Don't widen inferred return types when a contextual signature is available

[Daniel Rosenwasser (DanielRosenwasser)]

Background

This PR has two high-level objectives:

• to avoid anys in type argument inference outside of strictNullChecks
• to ensure that excess property checks are more broadly applied in callback return positions

This PR achieves it by avoiding widening return types for function expressions that have been inferred from the body in cases where the current function has a contextual signature.

Widening Poisoning Inference With any

declarefunctionpickRandom<T>(f: ()=>T,g: ()=>T): T;// Returned type is `any`pickRandom(()=>1,()=>null);

Here, we have two function expressions passed into pickRandom. As it currently behaves, TypeScript will try to figure out the type of each function from its body, and in the function () => null, it will find the type null. Outside of strictNullChecks, the type system then widens that null to any. When it has to infer a type for T, it finds the candidates number and any, which just becomes any. This is a result of widening too early, and it's undesirable to let any absorb the number type instead of letting the number type absorb the null type.

Note that that function's return type isn't an implicit any because the function has a contextual signature (i.e. it's being contextually typed). We make an exception in this case. The rationale for that behavior is that it makes very little sense in many cases to issue an implicit any error when there's a contextual type and we know that the any isn't going to escape; however, in the presence of generics, that anydoes escape.

This behavior is bad, but it's not as bad as it used to be because we live in a world with strictNullChecks.

No Excess Property Checking

Today, if you have the following

interfaceOptions{foo?: string,bar?: string,}declarefunctionconfig(getOpts: ()=>Options): void;// This errors.letopts: Options={foo: "hallo",ba: "oh no",};// This doesn't error.config(()=>({foo: "hallo",ba: "oh no",}));

Here, TypeScript gives an excess property error on the assignment to opts, but not within the callback to config.

The reason is that when it has to infer the type for () => ({ foo: "hallo", ba: "oh no", }), it picks up a fresh object type for { foo: "hallo", ba: "oh no", } as an initial candidate, and then widens that to its non-fresh version; however, freshness determines whether excess property checks are performed when determining compatibility against Options. This is another result of widening too early.

This seems to be the behavior most users we've heard are concerned with, which makes sense because users concerned with stricter checks might be the type to opt themselves into strictNullChecks.

Proposal

The proposal here is to only widen return types for function expressions when there is no contextual signature. The same is proposed for yielded and next'd types for generator expressions.

The key idea is that if a contextual type is found, we can be fairly sure that a return expression is going to need to be validated against that contextual type to some extent. In those cases, you really don't want to widen prematurely. In fact, this same logic is the reason we don't report a noImplicitAny.

Future Work

There are cases beyond checking whether there is no contextual signature (e.g. any argument position in a function call expression); however, those cases are no worse off than they are today.

interfaceObj{a: string;}declarefunctionfoo(obj: Obj): void;declarefunctionfoo(fn: ()=>Obj): void;foo(()=>({a: "hello",b: 123}));// no error on 'b' - bad

There are also cases where we'll still widen too early at type argument inference.

functionmap<T,U>(xs: T[],f: (x: T)=>U): U[];interfaceObj{a: string;}declareletstuff: string[];letobjs: Obj[]=map(stuff,_=>({a: "hello",b: 123}));// no error on 'b' - bad

We can do better in both these cases in the future.

Relevant Issues

Fixes#241

Subsumes #20976

Duplicates

Fixes#7547
Fixes#20859
Fixes#27237
Fixes#28634
Fixes#30096
Fixes#30198
Fixes#31235
Fixes#12632
Fixes#11895
Fixes#33908
Fixes#37017
Fixes#39635
Fixes#40270
Fixes#40307

Probable Duplicates

I don't feel like installing React/React Native to test these.

Fixes#29390
Fixes#29499

Related But Not Fixed

Does not fix the following:

#7220
#10245 (comment)
#20008
#26999
#31425
#36053
#36354
#31254

[Daniel Rosenwasser (DanielRosenwasser)]

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

[TypeScript Bot (typescript-bot)]

Daniel Rosenwasser (@DanielRosenwasser)
The results of the perf run you requested are in!

Here they are:

