Enum members aren't subtype reduced in unions

[DanielRosenwasser]

enum E {
    A,
    B,
    C,
}

function f(x: E | undefined) {
    return x ?? E.C;
}

Expected f to have type E, but instead it has type E | E.C

Playground

Doesn't have anything to do with ?? - can reproduce it with ||.

enum E {
    A = 1,
    B,
    C,
}

function f(x: E | undefined) {
    return x || E.C;
}

[jack-williams]

Related: #32621?

[DanielRosenwasser]

enum E {
    A,
    B,
    C,
}

function f(x: E | undefined) {
//       ^?
    return x ?? E.C;
}

Workbench Repro

[DanielRosenwasser]

I think this is fixed now. Would we have any volunteers willing to add a test case to prevent regressions?

[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.

---

Issue body code block by @DanielRosenwasser

‼️ Exception: Error - Did not find a source file for /home/runner/work/TypeScript/TypeScript/index.ts

Error: Did not find a source file for /home/runner/work/TypeScript/TypeScript/index.ts
    at Object.updateFile (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:6876:15)
    at _loop3 (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:1738:9)
    at twoslasher (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:1744:17)
    at /home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:777:44
    at runTwoslashRequests (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:744:56)
    at run (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:2169:75)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)

Historical Information

Version	Reproduction Outputs	Time
4.2.2, 4.3.2, 4.4.2, 4.5.2, 4.6.2	👍 Compiled	⚠️ Way slower

[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 this repro running against the nightly TypeScript.

---

Comment by @DanielRosenwasser

‼️ Exception: TypeError - Cannot read properties of undefined (reading 'getPositionOfLineAndCharacter')

TypeError: Cannot read properties of undefined (reading 'getPositionOfLineAndCharacter')
    at Object.getPositionOfLineAndCharacter (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/ts/nightly/lib/typescript.js:10384:27)
    at /home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:1679:25
    at Array.map (<anonymous>)
    at _loop3 (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:1677:40)
    at twoslasher (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:1744:17)
    at /home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:777:44
    at runTwoslashRequests (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:744:56)
    at run (/home/runner/work/_actions/microsoft/TypeScript-Twoslash-Repro-Action/master/dist/index.js:2169:75)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)

Historical Information

Version	Reproduction Outputs	Time
4.2.2, 4.3.2, 4.4.2, 4.5.2, 4.6.2	⚠️ Assertions: function f(x: E | undefined): E	⚠️ Way slower