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[NewErrors] 5.4.0-dev.20240121 vs 5.3.3 #57117

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[NewErrors] 5.4.0-dev.20240121 vs 5.3.3#57117

Description

@typescript-bot
TypeScript Bot (typescript-bot)
opened on Jan 21, 2024

The following errors were reported by 5.4.0-dev.20240121, but not by 5.3.3
Pipeline that generated this bug
Logs for the pipeline run
File that generated the pipeline

This run considered 200 popular TS repos from GH (after skipping the top 0).

Successfully analyzed 115 of 200 visited repos
Outcome Count
Detected interesting changes 12
Detected no interesting changes 103
Git clone failed 3
Package install failed 35
Project-graph error in old TS 5
Too many errors in old TS 38
Unknown failure 4

Investigation Status

Repo Errors Outcome
BuilderIO/qwik 2 #56004, #56598
chakra-ui/chakra-ui 1 Not a regression
drizzle-team/drizzle-orm 16 #56004
heyxyz/hey 1 Intentional isolatedModules correctness improvement
pixijs/pixijs 2 Routine DOM update #57027
prisma/prisma 10 Intentional isolatedModules correctness improvement
pubkey/rxdb 3 #54477, only showed up in editor for some reason
quilljs/quill 3 Unused ts-expect-error due to #56908, Intentional isolatedModules correctness improvement
react-hook-form/react-hook-form 1 #56004
reduxjs/react-redux 2 #56515
refined-github/refined-github 5 #56004
vuejs/core 3 #55015, #56004

Activity

  1. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    BuilderIO/qwik

    10 of 11 projects failed to build with the old tsc and were ignored

    tsconfig.json

  2. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    chakra-ui/chakra-ui

    4 of 28 projects failed to build with the old tsc and were ignored

    packages/components/tsconfig.build.json

    • error TS5056: Cannot write file '/mnt/ts_downloads/chakra-ui/packages/components/dist/types/menu/menu.stories.d.ts' because it would be overwritten by multiple input files.
      • Project Scope

  3. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    drizzle-team/drizzle-orm

    18 of 25 projects failed to build with the old tsc and were ignored

    drizzle-orm/tests/tsconfig.json

    • error TS2345: Argument of type 'SQLiteD1Session<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2345: Argument of type 'LibSQLSession<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'TFullSchema extends Record<string, never> ? DrizzleTypeError<"Seems like the schema generic is missing - did you forget to add it to your DB type?"> : { [K in keyof TSchema]: RelationalQueryBuilder<TResultKind, TFullSchema, TSchema, TSchema[K]>; }'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'this["query"]'.

    drizzle-orm/tsconfig.dts.json

    • error TS2345: Argument of type 'SQLiteD1Session<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2345: Argument of type 'LibSQLSession<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'TFullSchema extends Record<string, never> ? DrizzleTypeError<"Seems like the schema generic is missing - did you forget to add it to your DB type?"> : { [K in keyof TSchema]: RelationalQueryBuilder<TResultKind, TFullSchema, TSchema, TSchema[K]>; }'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'this["query"]'.

    drizzle-orm/tsconfig.json

    • error TS2345: Argument of type 'SQLiteD1Session<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2345: Argument of type 'LibSQLSession<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'TFullSchema extends Record<string, never> ? DrizzleTypeError<"Seems like the schema generic is missing - did you forget to add it to your DB type?"> : { [K in keyof TSchema]: RelationalQueryBuilder<TResultKind, TFullSchema, TSchema, TSchema[K]>; }'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'this["query"]'.

    drizzle-orm/type-tests/tsconfig.json

    • error TS2345: Argument of type 'SQLiteD1Session<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2345: Argument of type 'LibSQLSession<TSchema, ExtractTablesWithRelations<TSchema>>' is not assignable to parameter of type 'SQLiteSession<"async", unknown, Record<string, unknown>, TablesRelationalConfig>'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'TFullSchema extends Record<string, never> ? DrizzleTypeError<"Seems like the schema generic is missing - did you forget to add it to your DB type?"> : { [K in keyof TSchema]: RelationalQueryBuilder<TResultKind, TFullSchema, TSchema, TSchema[K]>; }'.
    • error TS2536: Type 'keyof TSchema' cannot be used to index type 'this["query"]'.