Comparison Report - master..40311

Metric	master	40311	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	344,546k (± 0.02%)	344,589k (± 0.02%)	+43k (+ 0.01%)	344,426k	344,697k
Parse Time	1.99s (± 0.49%)	2.00s (± 0.59%)	+0.01s (+ 0.45%)	1.98s	2.02s
Bind Time	0.82s (± 0.68%)	0.82s (± 0.94%)	-0.00s (- 0.12%)	0.81s	0.84s
Check Time	4.78s (± 0.66%)	4.79s (± 0.69%)	+0.01s (+ 0.19%)	4.72s	4.89s
Emit Time	5.17s (± 0.53%)	5.16s (± 0.62%)	-0.02s (- 0.31%)	5.11s	5.23s
Total Time	12.77s (± 0.29%)	12.77s (± 0.44%)	+0.00s (+ 0.02%)	12.66s	12.92s
Monaco - node (v10.16.3, x64)
Memory used	339,408k (± 0.03%)	339,287k (± 0.04%)	-122k (- 0.04%)	339,122k	339,623k
Parse Time	1.56s (± 0.49%)	1.55s (± 0.44%)	-0.01s (- 0.58%)	1.53s	1.56s
Bind Time	0.72s (± 0.51%)	0.71s (± 0.73%)	-0.01s (- 0.70%)	0.70s	0.72s
Check Time	4.97s (± 0.68%)	4.98s (± 0.63%)	+0.01s (+ 0.14%)	4.91s	5.06s
Emit Time	2.75s (± 0.66%)	2.73s (± 0.61%)	-0.02s (- 0.69%)	2.69s	2.76s
Total Time	10.00s (± 0.50%)	9.98s (± 0.42%)	-0.03s (- 0.27%)	9.86s	10.04s
TFS - node (v10.16.3, x64)
Memory used	302,337k (± 0.04%)	302,400k (± 0.04%)	+63k (+ 0.02%)	302,122k	302,649k
Parse Time	1.21s (± 0.72%)	1.21s (± 0.80%)	+0.00s (+ 0.08%)	1.19s	1.24s
Bind Time	0.67s (± 1.26%)	0.67s (± 0.83%)	-0.00s (- 0.45%)	0.65s	0.68s
Check Time	4.47s (± 0.91%)	4.47s (± 0.69%)	-0.00s (- 0.02%)	4.41s	4.56s
Emit Time	2.92s (± 0.79%)	2.90s (± 1.08%)	-0.02s (- 0.55%)	2.81s	2.97s
Total Time	9.26s (± 0.71%)	9.25s (± 0.60%)	-0.02s (- 0.19%)	9.12s	9.41s
material-ui - node (v10.16.3, x64)
Memory used	461,337k (± 0.01%)	461,277k (± 0.02%)	-60k (- 0.01%)	461,100k	461,408k
Parse Time	1.96s (± 0.51%)	1.95s (± 0.46%)	-0.01s (- 0.31%)	1.93s	1.98s
Bind Time	0.66s (± 1.14%)	0.66s (± 1.05%)	-0.01s (- 1.36%)	0.64s	0.67s
Check Time	13.52s (± 1.00%)	13.33s (± 0.64%)	-0.20s (- 1.45%)	13.22s	13.62s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	16.15s (± 0.87%)	15.94s (± 0.55%)	-0.21s (- 1.29%)	15.85s	16.25s
Angular - node (v12.1.0, x64)
Memory used	321,687k (± 0.03%)	321,696k (± 0.02%)	+8k (+ 0.00%)	321,605k	321,885k
Parse Time	1.99s (± 0.78%)	1.99s (± 0.87%)	-0.00s (- 0.10%)	1.96s	2.05s
Bind Time	0.81s (± 0.74%)	0.81s (± 0.59%)	+0.00s (+ 0.25%)	0.80s	0.82s
Check Time	4.67s (± 0.46%)	4.68s (± 0.46%)	+0.00s (+ 0.11%)	4.64s	4.72s
Emit Time	5.36s (± 1.08%)	5.33s (± 0.54%)	-0.03s (- 0.60%)	5.28s	5.40s
Total Time	12.83s (± 0.37%)	12.81s (± 0.35%)	-0.03s (- 0.23%)	12.72s	12.93s
Monaco - node (v12.1.0, x64)
Memory used	321,619k (± 0.01%)	321,605k (± 0.02%)	-14k (- 0.00%)	321,509k	321,781k
Parse Time	1.53s (± 0.80%)	1.54s (± 0.70%)	+0.01s (+ 0.78%)	1.52s	1.57s
Bind Time	0.69s (± 0.96%)	0.69s (± 0.83%)	-0.00s (- 0.43%)	0.68s	0.70s
Check Time	4.77s (± 0.31%)	4.76s (± 0.49%)	-0.01s (- 0.19%)	4.69s	4.81s
Emit Time	2.82s (± 0.90%)	2.79s (± 0.48%)	-0.02s (- 0.85%)	2.77s	2.82s
Total Time	9.81s (± 0.31%)	9.78s (± 0.45%)	-0.02s (- 0.25%)	9.68s	9.86s
TFS - node (v12.1.0, x64)
Memory used	286,688k (± 0.03%)	286,700k (± 0.03%)	+12k (+ 0.00%)	286,524k	287,016k
Parse Time	1.23s (± 0.76%)	1.23s (± 0.50%)	-0.00s (- 0.16%)	1.22s	1.24s
Bind Time	0.64s (± 1.09%)	0.65s (± 0.92%)	+0.01s (+ 0.78%)	0.63s	0.66s
Check Time	4.36s (± 0.59%)	4.38s (± 0.46%)	+0.02s (+ 0.53%)	4.34s	4.42s
Emit Time	2.92s (± 0.71%)	2.92s (± 0.77%)	+0.00s (+ 0.00%)	2.88s	2.99s
Total Time	9.15s (± 0.48%)	9.18s (± 0.42%)	+0.03s (+ 0.34%)	9.09s	9.27s
material-ui - node (v12.1.0, x64)
Memory used	439,707k (± 0.01%)	439,686k (± 0.01%)	-21k (- 0.00%)	439,579k	439,847k
Parse Time	1.98s (± 0.53%)	1.98s (± 0.33%)	+0.00s (+ 0.00%)	1.96s	1.99s
Bind Time	0.63s (± 0.91%)	0.63s (± 0.91%)	0.00s ( 0.00%)	0.62s	0.65s
Check Time	12.17s (± 1.26%)	11.95s (± 0.80%)	-0.22s (- 1.79%)	11.77s	12.24s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	14.77s (± 1.06%)	14.55s (± 0.67%)	-0.22s (- 1.49%)	14.37s	14.84s
Angular - node (v8.9.0, x64)
