Experiment: report functions with multiple returns that could be type predicates

[Dan Vanderkam (danvk)]

This is a side experiment for #58154. That PR extends type predicate inference to functions with multiple returns. It seems to have minimal perf impact and also minimal breakages. This makes you wonder… are there any such functions?

This fork of the PR should answer that question by reporting errors on all functions with 2+ return statements where it can infer a type predicate. To be clear, these aren't real errors! They're just identifying multi-return predicates in the wild.

There are three such function in TypeScript itself:

1. tryAddPropertyAssignment in nodeFactory.ts:

functiontryAddPropertyAssignment(properties: PropertyAssignment[],propertyName: string,expression: Expression|undefined){if(expression){properties.push(createPropertyAssignment(propertyName,expression));returntrue;}returnfalse;}

This gets an inferred return type of expression is Expression. This is accurate, though not consequential for any of the code that calls this function.

2. charIsPunctuation in patternMatcher.ts:

functioncharIsPunctuation(ch: number){switch(ch){caseCharacterCodes.exclamation:
caseCharacterCodes.doubleQuote:
// ...caseCharacterCodes._:
caseCharacterCodes.openBrace:
caseCharacterCodes.closeBrace:
returntrue;}returnfalse;}

This gets an inferred return type of ch is CharacterCodes._ | CharacterCodes.ampersand | CharacterCodes.asterisk | ... | CharacterCodes.slash. Again, this is accurate but not consequential for any of the calling code.

3. An anonymous function in getLinkedEditingRangeAtPosition in services.ts:

consttag=findAncestor(token.parent,n=>{if(isJsxOpeningElement(n)||isJsxClosingElement(n)){returntrue;}returnfalse;});if(!tag)returnundefined;Debug.assert(isJsxOpeningElement(tag)||isJsxClosingElement(tag),"tag should be opening or closing element");

The arrow function becomes a type predicate with #58154 which changes the type of tag. This is a positive change that eliminates the need for the Debug.assert statement.

Of course, the function could have been written more succinctly as n => isJsxOpeningElement(n) || isJsxClosingElement(n) and the existing code would infer a type predicate.

I'm curious what other errors this reports on the user/top/etc suites. This will characterize whether this is a common pattern in the wild.

[Ryan Cavanaugh (RyanCavanaugh)]

TypeScript Bot (@typescript-bot) test it

[TypeScript Bot (typescript-bot)]

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)]

Hey Ryan Cavanaugh (@RyanCavanaugh), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

[TypeScript Bot (typescript-bot)]

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the user tests comparing main and refs/pull/58173/merge:

Something interesting changed - please have a look.

Details

pyright

/mnt/ts_downloads/_/m/pyright/build.sh

• [NEW] error TS1499: Function with multiple returns is implicitly a type predicate.
  • /mnt/ts_downloads/_/m/pyright/pyright: ../pyright-internal/src/analyzer/parseTreeUtils.ts(2588,17)
  • /mnt/ts_downloads/_/m/pyright/pyright-internal: src/analyzer/parseTreeUtils.ts(2588,17)
  • /mnt/ts_downloads/_/m/pyright/vscode-pyright: ../pyright-internal/src/analyzer/parseTreeUtils.ts(2588,17)

[TypeScript Bot (typescript-bot)]

