Bring back return type narrowing + fixes by gabritto · Pull Request #61359 · microsoft/TypeScript

gabritto · 2025-03-05T22:04:28Z

This PR brings back conditional and indexed access return type narrowing from #56941, but with a few updates.
The main motivation behind those updates is described in https://gist.github.com/gabritto/b6ebd5f9fc2bb3cfc305027609e66bca,
but to put it shortly, type narrowing very often cannot distinguish between two non-primitive types,
and since return type narrowing depends on type narrowing to work, it often didn't.

Update 1: non-primitive restriction

To deal with those problematic type narrowing scenarios, the first update is to disallow narrowing of conditional return types that
attempt to distinguish between two non-primitive types.
So this example will not work:

type QuickPickReturn<T extends QuickPickOptions> =
    T extends { canSelectMultiple: true } ? string[] :
    T extends { canSelectMultiple: false } ? string :
    never;
    
type QuickPickOptions = {
    prompt: string;
    items: string[];
    canSelectMultiple: true;
} | {
    prompt: string;
    items: string[];
    canSelectMultiple: false;
}

function showQuickPick<T extends QuickPickOptions>(options: T): QuickPickReturn<T> {
    if (options.canSelectMultiple) {
        return options.items; // Error
    }
    return options.items[0]; // Error
}

That's because the conditional return type QuickPickReturn has one branch with type { canSelectMultiple: true }, which is non-primitive,
and another branch with type { canSelectMultiple: false }, which is also non-primitive.

However, the following will work:

type QuickPickOptions<T extends boolean> = {
    prompt: string;
    items: string[];
    canSelectMultiple: T;
};

type QuickPickReturn<T extends boolean> =
    T extends true ? string[] :
    T extends false ? string :
    never;

function showQuickPick<T extends boolean>(options: QuickPickOptions<T>): QuickPickReturn<T> {
    if (options.canSelectMultiple) {
        return options.items;
    }
    return options.items[0];
}

type QuickPickOptions = {
    prompt: string;
    items: string[];
};

type QuickPickReturn<T extends string | QuickPickOptions> =
    T extends string ? string :
    T extends QuickPickOptions ? string[] :
    never;

function showQuickPick<T extends string | QuickPickOptions>(optionsOrItem: T): QuickPickReturn<T> {
    if (typeof optionsOrItem === "string") {
        return optionsOrItem;
    }
    return optionsOrItem.items;
}

Distinguishing between two primitive types or between a primitive and non-primitive type in the conditional type's branches is allowed.

Update 2: type parameter embedding

We can now detect that a type parameter is a candidate for being used in return type narrowing (i.e. it's a narrowable type parameter) in
cases where the type parameter is indirectly used as a type of a parameter or property.
Before, only this type of usage of T in a parameter type annotation would be recognized:

type PickNumberRet<T> =
    T extends true ? 1 :
    T extends false ? 2 :
    never;

function pickNumber<T extends boolean>(b: T): PickNumberRet<T> {
    return b ? 1 : 2;
}

Now, the following also work:

type PickNumberRet<T> =
    T extends true ? 1 :
    T extends false ? 2 :
    never;

function pickNumber<T extends boolean>({ b }: { b: T }): PickNumberRet<T> {
    return b ? 1 : 2;
}

function pickNumber<T extends boolean>(opts: { b: T }): PickNumberRet<T> {
    return opts.b ? 1 : 2;
}

Combined with the non-primitive restriction mentioned above, this enables users to place narrowable type parameters inside object types
at the exact property that is used for narrowing, so instead of writing this: function fun<T extends { prop: true } | { prop: false }>(param: T): ...,
users can write this: function fun<T extends true | false>(param: { prop: T }): ....

Note that the analysis done to decide if a type parameter is used the parameter in a way that allows it to be narrowed is a syntactical one.
We want to avoid resolving and inspecting actual types during the analysis, because a lot of types are lazy in some sense, and we don't
want this analysis to cause unintended side effects, e.g. circularity errors.
But this means that any usage of a type parameter that requires semantically resolving types to validate it is not going to work.

