Add explicit type variance specifiers to Promise, Map, Set #62973

akaltar · 2026-01-10T17:44:49Z

In theory this could improve the performance of type checking in certain scenarios by avoiding a call to getVariancesWorker as measured in the perfetto profiles.

I've opted to only include some of the most commonly used types, so the added parsing time doesn't outweigh the benefits

I wasn't sure if the explicit specifier needs to be on every expansion of an interface or only the first one, so I've added it to all of them from es5 to esnext, but depending on how the compiler handles it, it could be applied only to the first occurrence of the interface

…erformance counters

typescript-bot · 2026-01-10T17:44:54Z

The TypeScript team hasn't accepted the linked issue #62954. If you can get it accepted, this PR will have a better chance of being reviewed.

akaltar · 2026-01-10T17:46:08Z

@microsoft-github-policy-service agree

jakebailey · 2026-01-12T21:59:52Z

I'm 99% certain we can't do this, but, curious

@typescript-bot test it

typescript-bot · 2026-01-12T21:59:56Z

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

ritschwumm · 2026-01-12T22:09:52Z

@akaltar sorry, but this is wrong in so far as it subverts type-safety.

take Map for example:
get(key: K): V | undefined; is indeed covariant in V, so out V would make sense.
set(key: K, value: V): this; on the other hand is contravariant in V, so only in V would be correct.

more generally: as long as you have a mutable collection, you cannot define its values covariant.

typescript-bot · 2026-01-12T22:21:44Z

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

Everything looks the same!

You can check the log here.

typescript-bot · 2026-01-12T22:29:27Z

@jakebailey Here are the results of running the user tests with tsc comparing main and refs/pull/62973/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Package install failed"
1 instance of "Git clone failed"

Otherwise...

Something interesting changed - please have a look.

Details

webpack

tsconfig.types.test.json

[NEW] error TS2322: Type 'WeakMap<object, any>' is not assignable to type 'WeakMap<Source, CachedSource>'.
[NEW] error TS2345: Argument of type 'WeakMap<object, any>' is not assignable to parameter of type 'WeakMap<HashableObject, LazyHashedEtag>'.
- lib/cache/getLazyHashedEtag.js#L62
- lib/cache/getLazyHashedEtag.js#L71
[NEW] error TS2345: Argument of type 'WeakMap<object, any>' is not assignable to parameter of type 'WeakMap<Source, boolean>'.
- lib/util/source.js#L49
- lib/util/source.js#L57

tsconfig.types.json

[NEW] error TS2322: Type 'WeakMap<object, any>' is not assignable to type 'WeakMap<Source, CachedSource>'.
[NEW] error TS2345: Argument of type 'WeakMap<object, any>' is not assignable to parameter of type 'WeakMap<HashableObject, LazyHashedEtag>'.
- lib/cache/getLazyHashedEtag.js#L62
- lib/cache/getLazyHashedEtag.js#L71
[NEW] error TS2345: Argument of type 'WeakMap<object, any>' is not assignable to parameter of type 'WeakMap<Source, boolean>'.
- lib/util/source.js#L49
- lib/util/source.js#L57

tsconfig.json

[NEW] error TS2322: Type 'WeakMap<object, any>' is not assignable to type 'WeakMap<Source, CachedSource>'.
[NEW] error TS2345: Argument of type 'WeakMap<object, any>' is not assignable to parameter of type 'WeakMap<HashableObject, LazyHashedEtag>'.
- lib/cache/getLazyHashedEtag.js#L62
- lib/cache/getLazyHashedEtag.js#L71
[NEW] error TS2345: Argument of type 'WeakMap<object, any>' is not assignable to parameter of type 'WeakMap<Source, boolean>'.
- lib/util/source.js#L49
- lib/util/source.js#L57

