Explicit undefined return type checked similar to explicit void return type

[Anders Hejlsberg (ahejlsberg)]

This PR (hopefully) completes the work started in #53092 and subsequently modified in #53490. With this and the other PRs, checking of functions changes as follows compared to 5.0:

• In functions with an explicit or contextual return type undefined, expression-less return statements and the implicit return and the end of the function are considered to have type undefined (as opposed to void). Note that this does not apply to functions with union return types that include undefined.
• Functions with an explicitly specified return type undefined aren't required to have return statements (similar to functions with an explicitly specified return type of void or any). Note that this does not apply to functions with union return types that include undefined.
• The --noImplicitReturns option doesn't apply to functions with return types that include undefined (similar to functions with return types that include void, any, or unknown).

Fixes#36288.
Fixes#53473.

[Anders Hejlsberg (ahejlsberg)]

TypeScript Bot (@typescript-bot) test this
TypeScript Bot (@typescript-bot) user test this inline
TypeScript Bot (@typescript-bot) run dt
TypeScript Bot (@typescript-bot) perf test faster
TypeScript Bot (@typescript-bot) test top100

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at 39ed3b8. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the diff-based user code test suite on this PR at 39ed3b8. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 39ed3b8. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the abridged perf test suite on this PR at 39ed3b8. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the diff-based top-repos suite on this PR at 39ed3b8. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user test suite comparing main and refs/pull/53607/merge:

Unfortunately, something went wrong, but it probably wasn't caused by your change.

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg)
The results of the perf run you requested are in!

Here they are:

Comparison Report - main..53607

Metric	main	53607	Delta	Best	Worst	p-value
Angular - node (v16.17.1, x64)
Memory used	361,673k (± 0.00%)	361,744k (± 0.01%)	+71k (+ 0.02%)	361,699k	361,785k	p=0.005 n=6
Parse Time	3.51s (± 0.43%)	3.53s (± 0.75%)	~	3.50s	3.56s	p=0.331 n=6
Bind Time	1.17s (± 0.76%)	1.18s (± 0.64%)	~	1.17s	1.19s	p=0.053 n=6
Check Time	9.49s (± 0.27%)	9.52s (± 0.20%)	+0.04s (+ 0.39%)	9.49s	9.54s	p=0.019 n=6
Emit Time	7.95s (± 0.75%)	8.03s (± 1.06%)	~	7.94s	8.17s	p=0.128 n=6
Total Time	22.12s (± 0.34%)	22.27s (± 0.41%)	+0.14s (+ 0.64%)	22.15s	22.40s	p=0.013 n=6
Compiler-Unions - node (v16.17.1, x64)
Memory used	192,523k (± 0.03%)	192,581k (± 0.03%)	~	192,485k	192,635k	p=0.199 n=6
Parse Time	1.59s (± 1.17%)	1.60s (± 0.39%)	~	1.59s	1.61s	p=1.000 n=6
Bind Time	0.83s (± 0.66%)	0.83s (± 1.27%)	~	0.81s	0.84s	p=1.000 n=6
Check Time	10.35s (± 0.67%)	10.36s (± 0.56%)	~	10.29s	10.42s	p=0.746 n=6
Emit Time	2.99s (± 0.70%)	3.00s (± 1.10%)	~	2.97s	3.06s	p=0.871 n=6
Total Time	15.75s (± 0.67%)	15.79s (± 0.50%)	~	15.70s	15.90s	p=0.872 n=6
Monaco - node (v16.17.1, x64)
Memory used	345,757k (± 0.00%)	345,677k (± 0.00%)	-81k (- 0.02%)	345,649k	345,691k	p=0.005 n=6
Parse Time	2.72s (± 0.54%)	2.74s (± 0.48%)	~	2.72s	2.75s	p=0.050 n=6
Bind Time	1.08s (± 0.50%)	1.08s (± 0.77%)	~	1.07s	1.09s	p=0.855 n=6
Check Time	7.77s (± 0.30%)	7.77s (± 0.56%)	~	7.73s	7.85s	p=0.567 n=6
Emit Time	4.46s (± 0.67%)	4.50s (± 0.70%)	~	4.45s	4.54s	p=0.127 n=6
Total Time	16.03s (± 0.24%)	16.09s (± 0.43%)	~	16.04s	16.22s	p=0.090 n=6
TFS - node (v16.17.1, x64)
Memory used	300,032k (± 0.01%)	300,061k (± 0.01%)	~	300,032k	300,114k	p=0.128 n=6
Parse Time	2.16s (± 0.63%)	2.17s (± 0.86%)	~	2.16s	2.21s	p=0.149 n=6
Bind Time	1.23s (± 0.80%)	1.24s (± 0.94%)	~	1.23s	1.26s	p=0.209 n=6
Check Time	7.18s (± 0.41%)	7.24s (± 0.68%)	+0.06s (+ 0.84%)	7.21s	7.34s	p=0.018 n=6
Emit Time	4.35s (± 0.86%)	4.34s (± 0.84%)	~	4.30s	4.39s	p=0.747 n=6
Total Time	14.92s (± 0.43%)	15.00s (± 0.63%)	~	14.92s	15.19s	p=0.199 n=6
material-ui - node (v16.17.1, x64)
Memory used	476,461k (± 0.01%)	476,455k (± 0.01%)	~	476,418k	476,488k	p=0.810 n=6
Parse Time	3.22s (± 0.38%)	3.24s (± 0.55%)	+0.02s (+ 0.73%)	3.21s	3.26s	p=0.042 n=6
Bind Time	0.95s (± 0.00%)	0.96s (± 0.43%)	+0.01s (+ 0.88%)	0.95s	0.96s	p=0.007 n=6
Check Time	18.13s (± 0.56%)	18.24s (± 0.43%)	~	18.14s	18.38s	p=0.149 n=6
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	~	0.00s	0.00s	p=1.000 n=6
Total Time	22.30s (± 0.46%)	22.44s (± 0.42%)	~	22.30s	22.59s	p=0.066 n=6
xstate - node (v16.17.1, x64)
Memory used	550,589k (± 0.02%)	550,621k (± 0.02%)	~	550,485k	550,758k	p=0.689 n=6
Parse Time	3.93s (± 0.21%)	3.94s (± 0.25%)	~	3.93s	3.95s	p=0.140 n=6
Bind Time	1.75s (± 0.86%)	1.76s (± 0.51%)	~	1.75s	1.77s	p=0.557 n=6
Check Time	3.06s (± 0.45%)	3.09s (± 0.96%)	~	3.06s	3.13s	p=0.084 n=6
Emit Time	0.09s (± 0.00%)	0.09s (± 0.00%)	~	0.09s	0.09s	p=1.000 n=6
Total Time	8.83s (± 0.23%)	8.88s (± 0.38%)	+0.04s (+ 0.47%)	8.84s	8.93s	p=0.029 n=6

