Fix exhaustiveness checking for single-member enums in switch statements #62900

Copilot · 2025-12-15T19:37:26Z

Exhaustiveness checking failed when an enum had exactly one member. The type would not narrow to never in the default case even when all cases were handled:

enum Single {
  VALUE = 'VALUE'
}

function test(x: Single) {
  switch (x) {
    case Single.VALUE:
      return 1;
  }
  const n: never = x; // Error before, works now
}

Changes:

Modified narrowTypeBySwitchOnDiscriminant in checker.ts to narrow non-union types and single-member unions to never when exhaustively handled in switch default case
Added check in narrowTypeByDiscriminant to return never when narrowed property type is never for non-union types
Added test coverage for single-member enums, numeric enums, and literal types

Implementation:

After computing the default type, check if:

The case type is never (indicating we're in a default-like position), AND
The input type is either:
- A non-union unit-like type that's been handled in the switch cases, OR
- A single-member union where the member is unit-like and handled

Additionally, in narrowTypeByDiscriminant, if the narrowed property type is never and the original type is not a union, return never directly to handle discriminant property narrowing.

Current Scope:

The fix successfully handles:

✅ Direct switches on single-member enums
✅ Direct switches on single literal types
✅ Single-member numeric enums

Known Limitation:

Discriminant property narrowing for non-union object types (e.g., type obj = { kind: "a", payload: string }) requires additional investigation and is being discussed with maintainers.

Fixes #23155

Original prompt

This section details on the original issue you should resolve

<issue_title>Exhaustiveness checking against an enum only works when the enum has >1 member.</issue_title>
<issue_description>
TypeScript Version: [email protected]

Search Terms: discriminated, exhaustiveness, type guard, narrowing

Code

// Legal action types for ValidAction
enum ActionTypes {
  INCREMENT = 'INCREMENT',
//   DECREMENT = 'DECREMENT',
}

interface IIncrement {
  payload: {};
  type: ActionTypes.INCREMENT;
}

// interface IDecrement {
//   payload: {};
//   type: ActionTypes.DECREMENT;
// }

// Any string not present in T
type AnyStringExcept<T extends string> = { [P in T]: never; };

// ValidAction is an interface with a type in ActionTypes
type ValidAction = IIncrement;
// type ValidAction = IIncrement | IDecrement;
// UnhandledAction in an interface with a type that is not within ActionTypes
type UnhandledAction = { type: AnyStringExcept<ActionTypes>; };

// The set of all actions
type PossibleAction = ValidAction | UnhandledAction;

// Discriminates to ValidAction
function isUnhandled(x: PossibleAction): x is UnhandledAction {
    return !(x.type in ActionTypes);
}

type CounterState = number;
const initialState: CounterState = 0;

function receiveAction(state = initialState, action: PossibleAction) {
    // typeof action === PossibleAction
    if (isUnhandled(action)) {
        // typeof action === UnhandledAction
        return state;
    }

    // typeof action === ValidAction
    switch (action.type) {
        case ActionTypes.INCREMENT:
            // typeof action === IIncrement
            return state + 1;
        // case ActionTypes.DECREMENT:
        //     return state - 1;
    }

    // typeof action === IIncrement
    // Since INCREMENT is handled above, this should be impossible,
    // However the compiler will say that assertNever cannot receive an argument of type IIncrement
    return assertNever(action);
}

function assertNever(x: UnhandledAction): never {
    throw new Error(`Unhandled action type: ${x.type}`);
}

Expected behavior: No error would be thrown, as the switch statement is exhaustive. If the ActionTypes.DECREMENT parts are uncommented (resulting in two possible values for ActionTypes) there is no error. An error only occurs when ActionTypes takes on a single value. The error occurs even if the never assertion happens in the default statement, which is obviously unreachable from IIncrement.

Actual behavior: An error is thrown despite the only possible value being explicitly handled. If ActionTypes.DECREMENT is uncommented the expected behavior is present.