For a more complete list of what's currently supported here in terms of usages of type parameters, see test tests\cases\compiler\dependentReturnType11.ts.

Update 3: relax extends type restriction

This is a small improvement unrelated to the previous updates.
In the original PR, this was disallowed because we required that a conditional return type's extends types be identical to the types
in the type parameter constraint:

interface Dog {
    name: string;
    bark(): string;
}

type NormalizedRet<T> =
    T extends {} ? T :
    T extends null | undefined ? undefined :
    never;

function normalizeDog<T extends Dog | undefined | null>(dog: T): NormalizedRet<T> {
    if (dog == undefined) {
        return undefined; // Originally an error
    }
    return dog; // Originally an error
}

Note that in NormalizedRet, the first branch's extends type is {}, not Dog, so this wasn't allowed because those types are not identical.
With this PR, we only require that a type in the constraint is assignable to the extends type, i.e. that Dog is assignable to {},
so the code above is now allowed for return type narrowing.

This reverts commit c3ae7c4.

…ches

gabritto · 2025-03-20T18:23:26Z

@typescript-bot pack this

typescript-bot · 2025-03-20T18:23:31Z

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

Command	Status	Results
`pack this`	✅ Started	✅ Results

gabritto · 2025-03-20T18:23:32Z

@typescript-bot test it

typescript-bot · 2025-03-20T18:23:38Z

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 · 2025-03-20T18:27:47Z

Hey @gabritto, 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/164947/artifacts?artifactName=tgz&fileId=D8EFF21205462B38E502A6643448F71CEACCECECE108A3D1D461EC8F7E9FF1E702&fileName=/typescript-5.9.0-insiders.20250320.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/pr-build@5.9.0-pr-61359-4".;

typescript-bot · 2025-03-20T18:46:53Z

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

Everything looks the same!

You can check the log here.

typescript-bot · 2025-03-20T18:51:59Z

@gabritto Here are the results of running the user tests with tsc comparing main and refs/pull/61359/merge:

Everything looks good!

