Allow implicit `undefined` returns when the contextual union type contains it by Andarist · Pull Request #57912 · microsoft/TypeScript

Mateusz Burzyński (Andarist) · 2024-03-22T20:17:09Z

Ryan Cavanaugh (@RyanCavanaugh) triaged this as a bug here. I'm pretty sure that Anders Hejlsberg (@ahejlsberg) didn't want this to work this way though (as per the description of #53607 and the comment in tests that I had to touch here).

…tains it

Jake Bailey (jakebailey) · 2024-03-22T20:27:54Z

TypeScript Bot (@typescript-bot) test top400
TypeScript Bot (@typescript-bot) user test this
TypeScript Bot (@typescript-bot) run dt

TypeScript Bot (@typescript-bot) perf test this faster
TypeScript Bot (@typescript-bot) pack this

TypeScript Bot (typescript-bot) · 2024-03-22T20:27:57Z

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
`pack this`	✅ Started	✅ Results

TypeScript Bot (typescript-bot) · 2024-03-22T20:33:05Z

Hey Jake Bailey (@jakebailey), 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/160694/artifacts?artifactName=tgz&fileId=976D886DEE8ACEEF241EF5A35B6FFC03FD3FAC6E83579B8581E469403D8E9E5002&fileName=/typescript-5.5.0-insiders.20240322.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@5.5.0-pr-57912-2".;

TypeScript Bot (typescript-bot) · 2024-03-22T20:47:38Z

Hey Jake Bailey (@jakebailey), the results of running the DT tests are ready.

There were interesting changes:

Branch only errors:

Package: nodes7
Error:

Error: /mnt/vss/_work/1/DefinitelyTyped/types/nodes7/nodes7-tests.ts
62:1 error TypeScript@local compile error: Unused '@ts-expect-error' directive @definitelytyped/expect
✖ 1 problem (1 error, 0 warnings)
at combineErrorsAndWarnings (/mnt/vss/_work/1/DefinitelyTyped/node_modules/.pnpm/@definitelytyped+dtslint@0.2.18_typescript@5.5.0-dev.20240322/node_modules/@definitelytyped/dtslint/dist/index.js:194:28)
at runTests (/mnt/vss/_work/1/DefinitelyTyped/node_modules/.pnpm/@definitelytyped+dtslint@0.2.18_typescript@5.5.0-dev.20240322/node_modules/@definitelytyped/dtslint/dist/index.js:186:20)

You can check the log here.

TypeScript Bot (typescript-bot) · 2024-03-22T20:52:14Z

Jake Bailey (@jakebailey) Here are the results of running the user tests comparing main and refs/pull/57912/merge:

Everything looks good!