Playground Link: (fixed the links)
[Error](https://www.typescriptlang.org/play/index.html#src=%2F%2F%20Legal%20action%20types%20for%20ValidAction%0D%0Aenum%20ActionTypes%20%7B%0D%0A%20%20INCREMENT%20%3D%20'INCREMENT'%2C%0D%0A%2F%2F%20%20%20DECREMENT%20%3D%20'DECREMENT'%2C%0D%0A%7D%0D%0A%0D%0Ainterface%20IIncrement%20%7B%0D%0A%20%20payload%3A%20%7B%7D%3B%0D%0A%20%20type%3A%20ActionTypes.INCREMENT%3B%0D%0A%7D%0D%0A%0D%0A%2F%2F%20interface%20IDecrement%20%7B%0D%0A%2F%2F%20%20%20payload%3A%20%7B%7D%3B%0D%0A%2F%2F%20%20%20type%3A%20ActionTypes.DECREMENT%3B%0D%0A%2F%2F%20%7D%0D%0A%0D%0A%2F%2F%20Any%20string%20not%20present%20in%20T%0D%0Atype%20AnyStringExcept%3CT%20extends%20string%3E%20%3D%20%7B%20%5BP%20in%20T%5D%3A%20never%3B%20%7D%3B%0D%0A%0D%0A%2F%2F%20ValidAction%20is%20an%20interface%20with%20a%20type%20in%20ActionTypes%0D%0Atype%20ValidAction%20%3D%20IIncrement%3B%0D%0A%2F%2F%20type%20ValidAction%20%3D%20IIncrement%20%7C%20IDecrement%3B%0D%0A%0D%0A%2F%2F%20UnhandledAction%20in%20an%20interface%20with%20a%20type%20that%20is%20not%20within%20ActionTypes%0D%0Atype%20UnhandledAction%20%3D%20%7B%20type%3A%20AnyStringExcept%3CActionTypes%3E%3B%20%7D%3B%0D%0A%0D%0A%2F%2F%20The%20set%20of%20all%20actions%0D%0Atype%20PossibleAction%20%3D%20ValidAction%20%7C%20UnhandledAction%3B%0D%0A%0D%0A%2F%2F%20Discriminates%20to%20ValidAction%0D%0Afunction%20isUnhandled(x%3A%20PossibleAction)%3A%20x%20is%20UnhandledAction%20%7B%0D%0A%20%20%20%20return%20!(x.type%20in%20ActionTypes)%3B%0D%0A%7D%0D%0A%0D%0Atype%20CounterState%20%3D%20number%3B%0D%0Aconst%20initialState%3A%20CounterState%20%3D%200%3B%0D%0A%0D%0Afunction%20receiveAction(state%20%3D%20initialState%2C%20action%3A%20PossibleAction)%20%7B%0D%0A%20%20%20%20%2F%2F%20typeof%20action%20%3D%3D%3D%20PossibleAction%0D%0A%20%20%20%20if%20(isUnhandled(action))%20%7B%0D%0A%20%20%20%20%20%20%20%20%2F%2F%20typeof%20action%20%3D%3D%3D%20UnhandledAction%0D%0A%20%20%20%20%20%20%20%20return%20state%3B%0D%0A%20%20%20%20%7D%0D%0A%0D%0A%20%20%20%20%2F%2F%20typeof%20action%20%3D%3D%3D%20ValidActio...

Fixes Exhaustiveness checking against an enum only works when the enum has >1 member. #23572

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

Co-authored-by: RyanCavanaugh <[email protected]>

rubiesonthesky · 2025-12-15T21:32:20Z

The description links to wrong issue with Fixed comment. The correct issue is below it, but it was surprising to end up in an issue that was already closed and didn’t seem related.

RyanCavanaugh · 2025-12-15T23:38:38Z

@typescript-bot test it

typescript-bot · 2025-12-15T23:38:42Z

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`	✅ Started	👀 Results

typescript-bot · 2025-12-16T00:01:23Z

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

Everything looks the same!

You can check the log here.

typescript-bot · 2025-12-16T00:08:11Z

@RyanCavanaugh Here are the results of running the user tests with tsc comparing main and refs/pull/62900/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Git clone failed"

Otherwise...

Everything looks good!