System

Machine Name	ts-ci-ubuntu
Platform	linux 5.4.0-135-generic
Architecture	x64
Available Memory	16 GB
Available Memory	15 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

• node (v16.17.1, x64)

Scenarios

• Angular - node (v16.17.1, x64)
• Compiler-Unions - node (v16.17.1, x64)
• Monaco - node (v16.17.1, x64)
• TFS - node (v16.17.1, x64)
• material-ui - node (v16.17.1, x64)
• xstate - node (v16.17.1, x64)

Benchmark	Name	Iterations
Current	53607	6
Baseline	main	6

Developer Information:

Download Benchmark

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg) Here are the results of running the top-repos suite comparing main and refs/pull/53607/merge:

Everything looks good!

[TypeScript Bot (typescript-bot)]

Hey Anders Hejlsberg (@ahejlsberg), the results of running the DT tests are ready.
Everything looks the same!
You can check the log here.

[Gabriela Araujo Britto (gabritto)]

TypeScript Bot (@typescript-bot) pack this

[TypeScript Bot (typescript-bot)]

Heya Gabriela Araujo Britto (@gabritto), I've started to run the tarball bundle task on this PR at 39ed3b8. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Hey Gabriela Araujo Britto (@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/151418/artifacts?artifactName=tgz&fileId=B89711228EC2A48ADEDB42C169AA808EADFEBAC64380B790A7B2A3C7A4798BEE02&fileName=/typescript-5.1.0-insiders.20230331.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@5.1.0-pr-53607-12".;

[Anders Hejlsberg (ahejlsberg)]

TypeScript Bot (@typescript-bot) perf test faster

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the abridged perf test suite on this PR at 39ed3b8. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg)
The results of the perf run you requested are in!

Here they are:

Comparison Report - main..53607

Metric	main	53607	Delta	Best	Worst	p-value
Angular - node (v16.17.1, x64)
Memory used	361,701k (± 0.01%)	361,738k (± 0.01%)	+37k (+ 0.01%)	361,697k	361,754k	p=0.045 n=6
Parse Time	3.51s (± 1.00%)	3.51s (± 0.70%)	~	3.47s	3.53s	p=0.935 n=6
Bind Time	1.18s (± 0.54%)	1.18s (± 1.13%)	~	1.16s	1.20s	p=0.720 n=6
Check Time	9.52s (± 0.15%)	9.53s (± 0.52%)	~	9.48s	9.60s	p=0.936 n=6
Emit Time	7.98s (± 0.54%)	7.96s (± 0.67%)	~	7.88s	8.04s	p=0.517 n=6
Total Time	22.19s (± 0.27%)	22.18s (± 0.43%)	~	22.10s	22.33s	p=0.470 n=6
Compiler-Unions - node (v16.17.1, x64)
Memory used	193,105k (± 0.72%)	194,643k (± 0.85%)	~	192,497k	195,759k	p=0.298 n=6
Parse Time	1.59s (± 1.66%)	1.59s (± 0.47%)	~	1.58s	1.60s	p=0.805 n=6
Bind Time	0.83s (± 0.91%)	0.82s (± 0.50%)	-0.01s (- 1.21%)	0.81s	0.82s	p=0.024 n=6
Check Time	10.34s (± 0.74%)	10.33s (± 0.42%)	~	10.25s	10.37s	p=0.470 n=6
Emit Time	3.00s (± 0.46%)	2.99s (± 0.95%)	~	2.96s	3.04s	p=0.459 n=6
Total Time	15.75s (± 0.45%)	15.73s (± 0.41%)	~	15.62s	15.82s	p=0.373 n=6
Monaco - node (v16.17.1, x64)
Memory used	345,738k (± 0.01%)	345,680k (± 0.00%)	-57k (- 0.02%)	345,669k	345,695k	p=0.005 n=6
Parse Time	2.72s (± 0.46%)	2.71s (± 0.43%)	~	2.69s	2.72s	p=0.210 n=6
Bind Time	1.08s (± 0.50%)	1.08s (± 1.08%)	~	1.07s	1.10s	p=0.498 n=6
Check Time	7.81s (± 0.45%)	7.76s (± 0.44%)	-0.05s (- 0.66%)	7.70s	7.80s	p=0.036 n=6
Emit Time	4.46s (± 0.52%)	4.43s (± 0.78%)	~	4.39s	4.47s	p=0.220 n=6
Total Time	16.08s (± 0.33%)	15.99s (± 0.40%)	~	15.92s	16.07s	p=0.077 n=6
TFS - node (v16.17.1, x64)
Memory used	300,030k (± 0.00%)	300,051k (± 0.00%)	+21k (+ 0.01%)	300,034k	300,074k	p=0.045 n=6
Parse Time	2.17s (± 0.50%)	2.16s (± 1.03%)	~	2.14s	2.20s	p=0.251 n=6
Bind Time	1.23s (± 0.80%)	1.23s (± 0.42%)	~	1.22s	1.23s	p=0.348 n=6
Check Time	7.20s (± 0.20%)	7.22s (± 0.58%)	~	7.17s	7.29s	p=0.329 n=6
Emit Time	4.34s (± 0.38%)	4.33s (± 0.91%)	~	4.29s	4.38s	p=0.573 n=6
Total Time	14.96s (± 0.25%)	14.94s (± 0.53%)	~	14.88s	15.05s	p=0.571 n=6
material-ui - node (v16.17.1, x64)
Memory used	476,456k (± 0.00%)	476,461k (± 0.01%)	~	476,426k	476,521k	p=0.936 n=6
Parse Time	3.22s (± 0.46%)	3.21s (± 0.23%)	~	3.20s	3.22s	p=0.241 n=6
Bind Time	0.95s (± 0.43%)	0.95s (± 0.79%)	~	0.94s	0.96s	p=1.000 n=6
Check Time	18.15s (± 0.73%)	18.21s (± 0.38%)	~	18.12s	18.30s	p=0.297 n=6
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	~	0.00s	0.00s	p=1.000 n=6
Total Time	22.32s (± 0.66%)	22.37s (± 0.35%)	~	22.27s	22.48s	p=0.378 n=6
xstate - node (v16.17.1, x64)
Memory used	550,624k (± 0.03%)	550,615k (± 0.02%)	~	550,483k	550,782k	p=0.810 n=6
Parse Time	3.95s (± 0.49%)	3.92s (± 0.46%)	-0.03s (- 0.80%)	3.90s	3.95s	p=0.029 n=6
Bind Time	1.76s (± 0.23%)	1.76s (± 0.56%)	~	1.75s	1.77s	p=0.930 n=6
Check Time	3.08s (± 0.61%)	3.06s (± 0.45%)	~	3.04s	3.08s	p=0.089 n=6
Emit Time	0.09s (± 0.00%)	0.09s (± 0.00%)	~	0.09s	0.09s	p=1.000 n=6
Total Time	8.88s (± 0.40%)	8.83s (± 0.30%)	-0.05s (- 0.56%)	8.79s	8.86s	p=0.043 n=6

System

Machine Name	ts-ci-ubuntu
Platform	linux 5.4.0-135-generic
Architecture	x64
Available Memory	16 GB
Available Memory	15 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

• node (v16.17.1, x64)

Scenarios

• Angular - node (v16.17.1, x64)
• Compiler-Unions - node (v16.17.1, x64)
• Monaco - node (v16.17.1, x64)
• TFS - node (v16.17.1, x64)
• material-ui - node (v16.17.1, x64)
• xstate - node (v16.17.1, x64)

Benchmark	Name	Iterations
Current	53607	6
Baseline	main	6

Developer Information:

Download Benchmark

[Daniel Rosenwasser (DanielRosenwasser)]

I reviewed, but let's just answer outstanding comments from Gabriela Araujo Britto (@gabritto) and others first.