TypeScript Bot (typescript-bot) · 2024-03-22T20:57:45Z

Jake Bailey (@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
_{Angular - node (v18.15.0, x64)}
_{Memory used}	_{295,514k (± 0.01%)}	_{295,515k (± 0.00%)}	_~	_295,503k	_295,530k	_{p=0.199 n=6}
_{Parse Time}	_{2.66s (± 0.61%)}	_{2.67s (± 0.39%)}	_~	_2.65s	_2.68s	_{p=0.358 n=6}
_{Bind Time}	_{0.83s (± 0.66%)}	_{0.83s (± 1.41%)}	_~	_0.82s	_0.85s	_{p=0.859 n=6}
_{Check Time}	_{8.19s (± 0.23%)}	_{8.21s (± 0.31%)}	_~	_8.18s	_8.25s	_{p=0.226 n=6}
_{Emit Time}	_{7.05s (± 0.20%)}	_{7.06s (± 0.45%)}	_~	_7.03s	_7.12s	_{p=1.000 n=6}
_{Total Time}	_{18.73s (± 0.14%)}	_{18.76s (± 0.27%)}	_~	_18.72s	_18.85s	_{p=0.198 n=6}
_{Compiler-Unions - node (v18.15.0, x64)}
_{Memory used}	_{193,094k (± 0.94%)}	_{193,656k (± 1.01%)}	_~	_191,845k	_195,576k	_{p=0.810 n=6}
_{Parse Time}	_{1.35s (± 2.64%)}	_{1.36s (± 1.30%)}	_~	_1.33s	_1.38s	_{p=0.870 n=6}
_{Bind Time}	_{0.72s (± 0.00%)}	_{0.72s (± 0.00%)}	_~	_0.72s	_0.72s	_{p=1.000 n=6}
_{Check Time}	_{9.47s (± 0.68%)}	_{9.46s (± 0.34%)}	_~	_9.41s	_9.50s	_{p=0.935 n=6}
_{Emit Time}	_{2.62s (± 0.51%)}	_{2.62s (± 0.68%)}	_~	_2.59s	_2.64s	_{p=0.743 n=6}
_{Total Time}	_{14.15s (± 0.62%)}	_{14.15s (± 0.23%)}	_~	_14.10s	_14.19s	_{p=1.000 n=6}
_{Monaco - node (v18.15.0, x64)}
_{Memory used}	_{347,377k (± 0.01%)}	_{347,380k (± 0.01%)}	_~	_347,359k	_347,417k	_{p=0.810 n=6}
_{Parse Time}	_{2.48s (± 0.34%)}	_{2.48s (± 0.59%)}	_~	_2.46s	_2.50s	_{p=0.867 n=6}
_{Bind Time}	_{0.92s (± 0.56%)}	_{0.92s (± 0.59%)}	_~	_0.92s	_0.93s	_{p=0.640 n=6}
_{Check Time}	_{7.01s (± 0.42%)}	_{7.00s (± 0.28%)}	_~	_6.98s	_7.03s	_{p=0.324 n=6}
_{Emit Time}	_{4.06s (± 0.43%)}	_{4.06s (± 0.34%)}	_~	_4.04s	_4.08s	_{p=0.871 n=6}
_{Total Time}	_{14.49s (± 0.18%)}	_{14.47s (± 0.19%)}	_~	_14.42s	_14.49s	_{p=0.681 n=6}
_{TFS - node (v18.15.0, x64)}
_{Memory used}	_{302,689k (± 0.01%)}	_{302,713k (± 0.01%)}	_~	_302,681k	_302,754k	_{p=0.093 n=6}
_{Parse Time}	_{2.44s (± 0.48%)}	_{2.42s (± 0.89%)}	_{-0.03s (- 1.02%)}	_2.39s	_2.44s	_{p=0.034 n=6}
_{Bind Time}	_{1.20s (± 0.46%)}	_{1.20s (± 1.47%)}	_~	_1.18s	_1.23s	_{p=0.322 n=6}
_{Check Time}	_{7.46s (± 0.30%)}	_{7.45s (± 0.37%)}	_~	_7.42s	_7.50s	_{p=0.742 n=6}