Memory used	341,102k (± 0.02%)	341,057k (± 0.02%)	-45k (- 0.01%)	340,920k	341,224k
Parse Time	2.54s (± 0.58%)	2.54s (± 0.55%)	-0.00s (- 0.12%)	2.51s	2.57s
Bind Time	0.85s (± 0.58%)	0.86s (± 0.85%)	+0.01s (+ 1.42%)	0.84s	0.87s
Check Time	5.40s (± 0.48%)	5.44s (± 0.43%)	+0.04s (+ 0.72%)	5.38s	5.50s
Emit Time	5.92s (± 0.49%)	6.01s (± 1.40%)	+0.08s (+ 1.42%)	5.88s	6.22s
Total Time	14.72s (± 0.30%)	14.85s (± 0.61%)	+0.13s (+ 0.88%)	14.71s	15.13s
Monaco - node (v8.9.0, x64)
Memory used	340,627k (± 0.01%)	340,643k (± 0.01%)	+16k (+ 0.00%)	340,533k	340,724k
Parse Time	1.87s (± 0.41%)	1.88s (± 0.44%)	+0.01s (+ 0.27%)	1.86s	1.89s
Bind Time	0.89s (± 0.56%)	0.88s (± 0.63%)	-0.00s (- 0.45%)	0.87s	0.90s
Check Time	5.48s (± 0.53%)	5.50s (± 0.49%)	+0.02s (+ 0.35%)	5.42s	5.55s
Emit Time	3.21s (± 0.58%)	3.23s (± 0.92%)	+0.01s (+ 0.40%)	3.18s	3.34s
Total Time	11.45s (± 0.32%)	11.49s (± 0.43%)	+0.04s (+ 0.32%)	11.39s	11.65s
TFS - node (v8.9.0, x64)
Memory used	303,971k (± 0.03%)	303,940k (± 0.02%)	-31k (- 0.01%)	303,816k	304,030k
Parse Time	1.55s (± 0.57%)	1.55s (± 0.38%)	-0.00s (- 0.26%)	1.53s	1.56s
Bind Time	0.68s (± 0.33%)	0.68s (± 0.88%)	-0.00s (- 0.44%)	0.66s	0.69s
Check Time	5.22s (± 0.68%)	5.22s (± 0.73%)	0.00s ( 0.00%)	5.15s	5.31s
Emit Time	2.92s (± 0.94%)	2.94s (± 0.79%)	+0.02s (+ 0.55%)	2.86s	2.98s
Total Time	10.37s (± 0.42%)	10.38s (± 0.51%)	+0.01s (+ 0.12%)	10.27s	10.49s
material-ui - node (v8.9.0, x64)
Memory used	465,698k (± 0.01%)	465,640k (± 0.01%)	-58k (- 0.01%)	465,498k	465,733k
Parse Time	2.38s (± 0.47%)	2.39s (± 0.64%)	+0.01s (+ 0.42%)	2.36s	2.42s
Bind Time	0.78s (± 1.65%)	0.78s (± 1.20%)	-0.00s (- 0.38%)	0.76s	0.80s
Check Time	17.87s (± 1.28%)	17.87s (± 0.81%)	+0.00s (+ 0.01%)	17.58s	18.14s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	21.03s (± 1.08%)	21.04s (± 0.65%)	+0.01s (+ 0.04%)	20.74s	21.32s
Angular - node (v8.9.0, x86)
Memory used	195,669k (± 0.02%)	195,711k (± 0.01%)	+42k (+ 0.02%)	195,653k	195,769k
Parse Time	2.46s (± 0.72%)	2.47s (± 0.60%)	+0.01s (+ 0.45%)	2.43s	2.49s
Bind Time	0.99s (± 0.60%)	0.99s (± 0.77%)	-0.00s (- 0.10%)	0.97s	1.00s
Check Time	4.90s (± 0.44%)	4.88s (± 0.75%)	-0.01s (- 0.27%)	4.81s	4.95s
Emit Time	5.91s (± 0.73%)	5.94s (± 0.70%)	+0.04s (+ 0.64%)	5.85s	6.07s
Total Time	14.24s (± 0.28%)	14.28s (± 0.43%)	+0.03s (+ 0.23%)	14.13s	14.42s
Monaco - node (v8.9.0, x86)
Memory used	193,655k (± 0.01%)	193,689k (± 0.01%)	+34k (+ 0.02%)	193,607k	193,742k
Parse Time	1.90s (± 0.53%)	1.91s (± 0.94%)	+0.00s (+ 0.05%)	1.88s	1.95s
Bind Time	0.70s (± 0.82%)	0.70s (± 0.67%)	-0.00s (- 0.00%)	0.69s	0.71s
Check Time	5.60s (± 1.21%)	5.61s (± 0.44%)	+0.01s (+ 0.21%)	5.56s	5.66s
Emit Time	2.71s (± 2.76%)	2.65s (± 0.51%)	-0.06s (- 2.28%)	2.62s	2.69s
Total Time	10.92s (± 0.47%)	10.87s (± 0.35%)	-0.05s (- 0.46%)	10.79s	10.97s
TFS - node (v8.9.0, x86)
Memory used	173,889k (± 0.04%)	173,877k (± 0.02%)	-13k (- 0.01%)	173,824k	173,935k
Parse Time	1.59s (± 1.11%)	1.58s (± 0.81%)	-0.00s (- 0.25%)	1.56s	1.62s
Bind Time	0.65s (± 1.72%)	0.64s (± 0.46%)	-0.00s (- 0.62%)	0.64s	0.65s
Check Time	4.73s (± 0.66%)	4.70s (± 0.37%)	-0.03s (- 0.57%)	4.67s	4.74s
Emit Time	2.80s (± 1.40%)	2.79s (± 1.10%)	-0.01s (- 0.29%)	2.71s	2.87s
Total Time	9.76s (± 0.56%)	9.72s (± 0.44%)	-0.05s (- 0.47%)	9.64s	9.80s
material-ui - node (v8.9.0, x86)
Memory used	263,677k (± 0.01%)	263,694k (± 0.01%)	+17k (+ 0.01%)	263,616k	263,765k
Parse Time	2.44s (± 0.74%)	2.44s (± 0.79%)	-0.00s (- 0.12%)	2.41s	2.50s
Bind Time	0.68s (± 1.92%)	0.68s (± 0.85%)	+0.00s (+ 0.15%)	0.67s	0.69s
Check Time	16.59s (± 0.56%)	16.29s (± 0.34%)	-0.30s (- 1.78%)	16.15s	16.41s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	19.71s (± 0.50%)	19.42s (± 0.30%)	-0.30s (- 1.50%)	19.26s	19.58s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-166-generic
Architecture	x64
Available Memory	16 GB
Available Memory	1 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

