Empty intersection reduction by ahejlsberg · Pull Request #31838 · microsoft/TypeScript · GitHub

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.

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

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.

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.

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.

Does this still support branding via object literals?

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

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.

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.

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

Ah, that makes it clearer. Thank you!

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.

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.

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.

TypeScript Bot (@typescript-bot) run dt

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.

RWC and DT test runs are both clean now.

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.

Hi I wanted to check to see if this has already been resolved as I am still facing this issue.

Utilizing [TypeScript intersection](microsoft/TypeScript#31838) to simplify config types. Config keys and payload can now always be `undefined`.

Anders Hejlsberg (ahejlsberg) added 7 commits June 9, 2019 07:46

Intersections of disjoint types become never upon construction

d4c9f24

Update tests

9cc9fb9

Accept new baselines

d872b3a

Fix minor issue

d2397e0

Update fourslash tests

aa0ea51

Add tests

98bbb22

Accept new baselines

015f4c5

Wesley Wigham (weswigham) reviewed Jun 10, 2019

View reviewed changes

Anders Hejlsberg (ahejlsberg) mentioned this pull request Jun 10, 2019

Update Ember tests to treat 'string & number' and 'never' the same DefinitelyTyped/DefinitelyTyped#36095

Merged

Anders Hejlsberg (ahejlsberg) requested a review from Wesley Wigham (weswigham) June 11, 2019 17:11

Wesley Wigham (weswigham) approved these changes Jun 11, 2019

View reviewed changes

Anders Hejlsberg (ahejlsberg) merged commit ad322a5 into master Jun 11, 2019

Anders Hejlsberg (ahejlsberg) deleted the emptyIntersectionReduction branch June 11, 2019 19:25

Anders Hejlsberg (ahejlsberg) added the Breaking Change Would introduce errors in existing code label Jun 11, 2019

Anders Hejlsberg (ahejlsberg) added this to the TypeScript 3.6.0 milestone Jun 11, 2019

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) mentioned this pull request Aug 29, 2019

error TS2339: Property 'length' does not exist on type 'never' #33138

Closed

Jack Williams (jack-williams) mentioned this pull request Sep 6, 2019

Intersection of non-null and null is not deduced to be never #33274

Closed

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

Closed

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

Closed

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

Closed

Anders Hejlsberg (ahejlsberg) mentioned this pull request Feb 16, 2020

Reduce intersections by discriminants #36696

Merged

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Feb 19, 2020

Design Meeting Notes, 2/14/2020 #36877

Closed

Deyaa (deyaaeldeen) mentioned this pull request Oct 23, 2020

deserialize null faithfully Azure/azure-sdk-for-js#12009

Closed

Wes Gorgichuk (wdamien) mentioned this pull request May 26, 2022

Intersection Types that have overlapping properties can fail #49264

Closed

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

Erik Hughes (Swiftwork) mentioned this pull request Mar 19, 2025

feat: simplify types treating key and payload always as optional reflagcom/javascript#380

Merged

Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 16, 2025

TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Oct 16, 2025

+                                  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;

Empty intersection reduction by ahejlsberg · Pull Request #31838 · microsoft/TypeScript · GitHub

Uh oh!

Empty intersection reduction - #31838

Empty intersection reduction#31838Anders Hejlsberg (ahejlsberg) merged 7 commits intomastermicrosoft/TypeScript:masterfrom emptyIntersectionReductionmicrosoft/TypeScript:emptyIntersectionReductionCopy head branch name to clipboard

Conversation

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019

Uh oh!

TypeScript Bot (typescript-bot) commented Jun 9, 2019 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

TypeScript Bot (typescript-bot) commented Jun 9, 2019 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019

Uh oh!

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

Uh oh!

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019

Uh oh!

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

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

Uh oh!

Wesley Wigham (weswigham) Jun 10, 2019

Choose a reason for hiding this comment

Uh oh!

Bruce Pascoe (fatcerberus) Jun 10, 2019

Choose a reason for hiding this comment

Uh oh!

Anders Hejlsberg (ahejlsberg) Jun 11, 2019

Choose a reason for hiding this comment

Uh oh!

Anders Hejlsberg (ahejlsberg) commented Jun 11, 2019

Uh oh!

TypeScript Bot (typescript-bot) commented Jun 11, 2019 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Anders Hejlsberg (ahejlsberg) commented Jun 11, 2019

Uh oh!

Wesley Wigham (weswigham) left a comment

Choose a reason for hiding this comment

Uh oh!

dharnil (dharnil) commented May 31, 2024

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

6 participants

Empty intersection reduction#31838
Anders Hejlsberg (ahejlsberg) merged 7 commits into
masterfrom
emptyIntersectionReduction

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019 •
edited
Loading

TypeScript Bot (typescript-bot) commented Jun 9, 2019 •
edited
Loading

TypeScript Bot (typescript-bot) commented Jun 9, 2019 •
edited
Loading

Anders Hejlsberg (ahejlsberg) commented Jun 9, 2019 •
edited
Loading

TypeScript Bot (typescript-bot) commented Jun 11, 2019 •
edited
Loading