_{Emit Time}	_{4.28s (± 0.58%)}	_{4.29s (± 0.82%)}	_~	_4.24s	_4.33s	_{p=0.872 n=6}
_{Total Time}	_{15.37s (± 0.25%)}	_{15.35s (± 0.37%)}	_~	_15.26s	_15.43s	_{p=0.627 n=6}
_{material-ui - node (v18.15.0, x64)}
_{Memory used}	_{509,899k (± 0.00%)}	_{509,899k (± 0.00%)}	_~	_509,885k	_509,920k	_{p=1.000 n=6}
_{Parse Time}	_{2.64s (± 0.56%)}	_{2.65s (± 0.50%)}	_~	_2.63s	_2.66s	_{p=0.394 n=6}
_{Bind Time}	_{0.99s (± 0.76%)}	_{0.98s (± 1.19%)}	_~	_0.96s	_0.99s	_{p=0.383 n=6}
_{Check Time}	_{17.22s (± 0.45%)}	_{17.27s (± 0.71%)}	_~	_17.18s	_17.50s	_{p=0.520 n=6}
_{Emit Time}	_{0.00s (± 0.00%)}	_{0.00s (± 0.00%)}	_~	_0.00s	_0.00s	_{p=1.000 n=6}
_{Total Time}	_{20.85s (± 0.32%)}	_{20.90s (± 0.65%)}	_~	_20.78s	_21.15s	_{p=0.747 n=6}
_{mui-docs - node (v18.15.0, x64)}
_{Memory used}	_{1,737,546k (± 0.00%)}	_{1,737,562k (± 0.00%)}	_~	_1,737,506k	_1,737,594k	_{p=0.378 n=6}
_{Parse Time}	_{7.76s (± 0.29%)}	_{7.80s (± 0.59%)}	_~	_7.76s	_7.88s	_{p=0.122 n=6}
_{Bind Time}	_{2.79s (± 0.67%)}	_{2.81s (± 1.18%)}	_~	_2.76s	_2.86s	_{p=0.373 n=6}
_{Check Time}	_{66.65s (± 0.38%)}	_{66.61s (± 0.49%)}	_~	_66.14s	_66.99s	_{p=0.748 n=6}
_{Emit Time}	_{0.15s (± 3.53%)}	_{0.15s (± 2.69%)}	_~	_0.15s	_0.16s	_{p=0.282 n=6}
_{Total Time}	_{77.36s (± 0.34%)}	_{77.37s (± 0.39%)}	_~	_76.87s	_77.72s	_{p=1.000 n=6}
_{self-build-src - node (v18.15.0, x64)}
_{Memory used}	_{2,394,576k (± 0.03%)}	_{2,394,386k (± 0.02%)}	_~	_2,393,918k	_2,394,850k	_{p=0.378 n=6}
_{Parse Time}	_{6.05s (± 1.49%)}	_{6.05s (± 0.44%)}	_~	_6.02s	_6.08s	_{p=0.575 n=6}
_{Bind Time}	_{2.26s (± 0.97%)}	_{2.25s (± 0.96%)}	_~	_2.24s	_2.29s	_{p=0.931 n=6}
_{Check Time}	_{39.43s (± 0.22%)}	_{39.48s (± 0.39%)}	_~	_39.29s	_39.72s	_{p=0.689 n=6}
_{Emit Time}	_{3.14s (± 1.17%)}	_{3.13s (± 1.28%)}	_~	_3.09s	_3.19s	_{p=0.873 n=6}
_{Total Time}	_{50.89s (± 0.20%)}	_{50.92s (± 0.29%)}	_~	_50.70s	_51.13s	_{p=0.630 n=6}
_{self-compiler - node (v18.15.0, x64)}
_{Memory used}	_{415,193k (± 0.00%)}	_{415,235k (± 0.01%)}	_{+42k (+ 0.01%)}	_415,187k	_415,270k	_{p=0.031 n=6}
_{Parse Time}	_{3.41s (± 1.15%)}	_{3.40s (± 1.31%)}	_~	_3.34s	_3.46s	_{p=0.515 n=6}
_{Bind Time}	_{1.29s (± 0.94%)}	_{1.28s (± 0.77%)}	_~	_1.27s	_1.29s	_{p=0.507 n=6}