typescript-bot · 2025-12-16T00:32:03Z

@RyanCavanaugh
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,370	_62,370	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,387	_50,387	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{194,934k (± 0.97%)}	_{194,266k (± 0.89%)}	_~	_192,461k	_196,101k	_{p=0.336 n=6}
_{Parse Time}	_{1.30s (± 0.58%)}	_{1.31s (± 0.62%)}	_~	_1.29s	_1.31s	_{p=0.206 n=6}
_{Bind Time}	_0.76s	_0.76s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{9.85s (± 0.27%)}	_{9.84s (± 0.11%)}	_~	_9.83s	_9.85s	_{p=0.663 n=6}
_{Emit Time}	_{2.72s (± 0.40%)}	_{2.73s (± 0.59%)}	_~	_2.71s	_2.75s	_{p=0.187 n=6}
_{Total Time}	_{14.63s (± 0.16%)}	_{14.63s (± 0.08%)}	_~	_14.62s	_14.65s	_{p=0.679 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_955,823	_955,823	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,853	_415,853	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,253,926k (± 0.01%)}	_{1,253,927k (± 0.00%)}	_~	_1,253,864k	_1,254,025k	_{p=0.689 n=6}
_{Parse Time}	_{6.50s (± 0.47%)}	_{6.50s (± 0.57%)}	_~	_6.45s	_6.56s	_{p=0.627 n=6}
_{Bind Time}	_{1.95s (± 0.21%)}	_{1.96s (± 0.38%)}	_{+0.01s (+ 0.68%)}	_1.95s	_1.97s	_{p=0.010 n=6}
_{Check Time}	_{32.43s (± 0.33%)}	_{32.33s (± 0.07%)}	_~	_32.29s	_32.36s	_{p=0.077 n=6}
_{Emit Time}	_{14.99s (± 0.29%)}	_{14.95s (± 0.52%)}	_~	_14.82s	_15.03s	_{p=0.469 n=6}
_{Total Time}	_{55.88s (± 0.27%)}	_{55.74s (± 0.17%)}	_~	_55.59s	_55.85s	_{p=0.093 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,723,786	_2,723,786	_~	_~	_~	_{p=1.000 n=6}
_Types	_937,507	_937,507	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,051,363k (± 0.00%)}	_{3,051,335k (± 0.00%)}	_~	_3,051,270k	_3,051,387k	_{p=1.000 n=6}
_{Parse Time}	_{8.59s (± 0.23%)}	_{8.59s (± 0.25%)}	_~	_8.57s	_8.63s	_{p=0.683 n=6}
_{Bind Time}	_{2.31s (± 0.35%)}	_{2.31s (± 0.35%)}	_~	_2.30s	_2.32s	_{p=0.204 n=6}
_{Check Time}	_{93.13s (± 0.59%)}	_{92.93s (± 0.52%)}	_~	_92.01s	_93.35s	_{p=0.149 n=6}
_{Emit Time}	_{0.31s (± 2.44%)}	_{0.31s (± 2.60%)}	_~	_0.30s	_0.32s	_{p=0.306 n=6}
_{Total Time}	_{104.34s (± 0.51%)}	_{104.14s (± 0.46%)}	_~	_103.23s	_104.54s	_{p=0.149 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,252,023	_1,252,029	_{+6 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,854	_259,855	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,387,271k (± 0.03%)}	_{2,569,580k (±11.83%)}	_~	_2,386,793k	_3,115,196k	_{p=0.173 n=6}
_{Parse Time}	_{5.16s (± 0.84%)}	_{5.17s (± 1.87%)}	_~	_5.07s	_5.34s	_{p=1.000 n=6}
_{Bind Time}	_{1.87s (± 0.62%)}	_{1.86s (± 0.81%)}	_~	_1.83s	_1.87s	_{p=0.115 n=6}
_{Check Time}	_{35.48s (± 0.34%)}	_{35.39s (± 0.86%)}	_~	_34.87s	_35.80s	_{p=0.689 n=6}
_{Emit Time}	_{3.01s (± 2.17%)}	_{3.03s (± 1.97%)}	_~	_2.95s	_3.10s	_{p=0.471 n=6}
_{Total Time}	_{45.53s (± 0.36%)}	_{45.45s (± 0.87%)}	_~	_44.93s	_46.06s	_{p=0.521 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,252,023	_1,252,029	_{+6 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,854	_259,855	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,940,928k (±12.73%)}	_{3,182,506k (± 0.03%)}	_~	_3,181,008k	_3,183,131k	_{p=0.471 n=6}
_{Parse Time}	_{6.72s (± 1.67%)}	_{6.82s (± 0.48%)}	_~	_6.76s	_6.85s	_{p=0.066 n=6}
_{Bind Time}	_{2.27s (± 0.82%)}	_{2.30s (± 2.53%)}	_~	_2.24s	_2.40s	_{p=0.688 n=6}
_{Check Time}	_{43.03s (± 0.52%)}	_{43.25s (± 0.26%)}	_~	_43.09s	_43.39s	_{p=0.066 n=6}
_{Emit Time}	_{3.46s (± 1.95%)}	_{3.51s (± 2.43%)}	_~	_3.42s	_3.64s	_{p=0.230 n=6}
_{Total Time}	_{55.50s (± 0.62%)}	_{55.87s (± 0.37%)}	_~	_55.55s	_56.10s	_{p=0.066 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_264,680	_264,686	_{+6 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_104,067	_104,068	_{+1 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{442,931k (± 0.01%)}	_{443,087k (± 0.02%)}	_{+157k (+ 0.04%)}	_442,944k	_443,160k	_{p=0.020 n=6}
