Merge trivially mergeable intersection types for identity comparison

[MichaelMitchell-at]

Please verify that:

• There is an associated issue in the Backlog milestone (required) - No one has created an issue for this specifically, but it is a subset of a larger set of problems discussed in [Feature request]type level equal operator #27024 and is similar to Make enum literal type identical to the union of its values #60417.
• Code is up-to-date with the main branch
• You've successfully run hereby runtests locally
• There are new or updated unit tests validating the change

Does not explicitly fix any particular issues I can find, but addresses use cases in the comments of #27024

---

TL;DR: this PR makes {a: 1} & {b: 2} compare "equal" to {a: 1; b: 2}.

Use case:

Type testing libraries want to be able to compare types that are intuitively equal. Examples of such libraries include expect-type and type-plus. These try to leverage type comparisons that use the underlying isTypeIdenticalTo function in checker.ts. isTypeIdenticalTo has stronger guarantees than mutual assignability, like ensuring {a?: 1} != {} and {readonly a: 1} != {a: 1}.

Lack of these guarantees leads to deficiences like

import {z} from 'zod';
const mismatch1: z.ZodType<{a?: number}> = z.object({});  // type error desirable
const mismatch2: z.ZodType<{a: number}> = z.object({a: z.number()}).readonly();  // type error desirable

It's increasingly important to be able to trust explicit type annotations under isolated declarations.

Approach:

Unlike #60417, this PR does not achieve equality by changing how types are normalized. That's because we don't actually want to change how the type is represented. For example, if we were to normalize {a: 1} & {b: 2} to {a: 1; b: 2} the following code would break:

function example<T extends {} | 1>(t: T) {
    // Ok because {a: 1} relates to {a: number} and {b: 2} relates to {b: number}
    Object.assign(t, {a: 1}, {b: 2}) satisfies {a: number} & {b: number}

    // @ts-expect-error Neither {a: 1} nor {b: 2} relates to {a: number; b: number}
    Object.assign(t, {a: 1}, {b: 2}) satisfies {a: number; b: number}
}

Instead this approach updates the logic of isRelatedTo to directly handle this case.

Caveats

This PR only attempts to make "trivial cases" work. At a high level this includes only intersections of plain object literals. I'll try to explain why other cases are not covered. Also consult the new test cases I added as reference.

Supported - All intersection constituents are plain object literals

{a: 1} & {b: 2} & {c: 3} == {a: 1; b: 2; c: 3}

Supported - Recursive comparisons

{a: {x: 1}} & {a: {y: 2}} == {a: {x: 1; y: 2}}

Unsupported - Any objects have call or construct signatures

{a: 1} & {(): void} != {a: 1; (): void}

This is not supported right now because merging call signatures is more complicated. You would have to ensure that the ordering is preserved since the intersection of call signatures is equivalent to function overloading, where order matters. A subset of simpler call signature cases could be supported, but it doesn't seem worth the complexity for only a half-baked solution.

Unsupported - Any objects have index signatures

{a: 1} & {[key: number]: 2} != {a: 1; [key: number]: 2}

The main reason to not support this now is because recursive merging can be more complicated, e.g.
You can write {[key: number]: {x: 1}} & {[key: number]: {z: 1}} but writing {[key: number]: {x: 1}; [key: number]: {z: 1}} isn't allowed!

Unsupported - Intersection includes non-plain objects, notably generic type variables

T & {a: 1} & {b: 2} != T & {a: 1} & {b: 2}

The main reason to not support this is because the implementation just becomes complicated and causes more types to be instantiated. Also consider the type {a: 1} & T & {b: 2}. At first glance, you might think that it's ok to make this equal to T & {a: 1} & {b: 2} and hence equal to T & {a: 1; b: 2}, but it's not actually safe to reorder constituents of intersections. As mentioned above, changing the order of call signatures changes the semantics of the type:

function example<T extends {a: () => void}>(t: T, x: {a: () => 1}, y: {a: () => 2}) {
   return Object.assign(x, t, y);
   // => {a: () => 1} & T & {a: () => 2}
   // => {a: () => 1} & T & {a: () => void} & {a: () => 2}
}

Like other cases, we could try to handle a simpler subset of cases, but it doesn't seem worth it.

Testing

This patch has been tested in Airtable's codebase and does not introduce any visible regressions. I added several test cases in this PR.

Note

This PR does not automatically make libraries like expect-type work. For example, this will still not work:

import {expectTypeOf} from 'expect-type'
expectTypeOf<{a: 1} & {b: 2}>().toEqualTypeOf<{a: 1; b: 2}>()

Why? Because of the way that expect-type implements its equality check.

export type StrictEqualUsingTSInternalIdenticalToOperator<L, R> =
  (<T>() => T extends (L & T) | T ? true : false) extends <T>() => T extends (R & T) | T ? true : false
    ? IsNever<L> extends IsNever<R>
      ? true
      : false
    : false

Note the (L & T) and (R & T). These intersections contain generic type variables and hence fall under the caveat above. I do not know why expect-type implements type equality this way. I mean I know that it probably copy-pasted the recipe from this thread, but it's unclear why the recipe is defined that way. My proposal for libraries like expect-type to benefit from this PR is to simplify their implementation to:

export type StrictEqualUsingTSInternalIdenticalToOperator<L, R> =
  (<T>() => T extends L ? true : false) extends <T>() => T extends R ? true : false
    ? IsNever<L> extends IsNever<R>
      ? true
      : false
    : false

I don't know of a case that the former definition satisfies that the latter does not, so if anyone knows, feel free to chime in.