_{Check Time}	_{18.07s (± 0.36%)}	_{18.07s (± 0.34%)}	_~	_17.99s	_18.16s	_{p=0.936 n=6}
_{Emit Time}	_{1.34s (± 1.19%)}	_{1.31s (± 1.04%)}	_{-0.03s (- 2.11%)}	_1.29s	_1.33s	_{p=0.015 n=6}
_{Total Time}	_{24.11s (± 0.35%)}	_{24.07s (± 0.42%)}	_~	_23.97s	_24.23s	_{p=0.572 n=6}
_{vscode - node (v18.15.0, x64)}
_{Memory used}	_{2,890,201k (± 0.00%)}	_{2,890,195k (± 0.00%)}	_~	_2,890,082k	_2,890,313k	_{p=0.873 n=6}
_{Parse Time}	_{12.93s (± 0.52%)}	_{12.93s (± 0.27%)}	_~	_12.88s	_12.98s	_{p=0.810 n=6}
_{Bind Time}	_{4.12s (± 0.42%)}	_{4.12s (± 0.36%)}	_~	_4.11s	_4.14s	_{p=1.000 n=6}
_{Check Time}	_{71.71s (± 0.55%)}	_{71.52s (± 0.37%)}	_~	_71.21s	_71.86s	_{p=0.378 n=6}
_{Emit Time}	_{19.44s (± 0.36%)}	_{19.45s (± 0.52%)}	_~	_19.33s	_19.62s	_{p=1.000 n=6}
_{Total Time}	_{108.20s (± 0.44%)}	_{108.03s (± 0.30%)}	_~	_107.72s	_108.59s	_{p=0.689 n=6}
_{webpack - node (v18.15.0, x64)}
_{Memory used}	_{408,148k (± 0.01%)}	_{408,114k (± 0.01%)}	_~	_408,077k	_408,140k	_{p=0.092 n=6}
_{Parse Time}	_{3.91s (± 0.53%)}	_{3.90s (± 0.53%)}	_~	_3.87s	_3.93s	_{p=0.223 n=6}
_{Bind Time}	_{1.69s (± 0.61%)}	_{1.67s (± 1.05%)}	_~	_1.66s	_1.70s	_{p=0.249 n=6}
_{Check Time}	_{16.73s (± 0.43%)}	_{16.75s (± 0.41%)}	_~	_16.67s	_16.86s	_{p=0.748 n=6}
_{Emit Time}	_{0.00s (± 0.00%)}	_{0.00s (± 0.00%)}	_~	_0.00s	_0.00s	_{p=1.000 n=6}
_{Total Time}	_{22.34s (± 0.35%)}	_{22.33s (± 0.31%)}	_~	_22.25s	_22.43s	_{p=0.936 n=6}
_{xstate - node (v18.15.0, x64)}
_{Memory used}	_{513,082k (± 0.02%)}	_{512,993k (± 0.02%)}	_~	_512,925k	_513,136k	_{p=0.066 n=6}
_{Parse Time}	_{3.95s (± 0.41%)}	_{4.01s (± 4.24%)}	_~	_3.92s	_4.35s	_{p=0.746 n=6}
_{Bind Time}	_{1.86s (± 0.88%)}	_{1.84s (± 0.74%)}	_~	_1.82s	_1.86s	_{p=0.085 n=6}
_{Check Time}	_{3.39s (± 0.86%)}	_{3.36s (± 0.15%)}	_~	_3.36s	_3.37s	_{p=0.100 n=6}
_{Emit Time}	_{0.09s (± 4.62%)}	_{0.09s (± 6.44%)}	_~	_0.08s	_0.09s	_{p=0.282 n=6}
_{Total Time}	_{9.27s (± 0.38%)}	_{9.30s (± 1.74%)}	_~	_9.20s	_9.62s	_{p=0.336 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Angular - node (v18.15.0, x64)
Compiler-Unions - node (v18.15.0, x64)
Monaco - node (v18.15.0, x64)
TFS - node (v18.15.0, x64)
material-ui - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