typescript-bot · 2025-03-20T19:09:50Z

@gabritto
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	₃₄	₃₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,390	_62,390	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,395	_50,395	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{194,123k (± 0.96%)}	_{193,824k (± 0.80%)}	_~	_193,125k	_196,973k	_{p=0.230 n=6}
_{Parse Time}	_{1.31s (± 0.79%)}	_{1.31s (± 0.75%)}	_~	_1.30s	_1.33s	_{p=0.591 n=6}
_{Bind Time}	_0.73s	_0.73s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{9.74s (± 0.43%)}	_{9.74s (± 0.18%)}	_~	_9.72s	_9.76s	_{p=0.935 n=6}
_{Emit Time}	_{2.74s (± 1.10%)}	_{2.72s (± 0.72%)}	_~	_2.70s	_2.75s	_{p=0.258 n=6}
_{Total Time}	_{14.53s (± 0.31%)}	_{14.51s (± 0.17%)}	_~	_14.46s	_14.53s	_{p=0.259 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₅₆	₅₆	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_948,670	_948,670	_~	_~	_~	_{p=1.000 n=6}
_Types	_410,947	_410,947	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,224,316k (± 0.00%)}	_{1,225,575k (± 0.00%)}	_{+1,259k (+ 0.10%)}	_1,225,549k	_1,225,617k	_{p=0.005 n=6}
_{Parse Time}	_{6.64s (± 0.98%)}	_{6.66s (± 0.59%)}	_~	_6.60s	_6.70s	_{p=0.810 n=6}
_{Bind Time}	_{1.88s (± 0.58%)}	_{1.88s (± 0.78%)}	_~	_1.86s	_1.90s	_{p=0.866 n=6}
_{Check Time}	_{31.88s (± 0.31%)}	_{31.99s (± 0.24%)}	_{+0.11s (+ 0.33%)}	_31.92s	_32.10s	_{p=0.037 n=6}
_{Emit Time}	_{15.23s (± 0.21%)}	_{15.25s (± 0.55%)}	_~	_15.11s	_15.33s	_{p=0.468 n=6}
_{Total Time}	_{55.63s (± 0.25%)}	_{55.77s (± 0.28%)}	_~	_55.58s	_56.01s	_{p=0.127 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,363,778	_2,363,778	_~	_~	_~	_{p=1.000 n=6}
_Types	_846,530	_846,530	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,133,827k (± 0.01%)}	_{2,138,080k (± 0.01%)}	_{+4,253k (+ 0.20%)}	_2,137,887k	_2,138,228k	_{p=0.005 n=6}
_{Parse Time}	_{7.22s (± 0.25%)}	_{7.24s (± 0.39%)}	_~	_7.21s	_7.29s	_{p=0.256 n=6}
_{Bind Time}	_{2.52s (± 0.39%)}	_{2.53s (± 0.41%)}	_~	_2.52s	_2.55s	_{p=0.103 n=6}
_{Check Time}	_{74.09s (± 0.26%)}	_{74.34s (± 0.45%)}	_~	_73.88s	_74.77s	_{p=0.173 n=6}
_{Emit Time}	_{0.16s (± 3.29%)}	_{0.16s (± 4.75%)}	_~	_0.15s	_0.17s	_{p=0.784 n=6}
_{Total Time}	_{83.99s (± 0.21%)}	_{84.27s (± 0.38%)}	_~	_83.82s	_84.68s	_{p=0.128 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,226,713	_1,227,156	_{+443 (+ 0.04%)}	_~	_~	_{p=0.001 n=6}
_Types	_266,991	_267,114	_{+123 (+ 0.05%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,357,851k (± 0.02%)}	_{2,360,494k (± 0.02%)}	_{+2,643k (+ 0.11%)}	_2,359,892k	_2,361,297k	_{p=0.005 n=6}
_{Parse Time}	_{5.19s (± 1.21%)}	_{5.22s (± 0.66%)}	_~	_5.19s	_5.28s	_{p=0.521 n=6}
_{Bind Time}	_{1.80s (± 1.82%)}	_{1.81s (± 0.57%)}	_~	_1.80s	_1.82s	_{p=0.806 n=6}
_{Check Time}	_{35.31s (± 0.34%)}	_{35.38s (± 0.17%)}	_~	_35.30s	_35.45s	_{p=0.297 n=6}
_{Emit Time}	_{2.99s (± 1.21%)}	_{3.00s (± 1.38%)}	_~	_2.96s	_3.06s	_{p=0.574 n=6}
_{Total Time}	_{45.29s (± 0.40%)}	_{45.41s (± 0.17%)}	_~	_45.32s	_45.54s	_{p=0.230 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,226,713	_1,227,156	_{+443 (+ 0.04%)}	_~	_~	_{p=0.001 n=6}
_Types	_266,991	_267,114	_{+123 (+ 0.05%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,910,623k (±12.87%)}	_{3,037,919k (± 9.77%)}	_{🔻+127,296k (+ 4.37%)}	_2,431,304k	_3,160,174k	_{p=0.031 n=6}
_{Parse Time}	_{6.94s (± 1.85%)}	_{7.00s (± 1.66%)}	_~	_6.78s	_7.12s	_{p=0.471 n=6}
_{Bind Time}	_{2.14s (± 0.71%)}	_{2.16s (± 1.07%)}	_~	_2.14s	_2.20s	_{p=0.126 n=6}
_{Check Time}	_{42.89s (± 0.52%)}	_{43.02s (± 0.72%)}	_~	_42.42s	_43.26s	_{p=0.230 n=6}
_{Emit Time}	_{3.57s (± 1.32%)}	_{3.53s (± 2.18%)}	_~	_3.44s	_3.65s	_{p=0.336 n=6}
