Revert return type narrowing by gabritto · Pull Request #61136 · microsoft/TypeScript

Gabriela Araujo Britto (gabritto) · 2025-02-07T02:26:03Z

As discussed in design meeting on Feb 4th, the current design for return type narrowing doesn't work well with object types, and that aspect needs to be fixed before the feature can be released.
My notes on the problem are here: https://gist.github.com/gabritto/b6ebd5f9fc2bb3cfc305027609e66bca

This PR reverts return type narrowing, but keeps the change where we check each branch of a conditional return expression separately, since that change, although a breaking one, uncovered bugs in vscode and Google.

My plan is to later fix return type narrowing and get the fixed version in TS again for the 5.9. The planned fix is to restrict return type narrowing for cases where we the function/return type distinguishes between primitive types or primitive and object types, e.g. cases where the parameter has type string | number or string | string[]. These are the cases where regular type narrowing already works well, and avoids the problems mentioned in the notes. Then, on top of that restriction, we'll enable embedding the "narrowable" type parameter in an object type (see https://gist.github.com/gabritto/b6ebd5f9fc2bb3cfc305027609e66bca#alternative-2-embedding).

Gabriela Araujo Britto (gabritto) · 2025-02-07T02:26:20Z

TypeScript Bot (@typescript-bot) test it

TypeScript Bot (typescript-bot) · 2025-02-07T02:26:24Z

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 (typescript-bot) · 2025-02-07T02:49:14Z

Hey Gabriela Araujo Britto (@gabritto), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

TypeScript Bot (typescript-bot) · 2025-02-07T02:49:34Z

Gabriela Araujo Britto (@gabritto) Here are the results of running the user tests with tsc comparing main and refs/pull/61136/merge:

Everything looks good!