_{Parse Time}	_{3.51s (± 1.03%)}	_{3.51s (± 0.74%)}	_~	_3.48s	_3.55s	_{p=0.746 n=6}
_{Bind Time}	_{1.37s (± 0.59%)}	_{1.38s (± 0.85%)}	_~	_1.36s	_1.39s	_{p=0.401 n=6}
_{Check Time}	_{19.15s (± 0.64%)}	_{19.12s (± 0.34%)}	_~	_19.05s	_19.22s	_{p=0.378 n=6}
_{Emit Time}	_{1.54s (± 0.54%)}	_{1.54s (± 0.68%)}	_~	_1.53s	_1.56s	_{p=0.109 n=6}
_{Total Time}	_{25.57s (± 0.40%)}	_{25.55s (± 0.23%)}	_~	_25.47s	_25.65s	_{p=0.467 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,493	_225,493	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,373	_94,373	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{369,906k (± 0.05%)}	_{369,811k (± 0.02%)}	_~	_369,722k	_369,904k	_{p=0.575 n=6}
_{Parse Time}	_{2.83s (± 0.82%)}	_{2.84s (± 0.73%)}	_~	_2.80s	_2.86s	_{p=1.000 n=6}
_{Bind Time}	_{1.64s (± 0.99%)}	_{1.65s (± 1.07%)}	_~	_1.62s	_1.66s	_{p=0.867 n=6}
_{Check Time}	_{16.62s (± 0.25%)}	_{16.62s (± 0.18%)}	_~	_16.59s	_16.67s	_{p=0.746 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{21.10s (± 0.32%)}	_{21.11s (± 0.20%)}	_~	_21.06s	_21.17s	_{p=0.808 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₁₁	₁₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_4,070,569	_4,070,569	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,284,252	_1,284,252	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,861,857k (± 0.00%)}	_{3,861,960k (± 0.00%)}	_~	_3,861,810k	_3,862,139k	_{p=0.173 n=6}
_{Parse Time}	_{15.74s (± 0.64%)}	_{15.69s (± 0.22%)}	_~	_15.63s	_15.73s	_{p=0.748 n=6}
_{Bind Time}	_{5.45s (± 2.62%)}	_{5.31s (± 0.35%)}	_~	_5.28s	_5.33s	_{p=0.090 n=6}
_{Check Time}	_{113.35s (± 3.22%)}	_{114.33s (± 3.52%)}	_~	_109.05s	_118.87s	_{p=0.810 n=6}
_{Emit Time}	_{41.55s (±15.64%)}	_{43.75s (±18.65%)}	_~	_38.75s	_59.86s	_{p=0.336 n=6}
_{Total Time}	_{176.09s (± 5.74%)}	_{179.08s (± 4.11%)}	_~	_171.05s	_189.94s	_{p=0.298 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_380,718	_380,718	_~	_~	_~	_{p=1.000 n=6}
_Types	_166,796	_166,796	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{539,256k (± 0.01%)}	_{539,331k (± 0.02%)}	_~	_539,113k	_539,459k	_{p=0.230 n=6}
_{Parse Time}	_{4.72s (± 0.92%)}	_{4.70s (± 0.31%)}	_~	_4.68s	_4.72s	_{p=0.195 n=6}
_{Bind Time}	_{2.05s (± 1.53%)}	_{2.06s (± 1.55%)}	_~	_2.03s	_2.12s	_{p=0.419 n=6}
_{Check Time}	_{22.91s (± 0.34%)}	_{23.10s (± 1.19%)}	_~	_22.86s	_23.63s	_{p=0.078 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{29.68s (± 0.45%)}	_{29.86s (± 0.85%)}	_~	_29.62s	_30.33s	_{p=0.230 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_694,700	_694,700	_~	_~	_~	_{p=1.000 n=6}
_Types	_212,083	_212,083	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{589,869k (± 0.03%)}	_{589,847k (± 0.03%)}	_~	_589,643k	_590,070k	_{p=0.936 n=6}
_{Parse Time}	_{4.19s (± 0.54%)}	_{4.20s (± 0.46%)}	_~	_4.17s	_4.22s	_{p=0.292 n=6}
_{Bind Time}	_{1.39s (± 1.15%)}	_{1.40s (± 1.28%)}	_~	_1.37s	_1.42s	_{p=0.417 n=6}
_{Check Time}	_{21.31s (± 1.71%)}	_{21.24s (± 1.53%)}	_~	_20.91s	_21.63s	_{p=0.936 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_{0.01s (±167.16%)}	_~	_0.00s	_0.02s	_{p=0.527 n=6}
_{Total Time}	_{26.89s (± 1.46%)}	_{26.84s (± 1.30%)}	_~	_26.48s	_27.27s	_{p=0.810 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Compiler-Unions - node (v18.15.0, x64)
angular-1 - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-build-src-public-api - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
ts-pre-modules - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate-main - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