Ryan Cavanaugh (RyanCavanaugh) · 2024-03-22T21:35:22Z

I think there's a possible parsing ambiguity in the referenced comment

Note that this does not apply to functions with union return types that include undefined

Foo | (() => undefined) vs () => (Foo | undefined)

TypeScript Bot (typescript-bot) · 2024-03-22T22:00:22Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos comparing main and refs/pull/57912/merge:

Everything looks good!

Mateusz Burzyński (Andarist) · 2024-03-23T14:55:25Z

The single reported failure here comes from those lines here. We can quickly verify on this TS playground that the contextual type of this function is (name: string) => string | undefined. So it makes sense that the error went away with this PR.

…turn-contextual-union # Conflicts: # tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types # tests/baselines/reference/inferenceDoesNotAddUndefinedOrNull.types

Andrew Branch (andrewbranch) · 2024-06-18T18:34:34Z

 const f21: () => undefined | number = () => {
- ~~~
-!!! error TS2322: Type '() => void' is not assignable to type '() => number | undefined'.
-!!! error TS2322: Type 'void' is not assignable to type 'number | undefined'.
- // Error, regular void function because contextual type for implicit return isn't just undefined
+ // Ok, contextual type for implicit return contains undefined
 }


I’m not the right person to review this, because I personally lean strongly toward the belief that distinctions between return, return undefined, and no return belong in a linter and not in TS, which I think is not the consensus view on the team. I can’t tell whether the design meeting notes on this were meant to be descriptive or prescriptive:
But no implicit returns when you have 4 | undefined
If it was intended to be prescriptive, this baseline change violates the intention that was captured in the notes. However, moving in this direction is my preference, so I’m going to approve in hopes that it moves the review along 🤷‍♂️

I believe that was descriptive, stating what the current behavior is and that it's inconsistent.

…turn-contextual-union

Allow implicit undefined returns when the contextual union type con…
c309c6f
…tains it

TypeScript Bot (typescript-bot) added the For Backlog Bug PRs that fix a backlog bug label Mar 22, 2024

Merge remote-tracking branch 'origin/main' into implicit-undefined-re…
4042168
…turn-contextual-union # Conflicts: # tests/baselines/reference/functionsMissingReturnStatementsAndExpressionsStrictNullChecks.types # tests/baselines/reference/inferenceDoesNotAddUndefinedOrNull.types

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request May 2, 2024
Design Meeting Notes, 3/29/2024 #58412
Closed

Nathan Shively-Sanders (sandersn) requested a review from Anders Hejlsberg (ahejlsberg)June 14, 2024 17:44

Nathan Shively-Sanders (sandersn) assigned Anders Hejlsberg (ahejlsberg)Jun 14, 2024

Nathan Shively-Sanders (sandersn) requested a review from Andrew Branch (andrewbranch)June 14, 2024 17:45

Andrew Branch (andrewbranch) approved these changes Jun 18, 2024
View reviewed changes

Merge remote-tracking branch 'origin/main' into implicit-undefined-re…
7b63236
…turn-contextual-union

Andrew Branch (andrewbranch) approved these changes Oct 30, 2025
View reviewed changes

Jake Bailey (jakebailey) approved these changes Oct 30, 2025
View reviewed changes

Jake Bailey (jakebailey) merged commit f6f4eab into microsoft:mainOct 30, 2025
31 checks passed

github-project-automationBot moved this from Needs merge to Done in PR Backlog Oct 30, 2025

Jake Bailey (jakebailey) mentioned this pull request Nov 3, 2025
[nodes7] Fix for TS 6.0 DefinitelyTyped/DefinitelyTyped#74012
Merged

Microsoft (microsoft) locked as resolved and limited conversation to collaborators Apr 28, 2026

Navigation Menu

Uh oh!

Allow implicit `undefined` returns when the contextual union type contains it - #57912

Allow implicit `undefined` returns when the contextual union type contains it#57912
Jake Bailey (jakebailey) merged 3 commits into
microsoft:mainfrom
Andarist:implicit-undefined-return-contextual-union

Mateusz Burzyński (Andarist) commented Mar 22, 2024

Uh oh!

Jake Bailey (jakebailey) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024•
edited
Loading

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024•
edited
Loading

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

tsc

Uh oh!

Ryan Cavanaugh (RyanCavanaugh) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

Uh oh!

Mateusz Burzyński (Andarist) commented Mar 23, 2024

Uh oh!

Andrew Branch (andrewbranch)Jun 18, 2024

Uh oh!

Jake Bailey (jakebailey)Jun 18, 2024

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

7 participants

Uh oh!

Conversation

Mateusz Burzyński (Andarist) commented Mar 22, 2024

Uh oh!

Jake Bailey (jakebailey) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

tsc

Uh oh!

Ryan Cavanaugh (RyanCavanaugh) commented Mar 22, 2024

Uh oh!

TypeScript Bot (typescript-bot) commented Mar 22, 2024

Uh oh!

Mateusz Burzyński (Andarist) commented Mar 23, 2024

Uh oh!

Andrew Branch (andrewbranch)Jun 18, 2024

Choose a reason for hiding this comment

Uh oh!

Jake Bailey (jakebailey)Jun 18, 2024

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

7 participants

TypeScript Bot (typescript-bot) commented Mar 22, 2024•
edited
Loading

TypeScript Bot (typescript-bot) commented Mar 22, 2024•
edited
Loading