_{Total Time}	_{55.54s (± 0.64%)}	_{55.70s (± 0.71%)}	_~	_54.98s	_56.18s	_{p=0.378 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,674	_263,095	_{+421 (+ 0.16%)}	_~	_~	_{p=0.001 n=6}
_Types	_106,849	_106,972	_{+123 (+ 0.12%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{440,508k (± 0.01%)}	_{441,629k (± 0.02%)}	_{+1,120k (+ 0.25%)}	_441,562k	_441,739k	_{p=0.005 n=6}
_{Parse Time}	_{3.55s (± 0.66%)}	_{3.56s (± 0.54%)}	_~	_3.54s	_3.59s	_{p=0.465 n=6}
_{Bind Time}	_{1.30s (± 0.64%)}	_{1.30s (± 0.42%)}	_~	_1.30s	_1.31s	_{p=0.855 n=6}
_{Check Time}	_{18.93s (± 0.57%)}	_{19.00s (± 0.44%)}	_~	_18.90s	_19.10s	_{p=0.520 n=6}
_{Emit Time}	_{1.52s (± 1.15%)}	_{1.51s (± 1.08%)}	_~	_1.49s	_1.54s	_{p=0.934 n=6}
_{Total Time}	_{25.31s (± 0.53%)}	_{25.38s (± 0.32%)}	_~	_25.29s	_25.47s	_{p=0.688 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₁	₇₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,981	_225,981	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,356	_94,356	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{371,202k (± 0.02%)}	_{371,814k (± 0.04%)}	_{+612k (+ 0.16%)}	_371,656k	_372,006k	_{p=0.005 n=6}
_{Parse Time}	_{2.89s (± 0.63%)}	_{2.89s (± 0.87%)}	_~	_2.86s	_2.93s	_{p=0.807 n=6}
_{Bind Time}	_{1.61s (± 2.02%)}	_{1.61s (± 1.27%)}	_~	_1.58s	_1.64s	_{p=0.871 n=6}
_{Check Time}	_{16.49s (± 0.37%)}	_{16.50s (± 0.47%)}	_~	_16.40s	_16.60s	_{p=0.936 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{20.99s (± 0.31%)}	_{20.99s (± 0.32%)}	_~	_20.89s	_21.09s	_{p=1.000 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₈	₈	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,320,535	_3,320,535	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,125,763	_1,125,763	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,382,323k (± 0.00%)}	_{3,387,326k (± 0.01%)}	_{+5,003k (+ 0.15%)}	_3,386,997k	_3,387,570k	_{p=0.005 n=6}
_{Parse Time}	_{14.78s (± 3.65%)}	_{14.66s (± 0.63%)}	_~	_14.56s	_14.80s	_{p=0.229 n=6}
_{Bind Time}	_{4.65s (± 0.42%)}	_{4.71s (± 2.25%)}	_~	_4.65s	_4.93s	_{p=0.075 n=6}
_{Check Time}	_{91.25s (± 1.44%)}	_{92.39s (± 3.01%)}	_~	_88.97s	_96.95s	_{p=0.471 n=6}
_{Emit Time}	_{29.29s (± 0.55%)}	_{29.91s (± 2.63%)}	_~	_29.10s	_30.93s	_{p=0.128 n=6}
_{Total Time}	_{139.96s (± 0.96%)}	_{141.67s (± 1.59%)}	_~	_138.32s	_145.29s	_{p=0.173 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_308,237	_308,237	_~	_~	_~	_{p=1.000 n=6}
_Types	_134,948	_134,948	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{463,416k (± 0.03%)}	_{463,566k (± 0.02%)}	_~	_463,395k	_463,653k	_{p=0.078 n=6}
_{Parse Time}	_{4.14s (± 0.47%)}	_{4.11s (± 0.78%)}	_~	_4.06s	_4.14s	_{p=0.088 n=6}
_{Bind Time}	_{1.80s (± 1.24%)}	_{1.80s (± 1.35%)}	_~	_1.75s	_1.82s	_{p=0.622 n=6}
_{Check Time}	_{20.40s (± 0.16%)}	_{20.46s (± 0.48%)}	_~	_20.29s	_20.57s	_{p=0.127 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_0.00s	_~	_~	_~	_{p=0.405 n=6}
_{Total Time}	_{26.34s (± 0.21%)}	_{26.36s (± 0.27%)}	_~	_26.24s	_26.45s	_{p=0.226 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₅	₅	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_566,532	_566,532	_~	_~	_~	_{p=1.000 n=6}
_Types	_189,740	_189,740	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{497,449k (± 0.01%)}	_{497,946k (± 0.02%)}	_{+498k (+ 0.10%)}	_497,832k	_498,093k	_{p=0.005 n=6}
_{Parse Time}	_{4.19s (± 0.49%)}	_{4.17s (± 0.70%)}	_~	_4.12s	_4.20s	_{p=0.191 n=6}
_{Bind Time}	_{1.50s (± 1.19%)}	_{1.50s (± 1.03%)}	_~	_1.48s	_1.52s	_{p=1.000 n=6}
_{Check Time}	_{24.73s (± 0.25%)}	_{24.85s (± 0.27%)}	_{+0.11s (+ 0.46%)}	_24.74s	_24.93s	_{p=0.020 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{30.43s (± 0.29%)}	_{30.52s (± 0.24%)}	_~	_30.41s	_30.62s	_{p=0.077 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 · 2025-03-20T20:01:24Z

