Order of ReadonlySet/ReadonlyMap in union causes differing key inference

[jakebailey]

Bug Report

🔎 Search Terms

ReadonlyMap ReadonlySet key inference

🕗 Version & Regression Information

• This is the behavior in every version I tried.

⏯ Playground Link

Playground Link

💻 Code

declare const map: Map<string, number>;


declare function forEachKey<K, T>(map: ReadonlySet<K> | ReadonlyMap<K, any>, callback: (key: K) => T | undefined): T | undefined;

forEachKey(map, (key) => {}) // Error passing "map", key type is wrong (number??)

forEachKey<string, void>(map, (key) => {}) // OK; no error, key type is string


// Now, flip the union order in the parameter's type.
declare function forEachKey2<K, T>(map: ReadonlyMap<K, any> | ReadonlySet<K>, callback: (key: K) => T | undefined): T | undefined;

forEachKey2(map, (key) => {}) // Okay?

forEachKey2<string, void>(map, (key) => {}) // Still okay.

🙁 Actual behavior

The order of the union appears to matter, breaking inference and erroring on valid code. Swapping the union order allows the code to compile.

🙂 Expected behavior

All of this should compile; the union ordering should not matter.

[typescript-bot]

👋 Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in the issue body running against the nightly TypeScript.

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Issue body code block by @jakebailey

‼️ Exception: Error - error TS5107: Option 'moduleResolution=node10' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error. Visit https://aka.ms/ts6 for migration information.

Error: error TS5107: Option 'moduleResolution=node10' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
  Visit https://aka.ms/ts6 for migration information.

    at Object.createVirtualTypeScriptEnvironment (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:8128:11)
    at twoslasher (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:7618:17)
    at /home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:439:44
    at runTwoslashRequests (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:406:56)
    at run (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:20096:75)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

⚠️ Way slower than historical runs

Historical Information

Version	Reproduction Outputs	Time
4.8.2, 4.9.3, 5.0.2, 5.1.3, 5.2.2	❌ Failed: - Argument of type 'Map' is not assignable to parameter of type 'ReadonlySet | ReadonlyMap'. Type 'Map' is not assignable to type 'ReadonlyMap'. Types of property 'forEach' are incompatible. Type '(callbackfn: (value: number, key: string, map: Map) => void, thisArg?: any) => void' is not assignable to type '(callbackfn: (value: any, key: number, map: ReadonlyMap) => void, thisArg?: any) => void'. Types of parameters 'callbackfn' and 'callbackfn' are incompatible. Types of parameters 'key' and 'key' are incompatible. Type 'string' is not assignable to type 'number'.	⚠️ Way slower