ45J85MtM3DOPLPumtdQN1xgtyPRnkAtBAsuQ6GVyvV7qj4P10w4idR9uJs4pnvp+4D55D1e50Obpvb6az2rlxdL5d+rrtzv71NHzOnl+lyHYAAmABmACANwIggA
### 💻 Code
```ts
function* test() {
while (true) yield 0;
}
let yieldsOnce = function*() {
yield 0;
}
let yieldsAlways = function*() {
while (true) yield 0;
}
yieldsOnce = test;
yieldsAlways = test;
// the last line shows error
// Type '() => Generator' is not assignable to type '() => Generator'.
// Call signature return types 'Generator' and 'Generator' are incompatible.
// The types returned by 'next(...)' are incompatible between these types.
// Type 'IteratorResult' is not assignable to type 'IteratorResult'.
// Type 'IteratorReturnResult' is not assignable to type 'IteratorResult'.
// Type 'IteratorReturnResult' is not assignable to type 'IteratorReturnResult'.
// Type 'void' is not assignable to type 'never'.(2322)
```
### 🙁 Actual behavior
Despite both having the same body, the function `test` has the type `Generator`, yet the variable `yieldsAlways` has the type `Generator`. The compiler seems to have understood the `test` function incorrectly, so the test code cannot do `yieldsAlways = test`.
### 🙂 Expected behavior
If the compiler knows that the lambda assigned to `yieldsAlways` never finishes, it should know that about the `test` function as well.
The code should be able to do `yieldsAlways = test`.
### Additional information about the issue
_No response_
Contributor guide
Research direction
Start by running the provided TypeScript Playground reproduction and compare the inferred generator types for the named function and lambda. Trace the generator return-type inference used for these two forms; done means both receive compatible return types and the `yieldsAlways = test` assignment no longer reports an error.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Active
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100