microsoft / microsoft/TypeScript
`Exclude<Exclude<T, null>, undefined>` is not assignable to `Exclude<T, null | undefined>` and vice versa
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Description
TypeScript Version: Nightly
Search Terms:
typescript nullness inference
Expected behavior:
They should be assignable to each other.
Actual behavior:
They are not considered assignable to each other.
Type 'Exclude<Exclude<T, null>, undefined>' is not assignable to type 'Exclude<T, null | undefined>'.
Type 'Exclude<T, null>' is not assignable to type 'Exclude<T, null | undefined>'.
Type 'T' is not assignable to type 'Exclude<T, null | undefined>'.(2322)
and
Type 'Exclude<T, null | undefined>' is not assignable to type 'Exclude<Exclude<T, null>, undefined>'.
Type 'T' is not assignable to type 'Exclude<Exclude<T, null>, undefined>'.(2322)
Related Issues:
N/A
Code
export function f<T>(v: Exclude<Exclude<T, null>, undefined>) {
const w: Exclude<T, null | undefined> = v;
}
export function g<T>(v: Exclude<T, null | undefined>) {
const w: Exclude<Exclude<T, null>, undefined> = v;
}
Output
export function f(v) {
const w = v;
}
export function g(v) {
const w = v;
}
Compiler Options
{
"compilerOptions": {
"noImplicitAny": true,
"strictNullChecks": true,
"strictFunctionTypes": true,
"strictPropertyInitialization": true,
"strictBindCallApply": true,
"noImplicitThis": true,
"noImplicitReturns": true,
"useDefineForClassFields": false,
"alwaysStrict": true,
"allowUnreachableCode": false,
"allowUnusedLabels": false,
"downlevelIteration": false,
"noEmitHelpers": false,
"noLib": false,
"noStrictGenericChecks": false,
"noUnusedLocals": false,
"noUnusedParameters": false,
"esModuleInterop": true,
"preserveConstEnums": false,
"removeComments": false,
"skipLibCheck": false,
"checkJs": false,
"allowJs": false,
"declaration": true,
"experimentalDecorators": false,
"emitDecoratorMetadata": false,
"target": "ES2017",
"module": "ESNext"
}
}
Playground Link: Provided
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the provided Playground link and the generic functions f and g under the listed strict compiler options. Trace how the compiler checks the two nested Exclude types, then verify that both assignments are accepted without the reported TS2322 errors while preserving the shown emitted output.
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
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 35/100