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[RyanCavanaugh]

@typescript-bot test it

[typescript-bot]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
test top400	✅ Started	👀 Results
user test this	✅ Started	✅ Results
run dt	✅ Started	👀 Results
perf test this faster	❌ Error: Error: {"$id":"1","innerException":null,"message":"TF400898: An Internal Error Occurred.","typeName":"Microsoft.VisualStudio.Services.CircuitBreaker.CircuitBreakerExceededConcurrencyException, Microsoft.VisualStudio.Services.Common","typeKey":"CircuitBreakerExceededConcurrencyException","errorCode"

[typescript-bot]

@RyanCavanaugh Here are the results of running the user tests with tsc comparing main and refs/pull/60726/merge:

Everything looks good!

[typescript-bot]

Hey @RyanCavanaugh, the results of running the DT tests are ready.

There were interesting changes:

Branch only errors:

Package: hasura
Error:

Error: 
/mnt/vss/_work/1/DefinitelyTyped/types/hasura/hasura-tests.ts
  44:23  error  TypeScript@local compile error: 
Object literal may only specify known properties, and '_eq' does not exist in type 'WhereBoolExp<ScalarJSON<{ foo: string; }>>'                        @definitelytyped/expect
  67:5   error  TypeScript@local compile error: 
Type 'string' is not assignable to type 'OrderBy<ScalarJSON<{ foo: string; }>>'.
  Type 'string' is not assignable to type '{ [key: string]: any; }'   @definitelytyped/expect
  68:5   error  TypeScript@local compile error: 
Type 'string' is not assignable to type 'OrderBy<ScalarJSONB<{ foo: string; }>>'.
  Type 'string' is not assignable to type '{ [key: string]: any; }'  @definitelytyped/expect

✖ 3 problems (3 errors, 0 warnings)

    at combineErrorsAndWarnings (/mnt/vss/_work/1/DefinitelyTyped/node_modules/.pnpm/@definitelytyped+dtslint@0.2.28_typescript@5.8.0-dev.20241212/node_modules/@definitelytyped/dtslint/dist/index.js:194:28)
    at runTests (/mnt/vss/_work/1/DefinitelyTyped/node_modules/.pnpm/@definitelytyped+dtslint@0.2.28_typescript@5.8.0-dev.20241212/node_modules/@definitelytyped/dtslint/dist/index.js:186:20)

You can check the log here.

[MichaelMitchell-at]

Hey @RyanCavanaugh, the results of running the DT tests are ready.

There were interesting changes:

Branch only errors:
Package: hasura Error:

Error: 
/mnt/vss/_work/1/DefinitelyTyped/types/hasura/hasura-tests.ts
  44:23  error  TypeScript@local compile error: 
Object literal may only specify known properties, and '_eq' does not exist in type 'WhereBoolExp<ScalarJSON<{ foo: string; }>>'                        @definitelytyped/expect
  67:5   error  TypeScript@local compile error: 
Type 'string' is not assignable to type 'OrderBy<ScalarJSON<{ foo: string; }>>'.
  Type 'string' is not assignable to type '{ [key: string]: any; }'   @definitelytyped/expect
  68:5   error  TypeScript@local compile error: 
Type 'string' is not assignable to type 'OrderBy<ScalarJSONB<{ foo: string; }>>'.
  Type 'string' is not assignable to type '{ [key: string]: any; }'  @definitelytyped/expect

✖ 3 problems (3 errors, 0 warnings)

    at combineErrorsAndWarnings (/mnt/vss/_work/1/DefinitelyTyped/node_modules/.pnpm/@definitelytyped+dtslint@0.2.28_typescript@5.8.0-dev.20241212/node_modules/@definitelytyped/dtslint/dist/index.js:194:28)
    at runTests (/mnt/vss/_work/1/DefinitelyTyped/node_modules/.pnpm/@definitelytyped+dtslint@0.2.28_typescript@5.8.0-dev.20241212/node_modules/@definitelytyped/dtslint/dist/index.js:186:20)

You can check the log here.

Will take a look at this this week

[typescript-bot]

@RyanCavanaugh Here are the results of running the top 400 repos with tsc comparing main and refs/pull/60726/merge:

Something interesting changed - please have a look.

Details

colinhacks/zod

1 of 5 projects failed to build with the old tsc and were ignored

tsconfig.json

• error TS2345: Argument of type 'true' is not assignable to parameter of type 'false'.
  • src/__tests__/branded.test.ts#L19

[MichaelMitchell-at]

I've sent PRs for @types/hasura and zod with forwards-compatible changes.

• [hasura] Update Equals type to use weaker comparison DefinitelyTyped/DefinitelyTyped#71426 - should be no behavior change for end users of @types/hasura.
• Update type in test for upcoming intersection equality behavior change colinhacks/zod#3910 - trivial change to a "type test"

[RyanCavanaugh]

Can't promise we'll get to this before the holidays, but we'll definitely discuss