TypeScript Bot (typescript-bot) · 2025-02-07T03:11:46Z

Gabriela Araujo Britto (@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}	_{194,943k (± 1.03%)}	_{194,118k (± 0.96%)}	_~	_192,889k	_196,543k	_{p=0.066 n=6}
_{Parse Time}	_{1.33s (± 0.39%)}	_{1.30s (± 0.64%)}	_{-0.02s (- 1.63%)}	_1.29s	_1.31s	_{p=0.004 n=6}
_{Bind Time}	_{0.73s (± 0.56%)}	_{0.73s (± 0.70%)}	_~	_0.73s	_0.74s	_{p=0.595 n=6}
_{Check Time}	_{9.80s (± 0.31%)}	_{9.79s (± 0.23%)}	_~	_9.75s	_9.81s	_{p=0.685 n=6}
_{Emit Time}	_{2.75s (± 0.78%)}	_{2.75s (± 0.44%)}	_~	_2.73s	_2.76s	_{p=0.934 n=6}
_{Total Time}	_{14.61s (± 0.16%)}	_{14.57s (± 0.18%)}	_~	_14.53s	_14.60s	_{p=0.060 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₃₇	₃₇	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_948,488	_948,488	_~	_~	_~	_{p=1.000 n=6}
_Types	_411,006	_411,006	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,225,423k (± 0.00%)}	_{1,224,194k (± 0.01%)}	_{-1,229k (- 0.10%)}	_1,224,114k	_1,224,328k	_{p=0.005 n=6}
_{Parse Time}	_{6.71s (± 0.52%)}	_{6.71s (± 0.70%)}	_~	_6.65s	_6.76s	_{p=0.222 n=6}
_{Bind Time}	_{1.93s (± 0.61%)}	_{1.92s (± 0.57%)}	_~	_1.91s	_1.94s	_{p=0.246 n=6}
_{Check Time}	_{32.25s (± 0.48%)}	_{31.95s (± 0.32%)}	_{-0.30s (- 0.92%)}	_31.84s	_32.11s	_{p=0.013 n=6}
_{Emit Time}	_{15.36s (± 0.17%)}	_{15.41s (± 0.57%)}	_~	_15.32s	_15.52s	_{p=0.520 n=6}
_{Total Time}	_{56.25s (± 0.30%)}	_{55.99s (± 0.19%)}	_{-0.26s (- 0.46%)}	_55.80s	_56.06s	_{p=0.024 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,371,940	_2,371,940	_~	_~	_~	_{p=1.000 n=6}
_Types	_846,141	_846,141	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,138,893k (± 0.01%)}	_{2,134,700k (± 0.00%)}	_{-4,194k (- 0.20%)}	_2,134,605k	_2,134,811k	_{p=0.005 n=6}
_{Parse Time}	_{7.31s (± 0.07%)}	_{7.30s (± 0.11%)}	_~	_7.29s	_7.31s	_{p=0.070 n=6}
_{Bind Time}	_{2.48s (± 0.94%)}	_{2.48s (± 0.90%)}	_~	_2.45s	_2.51s	_{p=0.743 n=6}
_{Check Time}	_{72.98s (± 0.34%)}	_{72.26s (± 1.81%)}	_~	_70.25s	_73.31s	_{p=0.873 n=6}
_{Emit Time}	_{0.14s (± 2.88%)}	_{0.14s (± 3.60%)}	_~	_0.14s	_0.15s	_{p=0.595 n=6}
_{Total Time}	_{82.91s (± 0.30%)}	_{82.19s (± 1.57%)}	_~	_80.19s	_83.25s	_{p=0.748 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,229,397	_1,229,161	_{-236 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_Types	_267,085	_267,043	_{-42 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,361,732k (± 0.03%)}	_{2,359,642k (± 0.02%)}	_{-2,090k (- 0.09%)}	_2,359,011k	_2,360,154k	_{p=0.005 n=6}
_{Parse Time}	_{5.22s (± 0.84%)}	_{5.25s (± 1.40%)}	_~	_5.15s	_5.34s	_{p=0.748 n=6}
_{Bind Time}	_{1.80s (± 0.96%)}	_{1.79s (± 1.20%)}	_~	_1.77s	_1.82s	_{p=0.685 n=6}
_{Check Time}	_{35.38s (± 0.40%)}	_{35.46s (± 0.15%)}	_~	_35.40s	_35.53s	_{p=0.521 n=6}
_{Emit Time}	_{3.07s (± 2.95%)}	_{3.07s (± 4.08%)}	_~	_2.98s	_3.32s	_{p=0.689 n=6}
_{Total Time}	_{45.46s (± 0.43%)}	_{45.60s (± 0.34%)}	_~	_45.45s	_45.87s	_{p=0.173 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,229,397	_1,229,161	_{-236 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_Types	_267,085	_267,043	_{-42 (- 0.02%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{2,796,547k (±14.25%)}	_{3,155,684k (± 0.03%)}	_~	_3,154,590k	_3,156,765k	_{p=1.000 n=6}
_{Parse Time}	_{6.93s (± 2.13%)}	_{7.02s (± 1.35%)}	_~	_6.90s	_7.13s	_{p=0.298 n=6}
_{Bind Time}	_{2.15s (± 1.96%)}	_{2.14s (± 1.13%)}	_~	_2.12s	_2.19s	_{p=0.421 n=6}
_{Check Time}	_{42.90s (± 0.53%)}	_{42.94s (± 0.31%)}	_~	_42.78s	_43.11s	_{p=0.810 n=6}
_{Emit Time}	_{3.51s (± 2.84%)}	_{3.52s (± 0.79%)}	_~	_3.49s	_3.57s	_{p=0.378 n=6}
_{Total Time}	_{55.48s (± 0.50%)}	_{55.61s (± 0.26%)}	_~	_55.33s	_55.72s	_{p=0.470 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_263,074	_262,860	_{-214 (- 0.08%)}	_~	_~	_{p=0.001 n=6}
_Types	_106,881	_106,839	_{-42 (- 0.04%)}	_~	_~	_{p=0.001 n=6}
_{Memory used}	_{441,257k (± 0.02%)}	_{440,613k (± 0.01%)}	_{-644k (- 0.15%)}	_440,563k	_440,649k	_{p=0.005 n=6}
_{Parse Time}	_{3.56s (± 1.37%)}	_{3.54s (± 0.93%)}	_~	_3.51s	_3.58s	_{p=0.466 n=6}
_{Bind Time}	_{1.30s (± 0.64%)}	_{1.31s (± 0.84%)}	_~	_1.30s	_1.32s	_{p=0.503 n=6}
_{Check Time}	_{18.97s (± 0.54%)}	_{18.92s (± 0.51%)}	_~	_18.82s	_19.09s	_{p=0.296 n=6}
_{Emit Time}	_{1.52s (± 1.31%)}	_{1.52s (± 0.77%)}	_~	_1.50s	_1.53s	_{p=0.740 n=6}
_{Total Time}	_{25.36s (± 0.53%)}	_{25.29s (± 0.44%)}	_~	_25.17s	_25.47s	_{p=0.228 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₀	₇₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_226,113	_226,113	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,488	_94,488	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{371,756k (± 0.01%)}	_{371,339k (± 0.02%)}	_{-417k (- 0.11%)}	_371,250k	_371,460k	_{p=0.005 n=6}
_{Parse Time}	_{2.89s (± 1.64%)}	_{2.90s (± 0.34%)}	_~	_2.89s	_2.91s	_{p=0.870 n=6}
_{Bind Time}	_{1.61s (± 1.74%)}	_{1.59s (± 1.02%)}	_~	_1.57s	_1.61s	_{p=0.514 n=6}
_{Check Time}	_{16.45s (± 0.21%)}	_{16.47s (± 0.29%)}	_~	_16.38s	_16.52s	_{p=0.261 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{20.95s (± 0.40%)}	_{20.96s (± 0.19%)}	_~	_20.90s	_21.01s	_{p=0.573 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_3,280,686	_3,280,686	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,128,284	_1,128,284	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,354,513k (± 0.01%)}	_{3,349,070k (± 0.01%)}	_{-5,443k (- 0.16%)}	_3,348,734k	_3,349,187k	_{p=0.005 n=6}
_{Parse Time}	_{14.57s (± 0.35%)}	_{14.58s (± 0.49%)}	_~	_14.52s	_14.71s	_{p=0.936 n=6}
_{Bind Time}	_{4.61s (± 0.61%)}	_{4.62s (± 0.32%)}	_~	_4.60s	_4.64s	_{p=0.806 n=6}
_{Check Time}	_{91.54s (± 2.63%)}	_{90.36s (± 2.03%)}	_~	_88.50s	_92.62s	_{p=0.378 n=6}
_{Emit Time}	_{28.89s (± 2.49%)}	_{27.01s (± 7.68%)}	_{🟩-1.87s (- 6.49%)}	_23.14s	_29.15s	_{p=0.013 n=6}
_{Total Time}	_{139.61s (± 2.02%)}	_{136.56s (± 1.03%)}	_~	_134.89s	_138.32s	_{p=0.066 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_293,378	_293,378	_~	_~	_~	_{p=1.000 n=6}
_Types	_119,581	_119,581	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{447,125k (± 0.02%)}	_{446,941k (± 0.01%)}	_{-185k (- 0.04%)}	_446,844k	_447,011k	_{p=0.005 n=6}
_{Parse Time}	_{4.11s (± 0.76%)}	_{4.09s (± 0.78%)}	_~	_4.06s	_4.15s	_{p=0.197 n=6}
_{Bind Time}	_{1.80s (± 1.24%)}	_{1.77s (± 0.42%)}	_~	_1.76s	_1.78s	_{p=0.051 n=6}
_{Check Time}	_{18.80s (± 0.76%)}	_{18.74s (± 0.49%)}	_~	_18.64s	_18.89s	_{p=0.297 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{24.72s (± 0.51%)}	_{24.60s (± 0.39%)}	_~	_24.46s	_24.72s	_{p=0.173 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₅	₅	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_555,375	_555,375	_~	_~	_~	_{p=1.000 n=6}
_Types	_186,146	_186,146	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{494,518k (± 0.01%)}	_{494,022k (± 0.01%)}	_{-497k (- 0.10%)}	_493,945k	_494,092k	_{p=0.005 n=6}
_{Parse Time}	_{3.42s (± 0.65%)}	_{3.40s (± 0.22%)}	_~	_3.39s	_3.41s	_{p=0.111 n=6}
_{Bind Time}	_{1.18s (± 0.87%)}	_{1.18s (± 1.42%)}	_~	_1.16s	_1.21s	_{p=0.549 n=6}
_{Check Time}	_{19.57s (± 0.16%)}	_{19.45s (± 0.55%)}	_~	_19.33s	_19.62s	_{p=0.077 n=6}
_{Emit Time}	_0.00s	_0.00s	_~	_~	_~	_{p=1.000 n=6}
_{Total Time}	_{24.17s (± 0.14%)}	_{24.03s (± 0.49%)}	_~	_23.90s	_24.20s	_{p=0.076 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 (typescript-bot) · 2025-02-07T04:01:45Z