• node (v10.16.3, x64)
• node (v12.1.0, x64)
• node (v8.9.0, x64)
• node (v8.9.0, x86)

Scenarios

• Angular - node (v10.16.3, x64)
• Angular - node (v12.1.0, x64)
• Angular - node (v8.9.0, x64)
• Angular - node (v8.9.0, x86)
• Monaco - node (v10.16.3, x64)
• Monaco - node (v12.1.0, x64)
• Monaco - node (v8.9.0, x64)
• Monaco - node (v8.9.0, x86)
• TFS - node (v10.16.3, x64)
• TFS - node (v12.1.0, x64)
• TFS - node (v8.9.0, x64)
• TFS - node (v8.9.0, x86)
• material-ui - node (v10.16.3, x64)
• material-ui - node (v12.1.0, x64)
• material-ui - node (v8.9.0, x64)
• material-ui - node (v8.9.0, x86)

Benchmark	Name	Iterations
Current	40311	10
Baseline	master	10

[Daniel Rosenwasser (DanielRosenwasser)]

Early results:

---

We correctly issue excess property errors as expected. In some cases, we switch from missing property errors to excess property errors.

e9b467a#diff-0e97ff0d65c086767a3444a59e9b9e28

---

We issue one new implicit any error!

e9b467a#diff-5a40e91166140ce9c4dc158237e985f1

---

This seems to have reverted a regression introduced in #29478, and explained in #29478 (comment). I don't really know why this fixes things, but my guess is that we are a little bit more lax about finding the best common type in fresh object types:

e9b467a#diff-87ebcaa183574f2823b6ee03abb3316b

---

In TSServer itself, we seem to have caught a bug where we weren't correctly including a source property in DiagnosticWithLinePosition (or we were incorrectly providing it).

src/server/session.ts(1123,17): error TS2322: Type '{ message: string; start: number; length: number; category: string; code: number; source: string | undefined; startLocation: protocol.Location; endLocation: protocol.Location; reportsUnnecessary: {} | undefined; reportsDeprecated: {} | undefined; relatedInformation: protocol.DiagnosticRelatedInformation[] | undefine...' is not assignable to type 'DiagnosticWithLinePosition'.
Object literal may only specify known properties, and 'source' does not exist in type 'DiagnosticWithLinePosition'.

https://github.com/microsoft/TypeScript/runs/1043841312#step:7:137

https://typescript.visualstudio.com/TypeScript/_build/results?buildId=83662&view=logs&j=fd490c07-0b22-5182-fac9-6d67fe1e939b&t=cb364164-b022-581f-97fd-158a20e3d408&l=178

---

It's unclear whether there's actually a speedup in check-time on material-ui because it's so noisy, but I thought that might be worth mentioning.

[Daniel Rosenwasser (DanielRosenwasser)]

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

[TypeScript Bot (typescript-bot)]

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the tarball bundle task on this PR at 4f30b40. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the perf test suite on this PR at 4f30b40. You can monitor the build here.

Update: The results are in!

[TypeScript Bot (typescript-bot)]

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the extended test suite on this PR at 4f30b40. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the parallelized Definitely Typed test suite on this PR at 4f30b40. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Hey Daniel Rosenwasser (@DanielRosenwasser), 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/83695/artifacts?artifactName=tgz&fileId=0A3E072802E458145BBA5B36AA37AABE4FB300F7BBD3A55715AA9AA077F8D69F02&fileName=/typescript-4.1.0-insiders.20200829.tgz"
}
}

and then running npm install.

[Daniel Rosenwasser (DanielRosenwasser)]

One deficiency that I've noticed is that this doesn't work well in the presence of overloads.

interfaceObj{a: string;}declarefunctionfoo(obj: Obj): void;declarefunctionfoo(fn: ()=>Obj): void;foo(()=>({a: "hello",b: 123}))

Right now this doesn't error under this PR (which is bad). Remove the first overload, and you'll an error (which is good).

[TypeScript Bot (typescript-bot)]

Daniel Rosenwasser (@DanielRosenwasser)
The results of the perf run you requested are in!

Here they are:

Comparison Report - master..40311

Metric	master	40311	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	344,479k (± 0.02%)	344,619k (± 0.03%)	+140k (+ 0.04%)	344,444k	344,774k
Parse Time	2.00s (± 0.51%)	1.99s (± 0.45%)	-0.01s (- 0.35%)	1.97s	2.01s
Bind Time	0.82s (± 0.70%)	0.82s (± 1.52%)	+0.00s (+ 0.49%)	0.80s	0.87s
Check Time	4.76s (± 0.32%)	4.80s (± 0.70%)	+0.04s (+ 0.84%)	4.75s	4.90s
Emit Time	5.17s (± 0.67%)	5.17s (± 0.49%)	+0.00s (+ 0.04%)	5.12s	5.22s
Total Time	12.75s (± 0.39%)	12.78s (± 0.27%)	+0.04s (+ 0.28%)	12.73s	12.85s
Monaco - node (v10.16.3, x64)
Memory used	339,354k (± 0.02%)	339,394k (± 0.04%)	+39k (+ 0.01%)	339,215k	339,756k
Parse Time	1.54s (± 0.44%)	1.55s (± 0.48%)	+0.00s (+ 0.19%)	1.53s	1.56s
Bind Time	0.71s (± 0.48%)	0.71s (± 0.83%)	+0.00s (+ 0.14%)	0.70s	0.73s
Check Time	4.96s (± 0.73%)	4.96s (± 0.67%)	+0.00s (+ 0.00%)	4.88s	5.03s
Emit Time	2.73s (± 0.33%)	2.74s (± 0.53%)	+0.01s (+ 0.26%)	2.70s	2.78s
Total Time	9.95s (± 0.39%)	9.96s (± 0.34%)	+0.02s (+ 0.16%)	9.90s	10.05s
TFS - node (v10.16.3, x64)
Memory used	302,328k (± 0.03%)	302,356k (± 0.03%)	+29k (+ 0.01%)	302,043k	302,519k
Parse Time	1.20s (± 0.51%)	1.21s (± 0.56%)	+0.01s (+ 0.42%)	1.19s	1.22s
Bind Time	0.66s (± 1.74%)	0.67s (± 1.30%)	+0.01s (+ 0.75%)	0.65s	0.69s
Check Time	4.47s (± 0.58%)	4.50s (± 0.59%)	+0.03s (+ 0.69%)	4.44s	4.58s
Emit Time	2.90s (± 1.04%)	2.91s (± 1.21%)	+0.01s (+ 0.17%)	2.84s	2.96s
Total Time	9.24s (± 0.40%)	9.28s (± 0.68%)	+0.05s (+ 0.49%)	9.18s	9.42s
material-ui - node (v10.16.3, x64)
Memory used	461,311k (± 0.01%)	461,289k (± 0.01%)	-23k (- 0.00%)	461,163k	461,426k
Parse Time	1.95s (± 0.32%)	1.95s (± 0.34%)	+0.00s (+ 0.21%)	1.94s	1.97s
Bind Time	0.65s (± 1.35%)	0.66s (± 0.84%)	+0.01s (+ 1.38%)	0.65s	0.67s
Check Time	13.34s (± 0.37%)	13.40s (± 0.73%)	+0.06s (+ 0.43%)	13.27s	13.72s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.94s (± 0.34%)	16.01s (± 0.62%)	+0.07s (+ 0.43%)	15.88s	16.33s
Angular - node (v12.1.0, x64)
Memory used	321,776k (± 0.03%)	321,804k (± 0.03%)	+28k (+ 0.01%)	321,617k	321,963k
Parse Time	1.98s (± 0.69%)	1.99s (± 0.54%)	+0.00s (+ 0.25%)	1.97s	2.02s
Bind Time	0.81s (± 0.69%)	0.81s (± 0.80%)	-0.00s (- 0.00%)	0.80s	0.82s
Check Time	4.66s (± 0.48%)	4.68s (± 0.54%)	+0.03s (+ 0.56%)	4.63s	4.74s
Emit Time	5.31s (± 0.53%)	5.34s (± 0.67%)	+0.03s (+ 0.62%)	5.28s	5.45s
Total Time	12.75s (± 0.36%)	12.82s (± 0.50%)	+0.07s (+ 0.53%)	12.68s	13.00s
Monaco - node (v12.1.0, x64)
Memory used	321,640k (± 0.02%)	321,660k (± 0.02%)	+20k (+ 0.01%)	321,528k	321,823k
Parse Time	1.51s (± 0.54%)	1.53s (± 0.80%)	+0.02s (+ 1.19%)	1.50s	1.55s
Bind Time	0.69s (± 0.81%)	0.69s (± 0.32%)	+0.00s (+ 0.44%)	0.69s	0.70s
Check Time	4.74s (± 0.35%)	4.77s (± 0.68%)	+0.03s (+ 0.65%)	4.73s	4.85s
Emit Time	2.78s (± 0.40%)	2.81s (± 0.81%)	+0.02s (+ 0.79%)	2.77s	2.88s
Total Time	9.72s (± 0.23%)	9.80s (± 0.42%)	+0.08s (+ 0.77%)	9.72s	9.87s
TFS - node (v12.1.0, x64)
Memory used	286,651k (± 0.04%)	286,710k (± 0.02%)	+60k (+ 0.02%)	286,592k	286,812k
Parse Time	1.23s (± 0.76%)	1.23s (± 0.50%)	-0.00s (- 0.08%)	1.22s	1.25s
Bind Time	0.64s (± 0.62%)	0.64s (± 1.44%)	+0.00s (+ 0.16%)	0.62s	0.66s
Check Time	4.36s (± 0.41%)	4.35s (± 0.46%)	-0.01s (- 0.25%)	4.29s	4.38s
Emit Time	2.92s (± 0.60%)	2.91s (± 1.06%)	-0.01s (- 0.27%)	2.83s	2.98s
Total Time	9.14s (± 0.25%)	9.13s (± 0.40%)	-0.02s (- 0.19%)	9.05s	9.22s
material-ui - node (v12.1.0, x64)
Memory used	439,532k (± 0.07%)	439,563k (± 0.04%)	+31k (+ 0.01%)	438,786k	439,744k
Parse Time	1.98s (± 0.61%)	1.97s (± 0.51%)	-0.01s (- 0.25%)	1.94s	1.99s
Bind Time	0.63s (± 1.03%)	0.63s (± 0.75%)	+0.00s (+ 0.32%)	0.62s	0.64s
Check Time	11.96s (± 0.55%)	12.03s (± 0.76%)	+0.06s (+ 0.54%)	11.86s	12.25s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	14.57s (± 0.46%)	14.63s (± 0.68%)	+0.06s (+ 0.41%)	14.44s	14.86s
Angular - node (v8.9.0, x64)