  4. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    heyxyz/hey

    1 of 12 projects failed to build with the old tsc and were ignored

    apps/web/tsconfig.json

  5. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    pixijs/pixijs

    tsconfig.json

    tsconfig.types.json

  6. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    prisma/prisma

    75 of 106 projects failed to build with the old tsc and were ignored

    packages/debug/tsconfig.build.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/debug/tsconfig.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/get-platform/tsconfig.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/engines/tsconfig.build.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/engines/tsconfig.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/generator-helper/tsconfig.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/internals/tsconfig.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/instrumentation/tsconfig.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/client/tsconfig.build.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

    packages/cli/tsconfig.build.json

    • error TS2865: Import 'Debug' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.

  7. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    pubkey/rxdb

    9 of 11 projects failed to build with the old tsc and were ignored

    tsconfig.json

  8. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    quilljs/quill

    packages/quill/tsconfig.json

    tsconfig.json

  9. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    react-hook-form/react-hook-form

    2 of 3 projects failed to build with the old tsc and were ignored

    tsconfig.json

  10. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    reduxjs/react-redux

    2 of 3 projects failed to build with the old tsc and were ignored

    test/typetests/tsconfig.json

  11. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    refined-github/refined-github

    tsconfig.json

  12. typescript-bot commented on Jan 21, 2024

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Jan 21, 2024
    ContributorAuthor

    vuejs/core

    1 of 3 projects failed to build with the old tsc and were ignored

    tsconfig.build.json

    tsconfig.json

  13. MichaelMitchell-at commented on Jan 22, 2024

    @MichaelMitchell-at
    michaelm (MichaelMitchell-at)
    on Jan 22, 2024
    Contributor

    Copying over my comment from #56972

    Minimal repro of a "cannot be used as an index type" error:

    type Type = {prop: 'a', otherProp: unknown}

type X = 'prop' extends keyof Type
    ? Type['prop']
           ^^^^^^ Type '"prop"' cannot be used as an index type.(2538)
    : never;

  14. RyanCavanaugh commented on Jan 22, 2024

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Jan 22, 2024
    Member

    michaelm (@MichaelMitchell-at) fix at #57113

  15. RyanCavanaugh commented on Jan 23, 2024

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Jan 23, 2024
    Member

    react-redux is trying to do the should-be-impossible and prevent function arguments from subtyping with this function

    export function expectExactType<T>(t: T) {
  return <U extends Equals<T, U>>(u: U) => {}
}

    demonstrated here:

    // @ts-expect-error
expectExactType('5' as string)('5' as const)

    and also to prevent any from bypassing errors

    // @ts-expect-error
expectExactType('5' as any)('5' as const)

    To the extent they once succeeded, they're now broken by #56515. However, this functionality was never used AFAICT, every call is in the form e.g.

    expectExactType<AppStore>(store)

    so I don't think this is a blocker for that project in particular.

  16. 2 remaining items

  17. Andarist commented on Jan 23, 2024

    @Andarist
    Mateusz Burzyński (Andarist)
    on Jan 23, 2024
    Contributor

    That was just a typo - I meant what you have confirmed 😉 That minimal repro didn't typecheck with 5.3 so it just continues to be an error with the current nightly of 5.4 and because of that it's unlikely to be related to any errors reported here.

  18. andrewbranch commented on Jan 24, 2024

    @andrewbranch
    Andrew Branch (andrewbranch)
    on Jan 24, 2024
    Member

    Anders Hejlsberg (@ahejlsberg) react-hook-form repro:

    type AsyncDefaultValues<TFieldValues> = (
  payload?: unknown,
) => Promise<TFieldValues>;

type DefaultValues<TFieldValues> =
  TFieldValues extends AsyncDefaultValues<TFieldValues>
    ? Awaited<TFieldValues>
    : TFieldValues;

function f<T extends Record<string, any>>(options: DefaultValues<T> | AsyncDefaultValues<T>) {
  if (typeof options === 'function') {
    options().then((data) => data);
  }
}