typescript-bot · 2026-01-12T23:03:29Z

@jakebailey
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	_62,400	_{+30 (+ 0.05%)}	_~	_~	_{p=0.001 n=6}
_Types	_50,387	_50,435	_{+48 (+ 0.10%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{194,982k (± 0.89%)}	_{193,334k (± 0.75%)}	_~	_192,650k	_196,293k	_{p=0.378 n=6}
_{Parse Time}	_{1.30s (± 0.40%)}	_{1.30s (± 0.58%)}	_~	_1.29s	_1.31s	_{p=0.241 n=6}
_{Bind Time}	_0.76s	_0.76s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{9.88s (± 0.47%)}	_{9.88s (± 0.42%)}	_~	_9.81s	_9.92s	_{p=0.627 n=6}
_{Emit Time}	_{2.75s (± 0.72%)}	_{2.74s (± 0.63%)}	_~	_2.72s	_2.77s	_{p=0.745 n=6}
_{Total Time}	_{14.69s (± 0.25%)}	_{14.68s (± 0.31%)}	_~	_14.62s	_14.74s	_{p=0.872 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_955,823	_955,416	_{-407 (- 0.04%)}	_~	_~	_{p=0.001 n=6}
_Types	_415,853	_415,617	_{-236 (- 0.06%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{1,253,923k (± 0.01%)}	_{1,253,594k (± 0.00%)}	_{-330k (- 0.03%)}	_1,253,528k	_1,253,647k	_{p=0.005 n=6}
_{Parse Time}	_{6.52s (± 0.68%)}	_{6.51s (± 0.30%)}	_~	_6.49s	_6.53s	_{p=0.685 n=6}
_{Bind Time}	_{1.96s (± 0.42%)}	_{1.96s (± 0.32%)}	_~	_1.95s	_1.97s	_{p=0.432 n=6}
_{Check Time}	_{32.43s (± 0.25%)}	_{32.44s (± 0.37%)}	_~	_32.29s	_32.59s	_{p=1.000 n=6}
_{Emit Time}	_{14.99s (± 0.17%)}	_{14.87s (± 0.33%)}	_{-0.12s (- 0.80%)}	_14.79s	_14.93s	_{p=0.005 n=6}
_{Total Time}	_{55.90s (± 0.19%)}	_{55.78s (± 0.21%)}	_~	_55.65s	_55.95s	_{p=0.109 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,699,165	_2,698,796	_{-369 (- 0.01%)}	_~	_~	_{p=0.001 n=6}
_Types	_937,297	_937,089	_{-208 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{3,028,591k (± 0.00%)}	_{3,028,267k (± 0.00%)}	_{-324k (- 0.01%)}	_3,028,189k	_3,028,358k	_{p=0.005 n=6}
_{Parse Time}	_{8.64s (± 0.37%)}	_{8.62s (± 0.28%)}	_~	_8.59s	_8.66s	_{p=0.368 n=6}
_{Bind Time}	_{2.29s (± 0.55%)}	_{2.28s (± 0.60%)}	_~	_2.26s	_2.30s	_{p=0.506 n=6}
_{Check Time}	_{93.63s (± 0.12%)}	_{93.77s (± 0.46%)}	_~	_93.28s	_94.46s	_{p=0.810 n=6}
_{Emit Time}	_0.31s	_{0.31s (± 1.31%)}	_~	_0.31s	_0.32s	_{p=0.405 n=6}
_{Total Time}	_{104.87s (± 0.10%)}	_{104.99s (± 0.41%)}	_~	_104.51s	_105.68s	_{p=0.873 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,252,015	_1,250,004	_{-2,011 (- 0.16%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,850	_258,728	_{-1,122 (- 0.43%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,509,153k (±11.87%)}	_{2,506,859k (±11.82%)}	_{-2,294k (- 0.09%)}	_2,385,348k	_3,112,520k	_{p=0.045 n=6}
_{Parse Time}	_{5.15s (± 1.47%)}	_{5.20s (± 1.02%)}	_~	_5.16s	_5.30s	_{p=0.229 n=6}
_{Bind Time}	_{1.87s (± 1.04%)}	_{1.85s (± 0.56%)}	_~	_1.84s	_1.87s	_{p=0.138 n=6}
_{Check Time}	_{35.55s (± 0.59%)}	_{35.41s (± 0.39%)}	_~	_35.28s	_35.66s	_{p=0.230 n=6}
_{Emit Time}	_{2.97s (± 1.14%)}	_{2.98s (± 2.44%)}	_~	_2.88s	_3.05s	_{p=0.748 n=6}