Memory used	341,064k (± 0.02%)	341,125k (± 0.02%)	+60k (+ 0.02%)	341,006k	341,244k
Parse Time	2.54s (± 0.44%)	2.54s (± 0.39%)	-0.01s (- 0.28%)	2.52s	2.57s
Bind Time	0.85s (± 0.61%)	0.85s (± 0.55%)	+0.00s (+ 0.12%)	0.84s	0.86s
Check Time	5.41s (± 0.69%)	5.43s (± 0.46%)	+0.02s (+ 0.31%)	5.38s	5.50s
Emit Time	5.93s (± 2.10%)	5.93s (± 1.41%)	-0.01s (- 0.10%)	5.72s	6.11s
Total Time	14.74s (± 0.96%)	14.75s (± 0.68%)	+0.00s (+ 0.02%)	14.55s	15.00s
Monaco - node (v8.9.0, x64)
Memory used	340,670k (± 0.02%)	340,684k (± 0.01%)	+14k (+ 0.00%)	340,590k	340,754k
Parse Time	1.87s (± 0.28%)	1.88s (± 0.37%)	+0.01s (+ 0.43%)	1.86s	1.90s
Bind Time	0.89s (± 0.54%)	0.89s (± 0.42%)	-0.00s (- 0.23%)	0.88s	0.89s
Check Time	5.46s (± 0.68%)	5.51s (± 0.45%)	+0.05s (+ 0.88%)	5.47s	5.58s
Emit Time	3.21s (± 0.96%)	3.21s (± 0.57%)	-0.00s (- 0.09%)	3.18s	3.27s
Total Time	11.43s (± 0.48%)	11.48s (± 0.33%)	+0.05s (+ 0.42%)	11.40s	11.57s
TFS - node (v8.9.0, x64)
Memory used	303,961k (± 0.01%)	303,902k (± 0.02%)	-59k (- 0.02%)	303,820k	304,031k
Parse Time	1.55s (± 0.54%)	1.54s (± 0.31%)	-0.00s (- 0.13%)	1.53s	1.55s
Bind Time	0.67s (± 0.66%)	0.67s (± 1.04%)	-0.00s (- 0.15%)	0.66s	0.69s
Check Time	5.18s (± 0.56%)	5.20s (± 0.54%)	+0.02s (+ 0.39%)	5.13s	5.26s
Emit Time	2.92s (± 0.87%)	2.92s (± 0.87%)	+0.00s (+ 0.07%)	2.84s	2.96s
Total Time	10.32s (± 0.44%)	10.34s (± 0.52%)	+0.02s (+ 0.17%)	10.17s	10.45s
material-ui - node (v8.9.0, x64)
Memory used	465,671k (± 0.01%)	465,680k (± 0.01%)	+10k (+ 0.00%)	465,626k	465,776k
Parse Time	2.38s (± 0.40%)	2.40s (± 0.81%)	+0.02s (+ 0.84%)	2.36s	2.46s
Bind Time	0.78s (± 1.28%)	0.79s (± 0.82%)	+0.00s (+ 0.51%)	0.77s	0.80s
Check Time	18.00s (± 0.79%)	17.93s (± 1.10%)	-0.07s (- 0.39%)	17.52s	18.40s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	21.16s (± 0.69%)	21.12s (± 0.91%)	-0.05s (- 0.22%)	20.71s	21.57s
Angular - node (v8.9.0, x86)
Memory used	195,721k (± 0.02%)	195,677k (± 0.02%)	-44k (- 0.02%)	195,587k	195,755k
Parse Time	2.43s (± 0.30%)	2.47s (± 0.70%)	+0.03s (+ 1.40%)	2.42s	2.50s
Bind Time	0.99s (± 0.59%)	1.00s (± 0.60%)	+0.01s (+ 0.91%)	0.98s	1.01s
Check Time	4.88s (± 0.54%)	4.88s (± 0.53%)	+0.00s (+ 0.06%)	4.84s	4.94s
Emit Time	5.97s (± 1.80%)	5.96s (± 1.17%)	-0.01s (- 0.23%)	5.83s	6.17s
Total Time	14.27s (± 0.86%)	14.30s (± 0.50%)	+0.03s (+ 0.21%)	14.13s	14.47s
Monaco - node (v8.9.0, x86)
Memory used	193,666k (± 0.01%)	193,667k (± 0.03%)	+1k (+ 0.00%)	193,554k	193,776k
Parse Time	1.91s (± 1.24%)	1.91s (± 0.79%)	-0.00s (- 0.10%)	1.88s	1.95s
Bind Time	0.70s (± 0.88%)	0.70s (± 0.68%)	-0.00s (- 0.29%)	0.69s	0.71s
Check Time	5.59s (± 1.22%)	5.58s (± 0.56%)	-0.02s (- 0.32%)	5.50s	5.65s
Emit Time	2.69s (± 2.98%)	2.66s (± 1.06%)	-0.03s (- 1.04%)	2.62s	2.74s
Total Time	10.89s (± 0.46%)	10.84s (± 0.32%)	-0.06s (- 0.51%)	10.77s	10.92s
TFS - node (v8.9.0, x86)
Memory used	173,892k (± 0.03%)	173,876k (± 0.02%)	-17k (- 0.01%)	173,787k	173,980k
Parse Time	1.58s (± 0.65%)	1.58s (± 0.60%)	+0.00s (+ 0.19%)	1.56s	1.61s
Bind Time	0.65s (± 1.38%)	0.65s (± 1.63%)	+0.00s (+ 0.15%)	0.63s	0.68s
Check Time	4.68s (± 0.47%)	4.72s (± 0.61%)	+0.05s (+ 1.01%)	4.67s	4.80s
Emit Time	2.77s (± 0.99%)	2.80s (± 1.32%)	+0.03s (+ 1.26%)	2.75s	2.92s
Total Time	9.67s (± 0.31%)	9.76s (± 0.47%)	+0.09s (+ 0.90%)	9.67s	9.86s
material-ui - node (v8.9.0, x86)
Memory used	263,649k (± 0.02%)	263,681k (± 0.02%)	+32k (+ 0.01%)	263,596k	263,751k
Parse Time	2.43s (± 0.41%)	2.45s (± 0.56%)	+0.02s (+ 0.82%)	2.42s	2.48s
Bind Time	0.67s (± 1.38%)	0.68s (± 1.77%)	+0.01s (+ 0.74%)	0.66s	0.71s
Check Time	16.47s (± 0.68%)	16.44s (± 0.91%)	-0.03s (- 0.19%)	16.25s	16.80s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	19.58s (± 0.58%)	19.57s (± 0.77%)	-0.02s (- 0.08%)	19.34s	19.95s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-166-generic
Architecture	x64
Available Memory	16 GB
Available Memory	1 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

