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T wider in lazy branch of T | (() => T) for known T #55277
Description
🔎 Search Terms
T | (() => T)
"T | (() => T)" inference
inference of lazy generic type
lazy generic inferred too wide
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about callbacks and functions and laziness and sum types.
⏯ Playground Link
💻 Code
type OptionallyLazy<T> = T | (() => T)
type MyType = { a: boolean }
const fn = (t: OptionallyLazy<MyType>) => {}
fn({ a: true, shouldFail: 'it does, good'})
fn(() => ({ a: true, shouldFail: 'this does not fail, which is bad' }))🙁 Actual behavior
No type error for shouldFail on last line above.
🙂 Expected behavior
Type error for shouldFail on last line above, same as line above it.
Activity
fatcerberus commentedon Aug 5, 2023
() => MyTypeWithExtraPropsis assignable to() => MyType, so this is working as intended. The error you see in the first case is due to the excess property check (EPC), which only applies to object literals directly assigned to a type. The error message for EPC is currently misleading; see #55152.RobertSandiford commentedon Aug 5, 2023
Discussed in some detail here #55113
Objects are allowed to have additional properties, however special checks are applied in some locations just to catch probable bugs. As said in other places, this is more of a warning than a type error.
Handbook: excess-property-checks
pckilgore commentedon Aug 5, 2023
Ahh, appreciate the notes. I wish the error message for those checks were more clear they are a lint, and not a type system property. I've been operating under this misunderstanding for years writing TS!
Any known good workarounds? I guess I can write a Lint rule to require annotation of a lazy value? This caused a very insidious misspelling bug.
Thanks again.
RobertSandiford commentedon Aug 5, 2023
I don't see any option other than explicit typings. There is an open suggestion (#12936) for Exact types that are always checked this way, maybe one day that will happen.
fatcerberus commentedon Aug 5, 2023
You’re certainly not alone. Thats why I’m glad for #55152.