    https://www.typescriptlang.org/play?module=1&ts=5.4.0-dev.20240124#code/C4TwDgpgBAggziAdgYwCIQGYEMCuAbYANSzxwjgB4AVAMQEsI8ATY08gPigF4oAKAKChQwWEHgD2WJgH4AXFByIA1onEB3RABp+ASm6cACgCdxAWzpwI1eoxYkycdgG5+-UJCjps+IvfLWGZlYHTi5BKFpAuzY4KAgAD2AIRCZY+CQ0TFwCYP9I21zHcKFpWDUsOiSmAIK-IqEheXygupd+DEVkYDpxRCgMajjE5NSoACUIZHEjarhgIzpEAHNNKCxEEHZ2XnEwbt64eS9s3xjqTgAfWAQUY59C870Ab3C6DD53CHF33f3EWK4gKgAHIOig-sDnsUoL8ev9eDoAHTAAAWyV4vCYWGAWD0XE4WJxOhcQgAvvxyUA

  19. Nathan Shively-Sanders (sandersn)
    added a commit that references this issue on Jan 24, 2024

    Revert microsoft#54477 but keep the tests

    10bbe3d

  20. sandersn commented on Jan 25, 2024

    @sandersn
    Nathan Shively-Sanders (sandersn)
    on Jan 25, 2024
    Member

    I double checked qwik. The errors below are definitely caused by #56598. I manually reverted that PR's checker diff and the errors went away. I'll try to make a small repro.

    error TS2345: Argument of type 'EventQRL<T>' is not assignable to parameter of type 'EventQRL<"copy" | "progress" | "reset" | "scroll" | "resize" | "drag" | "fullscreenchange" | "fullscreenerror" | "abort" | "animationcancel" | "animationend" | "animationiteration" | ... 112 more ... | "qvisible">'.

  21. ahejlsberg commented on Jan 25, 2024

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Jan 25, 2024
    Member

    Regarding the new react-hook-form error, the issue is the Record<string, any> constraint on T. We have rule that says an object with an [x: string]: any index signature is a supertype of any object type, even if that object type doesn't have a matching index signature. This includes function types, so in the example, the typeof options === "function" check can't just narrow the type to AsyncDefaultValues<T>. Indeed, options could be any arbitrary function. To wit, this call is permitted:

    f(() => "hello");

    The reason we issue an implicit any error on the data parameter is that, since the function check can really only narrow to type Function, we end up with an untyped function call.

    Things work fine when the constraint of T is changed to Record<string, unknown>. In that case we error on calls to f with arbitrary function types, and we narrow to AsyncDefaultValues<T> following the function check.

    Other than adding a type assertion, I'm not sure what the best fix is in the react-hook-form project since new errors pop up if I just change the constraint to Record<string, unknown>. But, certainly, I think our is correct and points out a latent issue in the code.

  22. sandersn commented on Jan 25, 2024

    @sandersn
    Nathan Shively-Sanders (sandersn)
    on Jan 25, 2024
    Member

    Here's an independent repro for qwik's use-on errors. This one fails after #56598 and passes before (where 'before' includes #56004 -- though both are required for the error to appear.)

    (I'll have a much smaller repro soon; this is just the first one that is independent of qwik)

    export type Q<TYPE = unknown> = {
  // Special type brand to let eslint that the Type is serializable
  __qwik_serializable__?: any;
  __brand__Q__: TYPE;

  /** Resolve the Q and return the actual value. */
  resolve(): Promise<TYPE>;
  /** The resolved value, once `resolve()` returns. */
  resolved: undefined | TYPE;

  getCaptured(): unknown[] | null;
  getSymbol(): string;
  getHash(): string;
}
type Handler<EV = Event, EL = {}> = {
  bivarianceHack(event: EV, element: EL): any;
}['bivarianceHack'];
type VirtualKeys = keyof WMap
interface E { }
interface WMap {
    "boo": E;
}
type AllEventsMap = WMap
type LcEvent<T extends string, C extends string = Lowercase<T>> = C extends keyof AllEventsMap
  ? AllEventsMap[C]
  : Event;
type VirtualEvent<T extends string = VirtualKeys> =
  | Q<Handler<LcEvent<T>, {}>>
  | undefined;
type Stringy<LiteralType, BaseType extends null | undefined | string | number | boolean | symbol | bigint> =
  | LiteralType
  | (BaseType & Record<never, never>);
type VirtualEventNames = Stringy<VirtualKeys, string>; // VirtualKeys | (string & { });
declare const _virtualOn: (eventQrl: VirtualEvent<VirtualKeys>) => void;
export const virtualOn = <T extends VirtualEventNames>(eventQrl: VirtualEvent<T>) => {
  _virtualOn(eventQrl); // error here where none was before
};