• node (v10.16.3, x64)
• node (v12.1.0, x64)
• node (v8.9.0, x64)
• node (v8.9.0, x86)

Scenarios

• Angular - node (v10.16.3, x64)
• Angular - node (v12.1.0, x64)
• Angular - node (v8.9.0, x64)
• Angular - node (v8.9.0, x86)
• Monaco - node (v10.16.3, x64)
• Monaco - node (v12.1.0, x64)
• Monaco - node (v8.9.0, x64)
• Monaco - node (v8.9.0, x86)
• TFS - node (v10.16.3, x64)
• TFS - node (v12.1.0, x64)
• TFS - node (v8.9.0, x64)
• TFS - node (v8.9.0, x86)
• material-ui - node (v10.16.3, x64)
• material-ui - node (v12.1.0, x64)
• material-ui - node (v8.9.0, x64)
• material-ui - node (v8.9.0, x86)

Benchmark	Name	Iterations
Current	40311	10
Baseline	master	10

[Daniel Rosenwasser (DanielRosenwasser)]

TypeScript Bot (@typescript-bot) user test this

[TypeScript Bot (typescript-bot)]

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the parallelized community code test suite on this PR at 4f30b40. You can monitor the build here.

[Daniel Rosenwasser (DanielRosenwasser)]

User test failures:

https://github.com/microsoft/fluentui/blob/bcf5ac28bfb4809a9097a8bec9bb26cba8d04ef6/packages/charting/src/components/VerticalStackedBarChart/examples/VerticalStackedBarChart.Styled.Example.tsx#L47-L49

src/components/VerticalStackedBarChart/examples/VerticalStackedBarChart.Styled.Example.tsx:47:9 - error TS2322: Type '() => { chart: { paddingBottom: string; }; chartLabel: { displayName?: string | undefined; selectors?: { [key: string]: IStyle; } | undefined; MsHighContrastAdjust?: string | undefined; MsOverflowStyle?: string | undefined; ... 277 more ...; fontWeight?: string | ... 9 more ... | undefined; }; xAxisText: { ...; }; }' is not assignable to type 'IStyleFunction<IVerticalStackedBarChartStyleProps, IVerticalStackedBarChartStyles> | DeepPartial<...> | undefined'.
@uifabric/charting: Type '() => { chart: { paddingBottom: string; }; chartLabel: { displayName?: string | undefined; selectors?: { [key: string]: IStyle; } | undefined; MsHighContrastAdjust?: string | undefined; MsOverflowStyle?: string | undefined; ... 277 more ...; fontWeight?: string | ... 9 more ... | undefined; }; xAxisText: { ...; }; }' is not assignable to type 'IStyleFunction<IVerticalStackedBarChartStyleProps, IVerticalStackedBarChartStyles>'.
@uifabric/charting: Type '{ chart: { paddingBottom: string; }; chartLabel: { displayName?: string | undefined; selectors?: { [key: string]: IStyle; } | undefined; MsHighContrastAdjust?: string | undefined; MsOverflowStyle?: string | undefined; ... 277 more ...; fontWeight?: string | ... 9 more ... | undefined; }; xAxisText: { ...; }; }' is not assignable to type 'DeepPartial<IVerticalStackedBarChartStyles>'.
@uifabric/charting: Object literal may only specify known properties, and 'chart' does not exist in type 'DeepPartial<IVerticalStackedBarChartStyles>'.

https://github.com/microsoft/fluentui/blob/bcf5ac28bfb4809a9097a8bec9bb26cba8d04ef6/packages/react-slider/src/next/Slider.tsx#L56

src/next/Slider.tsx:56:7 - error TS2322: Type '{ root: string; container: string; slideBox: string; line: string; thumb: string; activeSection: string; inactiveSection: string; lineContainer: string; valueLabel: string; titleLabel: string; showTransitions: string; zeroTick: string; }' is not assignable to type 'Required<ISliderStyles>'.
@fluentui/react-slider: Object literal may only specify known properties, and 'showTransitions' does not exist in type 'Required<ISliderStyles>'.
@fluentui/react-slider: 56 showTransitions: css(globalClassNames.showTransitions, ...propClasses),

https://github.com/microsoft/fluentui/blob/bcf5ac28bfb4809a9097a8bec9bb26cba8d04ef6/packages/date-time/src/components/Calendar/examples/Calendar.Inline.DateBoundaries.Example.tsx#L73