Ryan Cavanaugh (@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
Angular - node (v18.15.0, x64)
Memory used	296,989k (± 0.00%)	297,001k (± 0.01%)	~	296,971k	297,013k	p=0.298 n=6
Parse Time	2.69s (± 0.45%)	2.69s (± 0.38%)	~	2.67s	2.70s	p=0.408 n=6
Bind Time	0.82s (± 0.50%)	0.82s (± 0.00%)	~	0.82s	0.82s	p=0.405 n=6
Check Time	8.33s (± 0.42%)	8.35s (± 0.37%)	~	8.32s	8.40s	p=0.469 n=6
Emit Time	7.05s (± 0.31%)	7.05s (± 0.32%)	~	7.01s	7.07s	p=1.000 n=6
Total Time	18.89s (± 0.20%)	18.90s (± 0.26%)	~	18.83s	18.95s	p=0.629 n=6
Compiler-Unions - node (v18.15.0, x64)
Memory used	192,371k (± 0.74%)	194,332k (± 0.94%)	+1,961k (+ 1.02%)	191,962k	195,646k	p=0.013 n=6
Parse Time	2.01s (± 1.53%)	2.02s (± 0.60%)	~	2.01s	2.04s	p=0.416 n=6
Bind Time	1.07s (± 0.92%)	1.07s (± 0.97%)	~	1.05s	1.08s	p=0.788 n=6
Check Time	14.04s (± 0.18%)	14.06s (± 0.50%)	~	13.96s	14.13s	p=0.574 n=6
Emit Time	3.89s (± 2.32%)	3.87s (± 1.04%)	~	3.79s	3.90s	p=0.292 n=6
Total Time	21.01s (± 0.49%)	21.02s (± 0.42%)	~	20.88s	21.09s	p=0.747 n=6
Monaco - node (v18.15.0, x64)
Memory used	348,863k (± 0.01%)	348,866k (± 0.00%)	~	348,847k	348,882k	p=0.688 n=6
Parse Time	3.85s (± 0.86%)	3.85s (± 1.12%)	~	3.79s	3.90s	p=0.936 n=6
Bind Time	1.32s (± 0.62%)	1.32s (± 0.80%)	~	1.30s	1.33s	p=0.865 n=6
Check Time	10.26s (± 0.42%)	10.27s (± 0.49%)	~	10.18s	10.31s	p=0.872 n=6
Emit Time	5.99s (± 0.52%)	6.00s (± 0.61%)	~	5.94s	6.03s	p=0.809 n=6
Total Time	21.42s (± 0.32%)	21.42s (± 0.23%)	~	21.35s	21.50s	p=1.000 n=6
TFS - node (v18.15.0, x64)
Memory used	302,975k (± 0.01%)	302,982k (± 0.00%)	~	302,964k	303,001k	p=0.471 n=6
Parse Time	2.04s (± 0.51%)	2.02s (± 0.67%)	~	2.01s	2.04s	p=0.161 n=6
Bind Time	0.98s (± 0.77%)	0.98s (± 0.77%)	~	0.97s	0.99s	p=1.000 n=6
Check Time	6.47s (± 0.49%)	6.45s (± 0.51%)	~	6.41s	6.51s	p=0.519 n=6
Emit Time	3.58s (± 0.23%)	3.58s (± 0.51%)	~	3.56s	3.60s	p=1.000 n=6
Total Time	13.07s (± 0.27%)	13.04s (± 0.31%)	~	12.97s	13.08s	p=0.170 n=6
material-ui - node (v18.15.0, x64)
Memory used	511,624k (± 0.01%)	511,617k (± 0.00%)	~	511,579k	511,646k	p=0.575 n=6
Parse Time	4.01s (± 0.66%)	4.02s (± 1.04%)	~	3.99s	4.09s	p=0.681 n=6
Bind Time	1.46s (± 1.42%)	1.45s (± 2.02%)	~	1.42s	1.50s	p=0.872 n=6
Check Time	25.32s (± 0.24%)	25.27s (± 0.18%)	~	25.22s	25.35s	p=0.148 n=6
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	~	0.00s	0.00s	p=1.000 n=6
Total Time	30.79s (± 0.23%)	30.75s (± 0.22%)	~	30.69s	30.88s	p=0.335 n=6
mui-docs - node (v18.15.0, x64)
Memory used	1,748,693k (± 0.00%)	1,748,679k (± 0.00%)	~	1,748,618k	1,748,755k	p=0.471 n=6
Parse Time	9.96s (± 0.38%)	10.03s (± 0.88%)	~	9.93s	10.12s	p=0.261 n=6
Bind Time	3.35s (± 0.27%)	3.36s (± 0.87%)	~	3.32s	3.40s	p=0.370 n=6
Check Time	81.76s (± 0.27%)	82.04s (± 0.60%)	~	81.29s	82.68s	p=0.230 n=6
Emit Time	0.19s (± 2.81%)	0.20s (± 5.07%)	~	0.19s	0.22s	p=0.138 n=6
Total Time	95.26s (± 0.21%)	95.63s (± 0.47%)	~	95.00s	96.17s	p=0.229 n=6
self-build-src - node (v18.15.0, x64)
Memory used	2,306,333k (± 0.06%)	2,307,414k (± 0.03%)	~	2,306,245k	2,308,046k	p=0.093 n=6
Parse Time	6.04s (± 0.68%)	6.03s (± 0.69%)	~	5.97s	6.08s	p=0.872 n=6
Bind Time	2.23s (± 1.05%)	2.24s (± 1.28%)	~	2.20s	2.29s	p=0.520 n=6
Check Time	39.84s (± 0.28%)	39.90s (± 0.39%)	~	39.72s	40.17s	p=0.423 n=6
Emit Time	3.17s (± 3.58%)	3.19s (± 3.58%)	~	3.04s	3.35s	p=1.000 n=6
Total Time	51.30s (± 0.43%)	51.38s (± 0.38%)	~	51.15s	51.67s	p=0.688 n=6
self-build-src-public-api - node (v18.15.0, x64)
Memory used	2,380,781k (± 0.04%)	2,381,898k (± 0.03%)	~	2,381,032k	2,382,892k	p=0.066 n=6
Parse Time	7.63s (± 0.36%)	7.66s (± 0.82%)	~	7.56s	7.74s	p=0.230 n=6
Bind Time	2.52s (± 1.24%)	2.51s (± 1.12%)	~	2.48s	2.56s	p=0.368 n=6
Check Time	49.80s (± 0.92%)	49.52s (± 0.53%)	~	49.22s	49.97s	p=0.298 n=6
Emit Time	3.92s (± 1.70%)	3.96s (± 1.99%)	~	3.83s	4.03s	p=0.261 n=6
Total Time	63.87s (± 0.70%)	63.65s (± 0.40%)	~	63.29s	64.03s	p=0.575 n=6
self-compiler - node (v18.15.0, x64)
Memory used	419,069k (± 0.01%)	419,192k (± 0.01%)	+123k (+ 0.03%)	419,131k	419,245k	p=0.005 n=6
Parse Time	3.40s (± 0.76%)	3.37s (± 2.43%)	~	3.21s	3.44s	p=0.808 n=6
Bind Time	1.31s (± 1.01%)	1.33s (± 4.80%)	~	1.26s	1.45s	p=0.466 n=6
Check Time	18.04s (± 0.30%)	18.02s (± 0.41%)	~	17.94s	18.14s	p=0.686 n=6
Emit Time	1.37s (± 1.36%)	1.37s (± 2.62%)	~	1.33s	1.41s	p=0.872 n=6
Total Time	24.11s (± 0.20%)	24.10s (± 0.32%)	~	24.01s	24.22s	p=0.748 n=6
vscode - node (v18.15.0, x64)
Memory used	2,912,926k (± 0.00%)	2,912,839k (± 0.00%)	~	2,912,683k	2,912,925k	p=0.066 n=6
Parse Time	13.39s (± 0.21%)	13.42s (± 0.34%)	~	13.35s	13.49s	p=0.255 n=6
Bind Time	4.06s (± 0.40%)	4.10s (± 2.29%)	~	4.05s	4.29s	p=0.807 n=6
Check Time	72.46s (± 0.41%)	72.48s (± 0.27%)	~	72.33s	72.80s	p=0.689 n=6
Emit Time	19.48s (± 0.82%)	20.14s (± 7.49%)	~	19.37s	23.22s	p=0.172 n=6
Total Time	109.39s (± 0.39%)	110.15s (± 1.31%)	~	109.31s	113.06s	p=0.199 n=6
webpack - node (v18.15.0, x64)
Memory used	409,375k (± 0.01%)	409,385k (± 0.01%)	~	409,324k	409,429k	p=0.689 n=6
Parse Time	3.26s (± 0.82%)	3.27s (± 1.19%)	~	3.24s	3.32s	p=0.466 n=6
Bind Time	1.37s (± 0.75%)	1.38s (± 0.76%)	~	1.37s	1.40s	p=0.098 n=6
Check Time	14.38s (± 0.15%)	14.39s (± 0.29%)	~	14.35s	14.45s	p=0.872 n=6
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	~	0.00s	0.00s	p=1.000 n=6
Total Time	19.01s (± 0.23%)	19.04s (± 0.21%)	~	18.99s	19.08s	p=0.191 n=6
xstate - node (v18.15.0, x64)
Memory used	671,977k (± 0.01%)	672,047k (± 0.02%)	~	671,934k	672,265k	p=0.689 n=6
Parse Time	4.12s (± 0.84%)	4.12s (± 0.74%)	~	4.08s	4.17s	p=1.000 n=6
Bind Time	1.87s (± 0.83%)	1.88s (± 0.87%)	~	1.86s	1.90s	p=0.511 n=6
Check Time	3.43s (± 0.24%)	3.45s (± 0.57%)	~	3.42s	3.47s	p=0.081 n=6
Emit Time	0.03s (±19.35%)	0.03s (±14.39%)	~	0.02s	0.03s	p=0.595 n=6
Total Time	9.46s (± 0.48%)	9.47s (± 0.58%)	~	9.40s	9.56s	p=0.746 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-build-src-public-api - 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