[Anders Hejlsberg (ahejlsberg)]

TypeScript Bot (@typescript-bot) user test this inline

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the diff-based user code test suite on this PR at 39ed3b8. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user test suite comparing main and refs/pull/53607/merge:

Unfortunately, something went wrong, but it probably wasn't caused by your change.

[Jake Bailey (jakebailey)]

I'm looking into what's going on with those tests. Not sure what's going wrong; it looks green but then decides to say that something failed.

[Jake Bailey (jakebailey)]

It looks like one of the user tests no longer npm installs correctly; not totally sure why we fail the whole thing due to that, though. Otherwise, reading the logs, this PR shows no change in any of those user tests.

[Anders Hejlsberg (ahejlsberg)]

Gabriela Araujo Britto (@gabritto) Yeah, it would be more consistent for all of them to return the error. I will change the logic to have noImplicitReturns only exclude functions that have exactly undefined as their return type.

[Maria Solano (MariaSolOs)]

For these new cases, I would appreciate keeping the convention from the other tests of indicating if this will error :)

[Anders Hejlsberg (ahejlsberg)]

Sure, I can fix the comments.

[Anders Hejlsberg (ahejlsberg)]

In addition to addressing Gabriela Araujo Britto (@gabritto)'s feedback, latest commits align the rules for --noImplicitReturns to remove inconsistencies between functions with explicit unknown return types and functions with inferred unknown return types. Previously, only functions with explicit unknown return types would issue errors for implicit returns in --noImplicitReturns mode. Now both do.

[Anders Hejlsberg (ahejlsberg)]

TypeScript Bot (@typescript-bot) test this
TypeScript Bot (@typescript-bot) user test this inline
TypeScript Bot (@typescript-bot) run dt
TypeScript Bot (@typescript-bot) test top100

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 40f9512. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the diff-based user code test suite on this PR at 40f9512. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at 40f9512. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the diff-based top-repos suite on this PR at 40f9512. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user test suite comparing main and refs/pull/53607/merge:

Unfortunately, something went wrong, but it probably wasn't caused by your change.

[TypeScript Bot (typescript-bot)]

Hey Anders Hejlsberg (@ahejlsberg), the results of running the DT tests are ready.
Everything looks the same!
You can check the log here.

[TypeScript Bot (typescript-bot)]

Anders Hejlsberg (@ahejlsberg) Here are the results of running the top-repos suite comparing main and refs/pull/53607/merge:

Something interesting changed - please have a look.

Details

microsoft/vscode

4 of 53 projects failed to build with the old tsc and were ignored

extensions/references-view/tsconfig.json

• error TS7030: Not all code paths return a value.
  • extensions/references-view/src/tree.ts#L268

src/tsconfig.tsec.json

• error TS7030: Not all code paths return a value.
  • src/vs/workbench/contrib/notebook/browser/view/renderers/webviewPreloads.ts#L1646

vuetifyjs/vuetify

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

packages/vuetify/tsconfig.checks.json

• error TS7030: Not all code paths return a value.
  • packages/vuetify/src/util/injectSelf.ts#L5

packages/vuetify/tsconfig.dist.json

• error TS7030: Not all code paths return a value.
  • packages/vuetify/src/util/injectSelf.ts#L5

packages/vuetify/tsconfig.json

• error TS7030: Not all code paths return a value.
  • packages/vuetify/src/util/injectSelf.ts#L5

[Anders Hejlsberg (ahejlsberg)]

A few new errors in vscode and vuetify related to the change I mention here. I think these new errors are acceptable--the projects have noImplicitReturns enabled, the functions in question have inferred return types unknown (or Promise<unknown>), and the errors would have been reported if the functions had explicit return type annotations. It looks to me like all of the examples actually intended for the functions to return void but inadvertently returned values in some conditional code paths.

[Anders Hejlsberg (ahejlsberg)]

Otherwise things look good, so I'm going to merge this.

[stevenwdv]

Sorry for replying to an old thread, but can I ask a question about this?

Why is this not allowed for unions with undefined? E.g. for event listeners:

functionaddListener(listener: (ev: Event)=>undefined|boolean){/*...*/}addListener(ev=>{/*... (no return)*/});// Argument of type '(ev: Event) => void' is not assignable to parameter of type '(ev: Event) => boolean | undefined'.// Type 'void' is not assignable to type 'boolean | undefined'.

I think this could be a nice use case.