@gabritto Here are the results of running the top 400 repos with tsc comparing main and refs/pull/61359/merge:

Everything looks good!

gabritto · 2025-03-21T17:47:06Z

@typescript-bot perf test this faster

typescript-bot · 2025-03-21T17:47:10Z

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

Command	Status	Results
`perf test this faster`	✅ Started	👀 Results

typescript-bot · 2025-03-21T18:33:30Z

@gabritto
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	₃₄	₃₄	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,390	_62,390	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,395	_50,395	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{194,255k (± 0.92%)}	_{193,140k (± 0.01%)}	_~	_193,110k	_193,184k	_{p=0.378 n=6}
_{Parse Time}	_{1.31s (± 0.68%)}	_{1.32s (± 0.42%)}	_~	_1.31s	_1.32s	_{p=0.341 n=6}
_{Bind Time}	_0.73s	_0.73s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{9.74s (± 0.28%)}	_{9.74s (± 0.47%)}	_~	_9.69s	_9.80s	_{p=1.000 n=6}
_{Emit Time}	_{2.73s (± 0.55%)}	_{2.73s (± 0.51%)}	_~	_2.71s	_2.74s	_{p=0.461 n=6}
_{Total Time}	_{14.51s (± 0.23%)}	_{14.51s (± 0.32%)}	_~	_14.46s	_14.57s	_{p=0.872 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₅₆	₅₆	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_948,670	_948,670	_~	_~	_~	_{p=1.000 n=6}
_Types	_410,947	_410,947	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,224,347k (± 0.01%)}	_{1,225,556k (± 0.00%)}	_{+1,209k (+ 0.10%)}	_1,225,534k	_1,225,574k	_{p=0.005 n=6}
_{Parse Time}	_{6.62s (± 0.92%)}	_{6.67s (± 0.41%)}	_~	_6.63s	_6.70s	_{p=0.332 n=6}
_{Bind Time}	_{1.88s (± 0.43%)}	_{1.87s (± 0.28%)}	_{-0.01s (- 0.53%)}	_1.87s	_1.88s	_{p=0.050 n=6}
_{Check Time}	_{31.88s (± 0.45%)}	_{31.98s (± 0.62%)}	_~	_31.73s	_32.27s	_{p=0.575 n=6}
_{Emit Time}	_{15.31s (± 0.48%)}	_{15.19s (± 0.68%)}	_~	_15.08s	_15.35s	_{p=0.066 n=6}
_{Total Time}	_{55.69s (± 0.37%)}	_{55.71s (± 0.53%)}	_~	_55.39s	_56.07s	_{p=1.000 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,338,004	_2,338,004	_~	_~	_~	_{p=1.000 n=6}
_Types	_832,025	_832,025	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,105,904k (± 0.00%)}	_{2,109,988k (± 0.00%)}	_{+4,084k (+ 0.19%)}	_2,109,951k	_2,110,056k	_{p=0.005 n=6}
_{Parse Time}	_{7.21s (± 0.08%)}	_{7.21s (± 0.30%)}	_~	_7.20s	_7.25s	_{p=0.363 n=6}
_{Bind Time}	_{2.50s (± 0.30%)}	_{2.51s (± 0.48%)}	_~	_2.50s	_2.53s	_{p=0.652 n=6}
_{Check Time}	_{73.38s (± 0.38%)}	_{73.76s (± 0.53%)}	_~	_73.26s	_74.22s	_{p=0.173 n=6}
_{Emit Time}	_0.16s	_{0.16s (± 3.95%)}	_~	_0.15s	_0.17s	_{p=1.000 n=6}
