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GeezerVison is showing github.com — Empty intersection reduction by ahejlsberg · Pull Request #31838 · microsoft/TypeScript

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@ahejlsberg

@ahejlsberg Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019 •
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With this PR we reduce empty intersections to never immediately upon construction. For example, instead of preserving the type string & number and only reducing it away in union types we now immediately reduce it to never.

Some examples of types that now immediately reduce to never:

type T1 = 'a' & 'b';  // never
type T2 = { a: string } & null;  // never in strict mode, null in non-strict mode
type T3 = { a: string } & undefined;  // never in strict mode, undefined in non-strict mode
type T4 = string & number;  // never
type T5 = number & object;  // never
type T6 = symbol & string;  // never
type T7 = void & string;  // never

In #18438 we chose to preserve empty intersection types (a) to make it easier to understand how they originated and (b) to partially preserve the ability to "tag" primtive types using enum types (e.g. string & Tag where Tag is an enum). In practice this doesn't seem to outweigh the confusion these types bring about, in particular because there are subtle behavioral differences between never and types like string & number even though they really should be the same. The differences include:

  • never is assignable to anything whereas string & number is assignable only to string or number.
  • any is not assignable to never but is assignable to string & number.
  • For a type T with no index signatures, T[never] resolves to never whereas T[string & number] is an error.

These differences have no practical value. Particularly confusing is that string & boolean reduces to never (because it normalizes to string & true | string & false, which then reduces to never), whereas string & number sticks around and behaves differently.

Fixes #31663.

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Anders Hejlsberg (ahejlsberg) added 7 commits June 9, 2019 07:46
@ahejlsberg
Intersections of disjoint types become never upon construction
d4c9f24
@ahejlsberg
Update tests
9cc9fb9
@ahejlsberg
Accept new baselines
d872b3a
@ahejlsberg
Fix minor issue
d2397e0
@ahejlsberg
Update fourslash tests
aa0ea51
@ahejlsberg
Add tests
98bbb22
@ahejlsberg
Accept new baselines
015f4c5
@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019

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TypeScript Bot (@typescript-bot) test this
TypeScript Bot (@typescript-bot) run dt
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@typescript-bot

TypeScript Bot (typescript-bot) commented Jun 9, 2019 •
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 015f4c5. You can monitor the build here. It should now contribute to this PR's status checks.
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TypeScript Bot (typescript-bot) commented Jun 9, 2019 •
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at 015f4c5. You can monitor the build here. It should now contribute to this PR's status checks.
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@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019

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RWC test run is clean. DT test run has one minor difference in ember/test/detect-instance.ts. Types such as null & { foo: string } and undefined & { foo: string } now reduce to never but previously would stick around (yet disappear in union types) and allow properties to be accessed. The new behavior is definitely more consistent.
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@jack-williams

Jack Williams (jack-williams) commented Jun 9, 2019

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Does this still support branding via object literals?

type Tag<T> = T & { readonly brand: unique symbol };
type TaggedNumber = Tag<number>; // not never
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@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019

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Jack Williams (@jack-williams) It does, except you can't brand null or undefined. You sort of could before, but only in a limited sense. The minute you put null & { brand: xxx } in a union type it would disappear.
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@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019 •
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Jack Williams (@jack-williams) The only thing that changes with this PR is when we reduce away empty intersections. We now do it upon construction but we used to do it only when an intersection was put in a union type. The actual patterns we reduce away (or don't reduce away) remain the same.
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@jack-williams

Jack Williams (jack-williams) commented Jun 9, 2019

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The only thing that changes with this PR is when we reduce away empty intersections.

Ah, that makes it clearer. Thank you!
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weswigham
Wesley Wigham (weswigham) reviewed Jun 10, 2019
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Comment thread src/compiler/checker.ts
if (type.flags & TypeFlags.Unit && includes & TypeFlags.Unit) {
// We have seen two distinct unit types which means we should reduce to an
// empty intersection. Adding TypeFlags.NonPrimitive causes that to happen.
includes |= TypeFlags.NonPrimitive;

@weswigham Wesley Wigham (weswigham) Jun 10, 2019

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Counterexample:

enum A {
  Zero = 0,
  One = 1
}

type Zeroish<T> = T & 0;
type ShouldNotBeNever = Zeroish<A.Zero> | never;

we handle this incorrectly in master already - I'm just not a fan of propagating the logical error.

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@fatcerberus Bruce Pascoe (fatcerberus) Jun 10, 2019

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So is the reason for that that the code highlighted considers A.Zero and 0 to be distinct? I'm just trying to ascertain where in that example above the never creeps in.

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@ahejlsberg Anders Hejlsberg (ahejlsberg) Jun 11, 2019

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Wesley Wigham (@weswigham) I think it is debatable whether we want to consider literals and literal enum members with the same underlying value to be intersectable. But either way, it is an issue that should be covered in a separate PR. Here we're just concerned with changing when we reduce, not how.