Gabriela Araujo Britto (@gabritto) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/61136/merge:

Everything looks good!

Jake Bailey (jakebailey)

😢

Should we keep the tests behind for the future, rather than deleting them?

Gabriela Araujo Britto (gabritto) · 2025-02-07T18:51:27Z

😢

Should we keep the tests behind for the future, rather than deleting them?

I'll just revert this PR when I re-introduce narrowing for 5.9, so it will have the deleted tests again.

This reverts commit c3ae7c4.

revert return type narrowing

7bca43d

Jake Bailey (jakebailey) approved these changes Feb 7, 2025

View reviewed changes

Gabriela Araujo Britto (gabritto) merged commit c3ae7c4 into main Feb 7, 2025

Gabriela Araujo Britto (gabritto) deleted the gabritto/revert56941 branch February 7, 2025 18:55

Gabriela Araujo Britto (gabritto) mentioned this pull request Feb 28, 2025

Narrow generic conditional and indexed access return types when checking return statements #56941

Merged

This was referenced Feb 28, 2025

Relate control flow to conditional types in return types #33912

Open

Suggestion for Dependent-Type-Like Functions: Conservative Narrowing of Generic Indexed Access Result Type #33014

Open

Gabriela Araujo Britto (gabritto) added a commit that referenced this pull request Mar 5, 2025

Revert "Revert return type narrowing (#61136)"

aa38e34

This reverts commit c3ae7c4.

Jeremy Meng (jeremymeng) mentioned this pull request Mar 5, 2025

TS1166 reported for imported symbol as computed property name in generated d.ts #61360

Closed

Nathan Shively-Sanders (sandersn) removed this from PR Backlog Apr 22, 2025

Luke Deen Taylor (controversial) mentioned this pull request May 5, 2025

TypeScript 5.9 Iteration Plan #61648

Closed

Mateusz Burzyński (Andarist) mentioned this pull request Jun 6, 2025

Port return conditional expression checking improvements microsoft/typescript-go#1072

Merged

Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 15, 2025

TypeScript Bot (typescript-bot) assigned Gabriela Araujo Britto (gabritto) Oct 15, 2025

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Revert return type narrowing - #61136

Revert return type narrowing#61136
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