_{Total Time}	_{83.26s (± 0.34%)}	_{83.64s (± 0.47%)}	_~	_83.13s	_84.11s	_{p=0.173 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,226,713	_1,227,156	_{+443 (+ 0.04%)}	_~	_~	_{p=0.001 n=6}
_Types	_266,991	_267,114	_{+123 (+ 0.05%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,357,352k (± 0.03%)}	_{2,360,552k (± 0.02%)}	_{+3,200k (+ 0.14%)}	_2,359,990k	_2,361,021k	_{p=0.005 n=6}
_{Parse Time}	_{5.20s (± 0.47%)}	_{5.19s (± 1.09%)}	_~	_5.12s	_5.29s	_{p=0.688 n=6}
_{Bind Time}	_{1.80s (± 0.93%)}	_{1.80s (± 0.96%)}	_~	_1.78s	_1.83s	_{p=0.932 n=6}
_{Check Time}	_{35.32s (± 0.23%)}	_{35.35s (± 0.28%)}	_~	_35.20s	_35.44s	_{p=0.378 n=6}
_{Emit Time}	_{3.02s (± 0.99%)}	_{3.00s (± 0.81%)}	_~	_2.95s	_3.01s	_{p=0.166 n=6}
_{Total Time}	_{45.35s (± 0.21%)}	_{45.34s (± 0.26%)}	_~	_45.22s	_45.53s	_{p=0.873 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,226,713	_1,227,156	_{+443 (+ 0.04%)}	_~	_~	_{p=0.001 n=6}
_Types	_266,991	_267,114	_{+123 (+ 0.05%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,911,756k (±12.87%)}	_{2,916,546k (±12.87%)}	_~	_2,431,200k	_3,159,843k	_{p=0.128 n=6}
_{Parse Time}	_{6.91s (± 1.62%)}	_{6.92s (± 1.86%)}	_~	_6.78s	_7.07s	_{p=1.000 n=6}
_{Bind Time}	_{2.12s (± 2.05%)}	_{2.17s (± 1.28%)}	_~	_2.15s	_2.22s	_{p=0.170 n=6}
_{Check Time}	_{42.78s (± 0.51%)}	_{42.89s (± 0.68%)}	_~	_42.53s	_43.26s	_{p=0.471 n=6}
_{Emit Time}	_{3.46s (± 2.23%)}	_{3.42s (± 2.45%)}	_~	_3.35s	_3.58s	_{p=0.471 n=6}
_{Total Time}	_{55.29s (± 0.49%)}	_{55.39s (± 0.73%)}	_~	_54.90s	_55.84s	_{p=0.810 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_262,674	_263,095	_{+421 (+ 0.16%)}	_~	_~	_{p=0.001 n=6}
_Types	_106,849	_106,972	_{+123 (+ 0.12%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{440,467k (± 0.01%)}	_{441,611k (± 0.02%)}	_{+1,145k (+ 0.26%)}	_441,501k	_441,749k	_{p=0.005 n=6}
_{Parse Time}	_{3.53s (± 1.72%)}	_{3.57s (± 0.89%)}	_~	_3.54s	_3.61s	_{p=0.169 n=6}
_{Bind Time}	_{1.32s (± 0.57%)}	_{1.32s (± 0.88%)}	_~	_1.30s	_1.33s	_{p=0.396 n=6}
_{Check Time}	_{18.98s (± 0.22%)}	_{19.02s (± 0.47%)}	_~	_18.93s	_19.13s	_{p=0.809 n=6}
_{Emit Time}	_{1.53s (± 0.99%)}	_{1.52s (± 1.29%)}	_~	_1.50s	_1.55s	_{p=0.935 n=6}
_{Total Time}	_{25.36s (± 0.35%)}	_{25.43s (± 0.36%)}	_~	_25.31s	_25.57s	_{p=0.229 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₁	₇₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,981	_225,981	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,356	_94,356	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{371,187k (± 0.02%)}	_{371,720k (± 0.03%)}	_{+532k (+ 0.14%)}	_371,589k	_371,828k	_{p=0.005 n=6}
_{Parse Time}	_{2.89s (± 0.55%)}	_{2.88s (± 1.01%)}	_~	_2.83s	_2.90s	_{p=0.684 n=6}