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@ahejlsberg Anders Hejlsberg (ahejlsberg) mentioned this pull request Jun 10, 2019
Update Ember tests to treat 'string & number' and 'never' the same DefinitelyTyped/DefinitelyTyped#36095
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@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Jun 11, 2019

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TypeScript Bot (@typescript-bot) run dt
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TypeScript Bot (typescript-bot) commented Jun 11, 2019 •
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 015f4c5. You can monitor the build here. It should now contribute to this PR's status checks.
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@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Jun 11, 2019

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RWC and DT test runs are both clean now.
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@ahejlsberg
Anders Hejlsberg (ahejlsberg) requested a review from Wesley Wigham (weswigham) June 11, 2019 17:11
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Wesley Wigham (weswigham) approved these changes Jun 11, 2019
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@weswigham Wesley Wigham (weswigham) left a comment

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I expect we'll get some reports about the using-an-enum-to-tag break upon release - we should remember to include this in the breaking change log.

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Anders Hejlsberg (ahejlsberg) merged commit ad322a5 into master Jun 11, 2019
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Anders Hejlsberg (ahejlsberg) deleted the emptyIntersectionReduction branch June 11, 2019 19:25
@ahejlsberg Anders Hejlsberg (ahejlsberg) added the Breaking Change Would introduce errors in existing code label Jun 11, 2019
@ahejlsberg Anders Hejlsberg (ahejlsberg) added this to the TypeScript 3.6.0 milestone Jun 11, 2019
@karol-majewski Karol Majewski (karol-majewski) mentioned this pull request Jun 24, 2019
3.5 regression: invalid type for index lookup using constrained generic #32017
Closed
@jack-williams Jack Williams (jack-williams) mentioned this pull request Aug 29, 2019
error TS2339: Property 'length' does not exist on type 'never' #33138
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@jack-williams Jack Williams (jack-williams) mentioned this pull request Sep 6, 2019
Intersection of non-null and null is not deduced to be never #33274
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@jack-williams Jack Williams (jack-williams) mentioned this pull request Sep 16, 2019
Regression in 3.6: Combination of union and intersection types with literal types not resolving specific types properly #33448
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@jack-williams Jack Williams (jack-williams) mentioned this pull request Sep 28, 2019
Entries cannot be reduced to an interface due to any being assigned to never #33651
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@fatcerberus Bruce Pascoe (fatcerberus) mentioned this pull request Oct 26, 2019
「Bug」multiple candidates for the same type variable in contra-variant positions causes an intersection type to be inferred #34732
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@ahejlsberg Anders Hejlsberg (ahejlsberg) mentioned this pull request Feb 16, 2020
Reduce intersections by discriminants #36696
Merged
@DanielRosenwasser Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Feb 19, 2020
Design Meeting Notes, 2/14/2020 #36877
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@deyaaeldeen Deyaa (deyaaeldeen) mentioned this pull request Oct 23, 2020
deserialize null faithfully Azure/azure-sdk-for-js#12009
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@wdamien Wes Gorgichuk (wdamien) mentioned this pull request May 26, 2022
Intersection Types that have overlapping properties can fail #49264
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@RyanCavanaugh Ryan Cavanaugh (RyanCavanaugh) mentioned this pull request Mar 3, 2023
Impossible intersection is supposedly never but any can still be assigned to it #53027
Open
@dharnil

dharnil (dharnil) commented May 31, 2024

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Hi I wanted to check to see if this has already been resolved as I am still facing this issue.
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@Swiftwork Erik Hughes (Swiftwork) mentioned this pull request Mar 19, 2025
feat: simplify types treating key and payload always as optional reflagcom/javascript#380
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Erik Hughes (Swiftwork) added a commit to reflagcom/javascript that referenced this pull request Mar 20, 2025
@Swiftwork
feat: simplify types treating key and payload always as optional (#380)
2fa4704 
Utilizing [TypeScript
intersection](microsoft/TypeScript#31838) to
simplify config types. Config keys and payload can now always be
`undefined`.
@microsoft Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 16, 2025
@typescript-bot TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Oct 16, 2025
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Reviewers

@weswigham Wesley Wigham (weswigham) weswigham approved these changes
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@fatcerberus Bruce Pascoe (fatcerberus) fatcerberus left review comments
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@ahejlsberg Anders Hejlsberg (ahejlsberg)

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Breaking Change Would introduce errors in existing code

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TypeScript 3.6.0

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Successfully merging this pull request may close these issues.

Type 'any' is not assignable to type 'never' with boolean in interface in TS 3.5

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@ahejlsberg @typescript-bot @jack-williams @dharnil @weswigham @fatcerberus
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