    Smaller:

    // interesting things:
// 1. only errors if event in EMap is all lowercase.
// 2. bivariance hack is required
// 3. Lowercase<T> is required
type EMap = { event: {} }
type Keys = keyof EMap
type EPlusFallback<C> = C extends Keys
  ? EMap[C]
  : "unrecognised event";
type VirtualEvent<T extends string> = { bivarianceHack(event: EPlusFallback<Lowercase<T>>): any; }['bivarianceHack'];
type KeysPlusString = Keys | (string & Record<never, never>);
declare const _virtualOn: (eventQrl: VirtualEvent<Keys>) => void;
export const virtualOn = <T extends KeysPlusString>(eventQrl: VirtualEvent<T>) => {
  _virtualOn(eventQrl);
};

  23. sandersn commented on Jan 25, 2024

    @sandersn
    Nathan Shively-Sanders (sandersn)
    on Jan 25, 2024
    Member

    The error is the one Ryan Cavanaugh (@RyanCavanaugh) and Anders Hejlsberg (@ahejlsberg) found before--KeysPlusString is correctly not assignable to Keys in VIrtualEvent.

    The error seems to show that the conditional type EPlusFallback isn't assignable to its base contraint:

    Argument of type '(event: EPlusFallback<Lowercase<T>>) => any' is not assignable to parameter of type '(event: {}) => any'.
  Types of parameters 'event' and 'event' are incompatible.
    Type '{}' is not assignable to type 'EPlusFallback<Lowercase<T>>'.ts(2345)

    EPlusFallback<Lowercase<T>> should have the base constraint {} | "unrecognised event", but I guess that assignability doesn't try that.

    Also: I guess assignability to Lowercase<T> must go through isValidTypeForTemplateLiteralPlaceholder, because that's the only thing that looks relevant to #56598.

  24. ahejlsberg commented on Jan 25, 2024

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Jan 25, 2024
    Member

    Nathan Shively-Sanders (@sandersn) I tried reverting #56598 and it doesn't seem to have any effect. Your repro still errors in the same way. So I think the issue is just caused by #56004.

  25. sandersn commented on Jan 25, 2024

    @sandersn
    Nathan Shively-Sanders (sandersn)
    on Jan 25, 2024
    Member

    It repros for me, so maybe it's the tsconfig I'm using. I'll create a standalone repo.

    Example run of before/after showing no error/error 
    ✔ ~/ts [main|⚑ 1] 
14:30 $ git checkout e5513254a3
Note: switching to 'e5513254a3'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at e5513254a3 Fixed symbol lookup for binding expando properties in blocks (#56552)
✔ ~/ts [:e5513254a3|⚑ 1] 
14:31 $ b
Using ~/ts/Herebyfile.mjs to run tsc
Starting lib
Starting generate-diagnostics
> /home/nathansa/.nvm/versions/node/v20.11.0/bin/node scripts/processDiagnosticMessages.mjs src/compiler/diagnosticMessages.json
Reading diagnostics from src/compiler/diagnosticMessages.json
Finished lib in 33ms
Finished generate-diagnostics in 63ms
Starting bundle-tsc
Finished bundle-tsc in 179ms
Completed tsc in 242ms
✔ ~/ts [:e5513254a3|⚑ 1] 
14:31 $ pushd
~/src/test ~/ts
t✔ ~/src/test [master L|✚ 15…16] 
14:31 $ tsl
✔ ~/src/test [master L|✚ 15…16] 
14:31 $ pushd
~/ts ~/src/test
✔ ~/ts [:e5513254a3|⚑ 1] 
14:31 $ git checkout 68b9b07
Previous HEAD position was e5513254a3 Fixed symbol lookup for binding expando properties in blocks (#56552)
HEAD is now at 68b9b07264 Consistently check assignability to template literal placeholders (#56598)
✔ ~/ts [:68b9b07264|⚑ 1] 
14:31 $ b
Using ~/ts/Herebyfile.mjs to run tsc
Starting lib
Starting generate-diagnostics
> /home/nathansa/.nvm/versions/node/v20.11.0/bin/node scripts/processDiagnosticMessages.mjs src/compiler/diagnosticMessages.json
Reading diagnostics from src/compiler/diagnosticMessages.json
Finished lib in 32ms
Finished generate-diagnostics in 61ms
Starting bundle-tsc
puFinished bundle-tsc in 190ms
Completed tsc in 252ms
s✔ ~/ts [:68b9b07264|⚑ 1] 
14:31 $ pushd
~/src/test ~/ts
✔ ~/src/test [master L|✚ 15…16] 
14:31 $ tsl
welove.ts:22:14 - error TS2345: Argument of type '(event: EPlusFallback<Lowercase<T>>) => any' is not assignable to parameter of type '(event: {}) => any'.
  Types of parameters 'event' and 'event' are incompatible.
    Type '{}' is not assignable to type 'EPlusFallback<Lowercase<T>>'.