[Jake Bailey (jakebailey)]

The pyright one is: https://github.com/microsoft/pyright/blob/92f2284d2459fed8a9fdaab023527d8598749fcc/packages/pyright-internal/src/analyzer/parseTreeUtils.ts#L2588

Basically the same as charIsPunctuation.

[TypeScript Bot (typescript-bot)]

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the top 400 repos comparing main and refs/pull/58173/merge:

Something interesting changed - please have a look.

Details

facebook/lexical

3 of 7 projects failed to build with the old tsc and were ignored

tsconfig.json

• error TS1499: Function with multiple returns is implicitly a type predicate.
  • packages/lexical-playground/src/plugins/TableCellResizer/index.tsx#L147

Tencent/omi

6 of 19 projects failed to build with the old tsc and were ignored

packages/reactive-signal/tsconfig.json

• error TS1499: Function with multiple returns is implicitly a type predicate.
  • packages/reactive-signal/src/reactivity.ts#L76

packages/reactive-signal/examples/tsconfig.json

• error TS1499: Function with multiple returns is implicitly a type predicate.
  • packages/reactive-signal/src/reactivity.ts#L76

[Jake Bailey (jakebailey)]

I feel like it's telling that we're seeing so few of these, and that when we do, the code still compiles as it did before. That makes it seem like this isn't very valuable, and that the initial design was good enough.

[Dan Vanderkam (danvk)]

Closing this out since we've learned what we're going to from it.

[Jake Bailey (jakebailey)]

BTW thank you for sending these PRs in exactly the format we need to let the infra report something, super helpful.