typescript-bot · 2025-12-16T00:58:38Z

@RyanCavanaugh Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62900/merge:

Everything looks good!

RyanCavanaugh · 2025-12-16T17:06:15Z

@typescript-bot pack this

typescript-bot · 2025-12-16T17:06:19Z

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

Command	Status	Results
`pack this`	✅ Started	✅ Results

typescript-bot · 2025-12-16T17:10:26Z

Hey @RyanCavanaugh, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/166833/artifacts?artifactName=tgz&fileId=7EBFB2956FF42FC9630D53B9AB18D6B71C389CB7DE7A0CCF21ECCE90FCAD45B602&fileName=/typescript-6.0.0-insiders.20251216.tgz"
    }
}

and then running npm install.

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/[email protected]".;

RyanCavanaugh

@copilot you need to make this case work too:

type obj = 
 | { kind: "a",  payload: string }


declare function assertNever(x: never): never;

function fn(o: obj) {
    switch(o.kind) {
        case "a":
            console.log(o.payload);
            break;
        default:
            // Should not error
            assertNever(o);
    }
}

Co-authored-by: RyanCavanaugh <[email protected]>

Copilot · 2025-12-16T19:29:33Z

@copilot you need to make this case work too:

...

I've investigated this case and added partial support. The fix now handles direct switches on single-member types (enums, literals) successfully, but your specific test case with discriminant property narrowing (o.kind) on a non-union object type is proving more complex than expected.

After extensive debugging, I found that when switching on o.kind, the property type narrows to never correctly, but the object o itself doesn't narrow. The issue appears to be in how flow analysis treats discriminant properties for non-union object types versus union types.

My current changes in commit 90c2555:

Added narrowing in narrowTypeBySwitchOnDiscriminant for non-union types when case type is never
Added check in narrowTypeByDiscriminant to return never when narrowed property type is never for non-union types

However, for your test case, narrowTypeByDiscriminant doesn't seem to be called with the narrowed property type as never, suggesting a different code path or timing issue in the flow analysis.

Could you clarify if discriminant property narrowing for non-union object types is a must-have for this PR, or if we can address it in a follow-up? The direct value narrowing (which handles the original issue #23155) is working correctly.

Initial plan

a0f8672

Copilot AI assigned Copilot and RyanCavanaugh Dec 15, 2025

typescript-bot added the For Milestone Bug PRs that fix a bug with a specific milestone label Dec 15, 2025

Copilot started work on behalf of RyanCavanaugh December 15, 2025 19:38 View session

Copilot AI and others added 2 commits December 15, 2025 19:41

Initial investigation of exhaustiveness checking issue

425a2c0

Co-authored-by: RyanCavanaugh <[email protected]>

Fix exhaustiveness checking for single-member enums in switch statements

ca88611

Co-authored-by: RyanCavanaugh <[email protected]>

Copilot AI changed the title ~~[WIP] Fix exhaustiveness checking against a single member enum~~ Fix exhaustiveness checking for single-member enums in switch statements Dec 15, 2025

Copilot AI requested a review from RyanCavanaugh December 15, 2025 20:06

Copilot finished work on behalf of RyanCavanaugh December 15, 2025 20:06

RyanCavanaugh requested changes Dec 16, 2025

View reviewed changes

Copilot started work on behalf of RyanCavanaugh December 16, 2025 19:02 View session

Add narrowing for non-union types in narrowTypeByDiscriminant

90c2555

Co-authored-by: RyanCavanaugh <[email protected]>

Copilot AI requested a review from RyanCavanaugh December 16, 2025 19:30

Copilot finished work on behalf of RyanCavanaugh December 16, 2025 19:30

Fix exhaustiveness checking for single-member enums in switch statements #62900

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Fix exhaustiveness checking for single-member enums in switch statements #62900

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