22   _virtualOn(eventQrl);
                ~~~~~~~~


Found 1 error in welove.ts:22

✘-2 ~/src/test [master L|✚ 15…16]

  26. sandersn commented on Jan 25, 2024

    @sandersn
    Nathan Shively-Sanders (sandersn)
    on Jan 25, 2024
    Member

    Here's the standalone repo: https://github.com/sandersn/repro56598

  27. ahejlsberg commented on Jan 26, 2024

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Jan 26, 2024
    Member

    Ok, I see the same effects with your repro. The root cause is #56004, but the issue only shows up after #56598 because that PR fixes template literal placeholder matching such that `${string & Record<never, never>}` behaves the same as just `${string}` (which indeed it should). And apparently we use template literal matching when checking relations for two string mapping types (this happens in isMemberOfStringMapping). Crazy.

  28. ahejlsberg commented on Jan 26, 2024

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Jan 26, 2024
    Member

    Anyway, this means the repro can be reduced to:

    type EMap = { event: {} }
type Keys = keyof EMap
type EPlusFallback<C> = C extends Keys ? EMap[C] : "unrecognised event";
type VirtualEvent<T extends string> = { bivarianceHack(event: EPlusFallback<Lowercase<T>>): any; }['bivarianceHack'];
declare const _virtualOn: (eventQrl: VirtualEvent<Keys>) => void;
export const virtualOn = <T extends string>(eventQrl: VirtualEvent<T>) => {
  _virtualOn(eventQrl);
};

  29. ahejlsberg commented on Jan 26, 2024

    @ahejlsberg
    Anders Hejlsberg (ahejlsberg)
    on Jan 26, 2024
    Member

    Ok, I now see what's going on.

    The repro checks whether EPlusFallback<Lowercase<T>> is assignable to EPlusFallback<Keys> (which is just {}).

    With #56004, the constraint of EPlusFallback<Lowercase<T>> ends up correctly being type EMap["event" & Lowercase<T>] | "unrecognized event", where previously it would just be "unrecognized event".

    The constraint of EMap["event" & Lowercase<T>] in turn is reduced to EMap["event" & Lowercase<string>].

    This is where things go wrong. We currently lack the ability to reduce unions and intersections of string literal types (like "event") and string mapping types with non-generic placeholders (like Lowercase<string>). The intersection "event" & Lowercase<string> really should reduce to just "event", but it doesn't. So, we fail to consider it a valid index type in EMap["event" & Lowercase<string>], which therefore ends up being unknown. And unknown isn't assignable to {}, so therefore error.

    So, long story short, we can fix this issue by doing a better job reducing unions and intersections of string literal types and string mapping types with non-generic placeholders. I will look at putting up a PR.

  30. Ryan Cavanaugh (RyanCavanaugh)
    closed this as completedon Jan 29, 2024
  31. microsoft
    locked as resolved and limited conversation to collaborators on Oct 22, 2025
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