_{Total Time}	_{45.56s (± 0.45%)}	_{45.46s (± 0.40%)}	_~	_45.22s	_45.77s	_{p=0.521 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,252,015	_1,250,004	_{-2,011 (- 0.16%)}	_~	_~	_{p=0.001 n=6}
_Types	_259,850	_258,728	_{-1,122 (- 0.43%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{3,087,475k (± 7.49%)}	_{3,180,208k (± 0.01%)}	_~	_3,179,542k	_3,180,678k	_{p=0.066 n=6}
_{Parse Time}	_{8.37s (± 0.93%)}	_{8.39s (± 0.79%)}	_~	_8.29s	_8.49s	_{p=0.748 n=6}
_{Bind Time}	_{2.77s (± 1.42%)}	_{2.76s (± 0.59%)}	_~	_2.74s	_2.79s	_{p=0.810 n=6}
_{Check Time}	_{53.23s (± 0.77%)}	_{53.10s (± 0.26%)}	_~	_52.94s	_53.30s	_{p=0.128 n=6}
_{Emit Time}	_{4.37s (± 2.27%)}	_{4.32s (± 1.12%)}	_~	_4.26s	_4.39s	_{p=0.109 n=6}
_{Total Time}	_{68.76s (± 0.61%)}	_{68.59s (± 0.23%)}	_~	_68.38s	_68.81s	_{p=0.093 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_264,672	_264,244	_{-428 (- 0.16%)}	_~	_~	_{p=0.001 n=6}
_Types	_104,063	_103,870	_{-193 (- 0.19%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{443,069k (± 0.01%)}	_{442,582k (± 0.02%)}	_{-487k (- 0.11%)}	_442,425k	_442,706k	_{p=0.005 n=6}
_{Parse Time}	_{4.36s (± 0.60%)}	_{4.35s (± 0.84%)}	_~	_4.30s	_4.41s	_{p=0.466 n=6}
_{Bind Time}	_{1.70s (± 1.11%)}	_{1.71s (± 1.13%)}	_~	_1.69s	_1.74s	_{p=0.367 n=6}
_{Check Time}	_{23.82s (± 0.36%)}	_{23.67s (± 0.38%)}	_{-0.14s (- 0.61%)}	_23.54s	_23.76s	_{p=0.025 n=6}
_{Emit Time}	_{1.93s (± 1.07%)}	_{1.92s (± 1.27%)}	_~	_1.89s	_1.96s	_{p=0.170 n=6}
_{Total Time}	_{31.81s (± 0.23%)}	_{31.65s (± 0.32%)}	_{-0.16s (- 0.50%)}	_31.50s	_31.74s	_{p=0.020 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,493	_225,452	_{-41 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_Types	_94,373	_94,349	_{-24 (- 0.03%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{369,971k (± 0.06%)}	_{369,850k (± 0.06%)}	_~	_369,703k	_370,328k	_{p=0.378 n=6}
_{Parse Time}	_{2.83s (± 1.25%)}	_{2.83s (± 0.86%)}	_~	_2.81s	_2.87s	_{p=0.686 n=6}
_{Bind Time}	_{1.64s (± 0.84%)}	_{1.66s (± 1.56%)}	_~	_1.62s	_1.70s	_{p=0.325 n=6}
_{Check Time}	_{16.68s (± 0.29%)}	_{16.63s (± 0.18%)}	_~	_16.58s	_16.66s	_{p=0.088 n=6}
_{Emit Time}	_0.00s	_{0.00s (±154.76%)}	_~	_0.00s	_0.01s	_{p=0.174 n=6}
_{Total Time}	_{21.16s (± 0.27%)}	_{21.12s (± 0.24%)}	_~	_21.05s	_21.18s	_{p=0.809 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₁₁	₁₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_4,092,302	_4,092,428	_{+126 (+ 0.00%)}	_~	_~	_{p=0.001 n=6}
_Types	_1,292,805	_1,292,870	_{+65 (+ 0.01%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{3,885,124k (± 0.00%)}	_{3,885,329k (± 0.01%)}	_~	_3,885,025k	_3,885,803k	_{p=0.230 n=6}
_{Parse Time}	_{19.45s (± 0.49%)}	_{19.40s (± 0.34%)}	_~	_19.32s	_19.47s	_{p=0.520 n=6}
_{Bind Time}	_{6.61s (± 2.57%)}	_{6.68s (± 2.52%)}	_~	_6.49s	_6.87s	_{p=0.471 n=6}
_{Check Time}	_{137.98s (± 2.39%)}	_{137.17s (± 1.10%)}	_~	_134.68s	_138.76s	_{p=0.689 n=6}