@uifabric/date-time: src/components/Calendar/examples/Calendar.Inline.DateBoundaries.Example.tsx:73:9 - error TS2345: Argument of type '(prevState: ICalendarInlineExampleState) => { selectedDate: Date; selectedDateRange: Date[]; }' is not assignable to parameter of type 'ICalendarInlineExampleState | ((prevState: Readonly<ICalendarInlineExampleState>, props: Readonly<{}>) => ICalendarInlineExampleState | ... 1 more ... | null) | Pick<...> | null'.
@uifabric/date-time: Type '(prevState: ICalendarInlineExampleState) => { selectedDate: Date; selectedDateRange: Date[]; }' is not assignable to type '(prevState: Readonly<ICalendarInlineExampleState>, props: Readonly<{}>) => ICalendarInlineExampleState | Pick<...> | null'.
@uifabric/date-time: Type '{ selectedDate: Date; selectedDateRange: Date[]; }' is not assignable to type 'ICalendarInlineExampleState | Pick<ICalendarInlineExampleState, "selectedDate"> | null'.
@uifabric/date-time: Object literal may only specify known properties, and 'selectedDateRange' does not exist in type 'ICalendarInlineExampleState | Pick<ICalendarInlineExampleState, "selectedDate">'.
@uifabric/date-time: 73 selectedDateRange: dateRangeArray,
@uifabric/date-time: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

David Zearing (@dzearing)Jason Gore (@JasonGore) we might break some of your code in 4.1, but I think these are mostly bugs. On the upside, this fixes #27237. 😄

[Daniel Rosenwasser (DanielRosenwasser)]

TypeScript Bot (@typescript-bot) pack this

[TypeScript Bot (typescript-bot)]

Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the tarball bundle task on this PR at 4f30b40. You can monitor the build here.

[TypeScript Bot (typescript-bot)]

Hey Daniel Rosenwasser (@DanielRosenwasser), 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/84231/artifacts?artifactName=tgz&fileId=C84DB49EF41CE2A07C073835B44DCE033C5DD3BE0AB4AEBC5542FBABAD1F9E9702&fileName=/typescript-4.1.0-insiders.20200904.tgz"
}
}

and then running npm install.

---

There is also a playground for this build.

[Nathan Shively-Sanders (sandersn)]

is this change unrelated?

[Daniel Rosenwasser (DanielRosenwasser)]

Yes, I think Sheetal Nandi (@sheetalkamat) might have a better idea, but this un-breaks an excess property error we had in the server layer.

[Sheetal Nandi (sheetalkamat)]

#15296 added this so diagnostics origin can be distinguished.

[Nathan Shively-Sanders (sandersn)]

I'm still a bit wary that this is ad-hoc and will cause either confusion in the form of subtle by-design bugs.

But it does what what we agreed on at the design meeting.

[Wesley Wigham (weswigham)]

Yeah, as stated in the meeting I do think this is not a reasonable new error in a quite old test (and was dealt with in the prior iteration of this change), and this can be somewhat easily avoided by using the old behavior for named function expressions (which are generally only used when you want recursive references like this anyhow). A self-call witnessing the unwidened type is very odd, given our widening rules (no exterior calls can witness the unwidened type). I think this can manifest in even stranger behavior if you do something with the bar reference rather than just call it - like pass it into a function desiring a callback, eg handle(bar) where declare function handle(a: () => [any]): void, or worse yet declare function handle<T>(a: T): T where the unwidened type will simply propagate via inference - where then you'll get this widening error so far removed from anything that it's relevant to that it's quite clear something's gone wrong.

[Lauchlan Irwin (lauchlan105)]

Is merging this still a possibility or will this PR ultimately be abandoned?

[TypeScript Bot (typescript-bot)]

The TypeScript team hasn't accepted the linked issue #241. If you can get it accepted, this PR will have a better chance of being reviewed.

[Nathan Shively-Sanders (sandersn)]

Daniel Rosenwasser (@DanielRosenwasser) is this change still worth taking? If so, can you address Wesley Wigham (@weswigham)'s idea for named functions?

[Lubos (mrlubos)]

Hey everyone, is this pull request still on track to land any time soon?

[Nathan Shively-Sanders (sandersn)]

This PR hasn't seen any activity for quite a while, so I'm going to close it to keep the number of open PRs manageable.

[James Bromwell (thw0rted)]

Daniel Rosenwasser (@DanielRosenwasser)Nathan Shively-Sanders (@sandersn) I just skimmed the history here and it looks like this implements fixes for a large number of issues, including one that's over 7 years old (filed by Ryan Cavanaugh (@RyanCavanaugh) , with 58 upvotes). As far as I can tell the only thing that prevented merging was #40311 (comment) , but there was no further discussion about whether this would be easy to fix.

Is my assessment accurate? Is this likely to get picked back up at some later point by the current triage process?

[Nathan Shively-Sanders (sandersn)]

Looking at the history, it seems like Daniel Rosenwasser (@DanielRosenwasser) and Wesley Wigham (@weswigham) disagreed about the outcome of a design meeting. My guess is that we should bring it up there again. It's up to Daniel Rosenwasser (@DanielRosenwasser) though, since he wrote the code and puts together the agenda.

[Victor Hallberg (mogelbrod)]

Linking to a relevant StackOverflow question that would've been solved by this PR - I've yet to find any alternative solutions to this. Related test in the typescript playground.

[J Garcia (JGJP)]

Daniel Rosenwasser (@DanielRosenwasser)Nathan Shively-Sanders (@sandersn) it is now a year later, was this brought up for discussion internally? What will it take to get this resolved?

[Yam Borodetsky (yamcodes)]

We've encountered this issue many times before and would love this fix to be included in the next TypeScript release.