_{Bind Time}	_{1.59s (± 1.30%)}	_{1.58s (± 1.10%)}	_~	_1.57s	_1.61s	_{p=0.416 n=6}
_{Check Time}	_{16.52s (± 0.35%)}	_{16.47s (± 0.28%)}	_~	_16.41s	_16.52s	_{p=0.199 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{21.00s (± 0.27%)}	_{20.93s (± 0.22%)}	_~	_20.88s	_21.00s	_{p=0.064 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₈	₈	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,323,316	_3,323,316	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,126,439	_1,126,439	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,383,716k (± 0.01%)}	_{3,389,145k (± 0.01%)}	_{+5,430k (+ 0.16%)}	_3,388,828k	_3,389,364k	_{p=0.005 n=6}
_{Parse Time}	_{14.60s (± 0.59%)}	_{14.64s (± 0.49%)}	_~	_14.55s	_14.75s	_{p=0.376 n=6}
_{Bind Time}	_{4.66s (± 0.55%)}	_{4.65s (± 0.78%)}	_~	_4.61s	_4.70s	_{p=0.630 n=6}
_{Check Time}	_{91.25s (± 1.89%)}	_{90.75s (± 1.77%)}	_~	_89.52s	_93.90s	_{p=0.470 n=6}
_{Emit Time}	_{29.84s (± 2.63%)}	_{28.86s (± 6.33%)}	_~	_25.31s	_30.62s	_{p=0.199 n=6}
_{Total Time}	_{140.36s (± 1.79%)}	_{138.91s (± 0.57%)}	_~	_138.09s	_139.93s	_{p=0.298 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_308,237	_308,237	_~	_~	_~	_{p=1.000 n=6}
_Types	_134,948	_134,948	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{463,361k (± 0.03%)}	_{463,558k (± 0.02%)}	_{+198k (+ 0.04%)}	_463,429k	_463,661k	_{p=0.031 n=6}
_{Parse Time}	_{4.11s (± 0.50%)}	_{4.10s (± 0.82%)}	_~	_4.06s	_4.16s	_{p=0.572 n=6}
_{Bind Time}	_{1.80s (± 0.55%)}	_{1.80s (± 1.15%)}	_~	_1.76s	_1.82s	_{p=0.869 n=6}
_{Check Time}	_{20.46s (± 0.38%)}	_{20.43s (± 0.26%)}	_~	_20.37s	_20.52s	_{p=0.747 n=6}
_{Emit Time}	_0.00s	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=0.405 n=6}
_{Total Time}	_{26.36s (± 0.33%)}	_{26.33s (± 0.14%)}	_~	_26.28s	_26.38s	_{p=0.687 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₅	₅	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_566,532	_566,532	_~	_~	_~	_{p=1.000 n=6}
_Types	_189,740	_189,740	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{497,380k (± 0.04%)}	_{497,910k (± 0.02%)}	_{+530k (+ 0.11%)}	_497,738k	_498,004k	_{p=0.005 n=6}
_{Parse Time}	_{4.18s (± 0.49%)}	_{4.17s (± 0.91%)}	_~	_4.12s	_4.20s	_{p=0.685 n=6}
_{Bind Time}	_{1.51s (± 0.68%)}	_{1.50s (± 0.91%)}	_~	_1.48s	_1.52s	_{p=0.190 n=6}
_{Check Time}	_{24.95s (± 1.24%)}	_{24.77s (± 0.48%)}	_~	_24.58s	_24.88s	_{p=0.261 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_0.00s	_~	_~	_~	_{p=0.405 n=6}
_{Total Time}	_{30.64s (± 0.96%)}	_{30.43s (± 0.45%)}	_~	_30.25s	_30.60s	_{p=0.128 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