_{Emit Time}	_{50.74s (±11.29%)}	_{50.66s (±12.64%)}	_~	_46.88s	_62.83s	_{p=0.471 n=6}
_{Total Time}	_{214.78s (± 2.42%)}	_{213.91s (± 3.34%)}	_~	_207.71s	_227.37s	_{p=0.575 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₁	₄₉	_{🔻+8 (+19.51%)}	_~	_~	_{p=0.001 n=6}
_Symbols	_383,772	_383,946	_{+174 (+ 0.05%)}	_~	_~	_{p=0.001 n=6}
_Types	_168,183	_168,230	_{+47 (+ 0.03%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{542,342k (± 0.01%)}	_{542,444k (± 0.04%)}	_~	_542,193k	_542,796k	_{p=0.470 n=6}
_{Parse Time}	_{4.66s (± 0.68%)}	_{4.65s (± 0.75%)}	_~	_4.63s	_4.71s	_{p=0.624 n=6}
_{Bind Time}	_{2.02s (± 0.77%)}	_{2.01s (± 0.98%)}	_~	_1.99s	_2.04s	_{p=0.514 n=6}
_{Check Time}	_{23.84s (± 0.27%)}	_{23.82s (± 1.37%)}	_~	_23.25s	_24.11s	_{p=1.000 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{30.52s (± 0.27%)}	_{30.49s (± 1.09%)}	_~	_29.88s	_30.78s	_{p=0.748 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_696,322	_695,557	_{-765 (- 0.11%)}	_~	_~	_{p=0.001 n=6}
_Types	_212,496	_211,893	_{-603 (- 0.28%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{590,941k (± 0.02%)}	_{590,230k (± 0.02%)}	_{-711k (- 0.12%)}	_590,117k	_590,399k	_{p=0.005 n=6}
_{Parse Time}	_{4.17s (± 0.83%)}	_{4.15s (± 0.72%)}	_~	_4.11s	_4.19s	_{p=0.567 n=6}
_{Bind Time}	_{1.42s (± 1.46%)}	_{1.41s (± 0.94%)}	_~	_1.39s	_1.43s	_{p=0.934 n=6}
_{Check Time}	_{21.53s (± 1.39%)}	_{21.40s (± 1.72%)}	_~	_20.89s	_21.70s	_{p=0.471 n=6}
_{Emit Time}	_0.00s	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=0.405 n=6}
_{Total Time}	_{27.11s (± 1.11%)}	_{26.97s (± 1.38%)}	_~	_26.49s	_27.28s	_{p=0.422 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 · 2026-01-12T23:23:45Z

@jakebailey Here are the results of running the top 400 repos with tsc comparing main and refs/pull/62973/merge:

Something interesting changed - please have a look.

Details

microsoft/playwright

9 of 13 projects failed to build with the old tsc and were ignored

packages/trace-viewer/tsconfig.json

error TS2322: Type '{ viewport: ViewportSize; frames: WeakMap<WeakKey, any>; }' is not assignable to type 'FrameBoundingRectsInfo'.
- packages/playwright-core/src/utils/isomorphic/trace/snapshotRenderer.ts#L273

akaltar added 2 commits January 10, 2026 18:19

Add explicit variance specifiers to Promise, Map, Set and related types

809a00c

Update baselines to contain explicit variance specifiers and adjust p…

f276040

…erformance counters

github-project-automation bot added this to PR Backlog Jan 10, 2026

github-project-automation bot moved this to Not started in PR Backlog Jan 10, 2026

typescript-bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Jan 10, 2026

Add explicit type variance specifiers to Promise, Map, Set #62973

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Add explicit type variance specifiers to Promise, Map, Set #62973

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