gabritto · 2025-03-21T20:00:26Z

Note to self:

Add tests with typeof param as a way of accessing the narrowable type parameter
Support parameter properties

lgenzelis · 2025-08-26T23:36:28Z

hi! Not sure where it's the right place to ask this, but are you planning on having this feature re-added in the foreseeable future? Or did you decide to de-prioritize it because of the go port? :)

controversial · 2025-08-27T02:54:26Z

There’s some discussion on this in #61648 (comment):

We want to be more conservative about larger features that are more involved while so much of our attention is currently on the port.

we generally won't plan on new features, but it depends on priority, finding improvements that need to be backported during the port, etc. We just don't want to split our focus too much.

controversial · 2025-12-08T19:22:15Z

rip 💔🤕

great work on this @gabritto, and I hope we can see it land in Corsa soon!

gabritto added 10 commits March 5, 2025 10:59

Revert "Revert return type narrowing (#61136)"

aa38e34

This reverts commit c3ae7c4.

add restriction on narrowable type constraint

073d868

relax restriction on extends type of narrowable conditional type

17ed356

WIP: detect embedded references to a narrowable type parameter

fdb6bea

Detect and construct narrowable references for embedded type parameters

1849292

validate optional param/property

009d152

format

5cb7e90

rework non-primitive restriction to be based on conditional type bran…

3c5327e

…ches

update tests

7464af0

bring back conditional expression test

c96e01e

github-project-automation bot added this to PR Backlog Mar 5, 2025

github-project-automation bot moved this to Not started in PR Backlog Mar 5, 2025

typescript-bot assigned gabritto Mar 5, 2025

typescript-bot added Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Mar 5, 2025

gabritto requested review from DanielRosenwasser, andrewbranch, sandersn and weswigham March 5, 2025 22:07

controversial mentioned this pull request Mar 6, 2025

feat: implement typed Input/Output interface for resolvers react-hook-form/resolvers#753

Merged

This was referenced Mar 7, 2025

Allow extends type to be supertype of constraint types when narrowing conditional return type #61117

Closed

Fix detection of single usage of type parameter for return type narrowing #61115

Closed

Also support classes. TODO: support parameter properties

c872eb8

fix baselines

2084fa0

controversial mentioned this pull request May 5, 2025

TypeScript 5.9 Iteration Plan #61648

Closed

gabritto closed this Dec 8, 2025

github-project-automation bot moved this from Not started to Done in PR Backlog Dec 8, 2025

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gabritto commented Mar 5, 2025

Update 1: non-primitive restriction

Update 2: type parameter embedding

Update 3: relax extends type restriction

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