Simplify relationship check for conditional type on target side by ahejlsberg · Pull Request #46429 · microsoft/TypeScript · GitHub

This PR simplifies our relationship checking logic for conditional types on the target side. With this PR, a source type S is assignable to a target type A extends B ? C : D when

The conditional type is not distributive, or is distributive but A is never referenced in C or D, and
B has no infer positions, and
S is assignable to C (except if A never assignable to B for any instantiation), and
S is assignable to D (except if A is always assignable to B for any instantiation).

The PR revises some of the logic that was added in #30639, specifically the logic that determines if A is referenced in C or D in a distributive conditional type. There are no baseline changes as a result of this PR, but it fixes the performance degradation reported in #44851.

Fixes #44851.

I should mention that with this PR, the check time for this repro drops from ~10s to about 0.4s.

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TypeScript Bot (@typescript-bot) all the things please

Heya Andrew Branch (@andrewbranch), I've started to run the inline community code test suite on this PR at 75ab347. You can monitor the build here.

Update: The results are in!

Heya Andrew Branch (@andrewbranch), I've started to run the extended test suite on this PR at 75ab347. You can monitor the build here.

Heya Andrew Branch (@andrewbranch), I've started to run the parallelized Definitely Typed test suite on this PR at 75ab347. You can monitor the build here.

Heya Andrew Branch (@andrewbranch), I've started to run the tarball bundle task on this PR at 75ab347. You can monitor the build here.

Heya Andrew Branch (@andrewbranch), I've started to run the perf test suite on this PR at 75ab347. You can monitor the build here.

Update: The results are in!

Hurray, I helped find something maybe useful!

Andrew Branch (@andrewbranch)
Great news! no new errors were found between main..refs/pull/46429/merge

This one is interesting - is this actually a distributive conditional type? Why is it deferred?

We always defer if either of the check and extends types contains generics. This is somewhat of a blunt instrument, and we could possibly do better in cases like this.

And by "contains generics" I mean we defer when isGenericType returns true for either of the check and extends types.

If we're going to attempt to fix those, the fix really should be to have the conditional type construction logic decide to resolve the types instead of deferring them--but I think that has lower priority.

This is impossible in the general case - any condition with an infer has to be deferred so the infer can be instantiated with every possible input (since an infer result can't stand on its own) - if we don't reason about branch reachability in relationship checking, we can never handle conditional types that both filter and transform simultaneously like T extends () => infer I ? I : never. We knew that adding constraint checks on branch reachability would be expensive - but it's the typespace equivalent of doing control flow on genetics, which we also do wherever we can now.

Andrew Branch (@andrewbranch)
The results of the perf run you requested are in!

Here they are:

Comparison Report - main..46429

Metric	main	46429	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	354,063k (± 0.01%)	354,012k (± 0.02%)	-51k (- 0.01%)	353,868k	354,166k
Parse Time	1.96s (± 0.58%)	1.95s (± 0.50%)	-0.01s (- 0.46%)	1.93s	1.97s
Bind Time	0.84s (± 0.66%)	0.84s (± 0.81%)	+0.00s (+ 0.36%)	0.83s	0.86s
Check Time	5.47s (± 0.36%)	5.48s (± 0.81%)	+0.01s (+ 0.22%)	5.42s	5.61s
Emit Time	5.86s (± 0.67%)	5.88s (± 0.70%)	+0.01s (+ 0.20%)	5.78s	5.98s
Total Time	14.13s (± 0.34%)	14.15s (± 0.46%)	+0.02s (+ 0.12%)	14.04s	14.31s
Compiler-Unions - node (v10.16.3, x64)
Memory used	203,871k (± 0.03%)	203,893k (± 0.03%)	+22k (+ 0.01%)	203,721k	203,980k
Parse Time	0.78s (± 0.67%)	0.78s (± 0.63%)	+0.00s (+ 0.38%)	0.78s	0.80s
Bind Time	0.53s (± 1.13%)	0.52s (± 1.28%)	-0.01s (- 1.14%)	0.50s	0.53s
Check Time	7.95s (± 0.67%)	7.90s (± 0.45%)	-0.05s (- 0.62%)	7.83s	7.97s
Emit Time	2.45s (± 0.80%)	2.44s (± 0.65%)	-0.01s (- 0.45%)	2.40s	2.48s
Total Time	11.71s (± 0.46%)	11.65s (± 0.29%)	-0.06s (- 0.51%)	11.56s	11.72s
Monaco - node (v10.16.3, x64)
Memory used	342,091k (± 0.01%)	342,060k (± 0.03%)	-32k (- 0.01%)	341,822k	342,213k
Parse Time	1.48s (± 0.78%)	1.49s (± 0.46%)	+0.00s (+ 0.13%)	1.47s	1.50s
Bind Time	0.75s (± 0.82%)	0.75s (± 0.67%)	-0.00s (- 0.53%)	0.74s	0.76s
Check Time	5.46s (± 0.55%)	5.42s (± 0.83%)	-0.03s (- 0.59%)	5.33s	5.51s
Emit Time	3.17s (± 0.70%)	3.17s (± 0.83%)	+0.00s (+ 0.00%)	3.12s	3.25s
Total Time	10.86s (± 0.31%)	10.82s (± 0.31%)	-0.04s (- 0.33%)	10.74s	10.88s
TFS - node (v10.16.3, x64)
Memory used	304,751k (± 0.01%)	304,803k (± 0.02%)	+52k (+ 0.02%)	304,682k	304,886k
Parse Time	1.20s (± 0.68%)	1.19s (± 0.25%)	-0.01s (- 0.42%)	1.19s	1.20s
Bind Time	0.71s (± 0.70%)	0.71s (± 0.56%)	+0.00s (+ 0.71%)	0.70s	0.72s
Check Time	4.97s (± 0.51%)	4.95s (± 0.67%)	-0.02s (- 0.38%)	4.88s	5.03s
Emit Time	3.30s (± 1.07%)	3.32s (± 1.02%)	+0.02s (+ 0.45%)	3.26s	3.40s
Total Time	10.17s (± 0.45%)	10.17s (± 0.44%)	-0.01s (- 0.06%)	10.10s	10.28s
material-ui - node (v10.16.3, x64)
Memory used	471,094k (± 0.02%)	469,960k (± 0.01%)	-1,134k (- 0.24%)	469,861k	470,042k
Parse Time	1.77s (± 0.56%)	1.77s (± 0.42%)	-0.00s (- 0.17%)	1.76s	1.79s
Bind Time	0.66s (± 1.23%)	0.66s (± 1.22%)	-0.00s (- 0.30%)	0.64s	0.68s
Check Time	14.38s (± 0.41%)	14.34s (± 0.56%)	-0.04s (- 0.31%)	14.18s	14.58s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	16.82s (± 0.39%)	16.77s (± 0.52%)	-0.05s (- 0.32%)	16.59s	17.02s
Angular - node (v12.1.0, x64)
Memory used	331,719k (± 0.08%)	331,897k (± 0.02%)	+178k (+ 0.05%)	331,749k	332,046k
Parse Time	1.96s (± 0.67%)	1.94s (± 0.51%)	-0.02s (- 1.02%)	1.92s	1.97s
Bind Time	0.82s (± 1.34%)	0.82s (± 1.15%)	0.00s ( 0.00%)	0.80s	0.84s
Check Time	5.33s (± 0.81%)	5.31s (± 0.38%)	-0.02s (- 0.41%)	5.27s	5.36s
Emit Time	6.15s (± 0.82%)	6.09s (± 0.44%)	-0.06s (- 0.99%)	6.05s	6.18s
Total Time	14.26s (± 0.59%)	14.16s (± 0.28%)	-0.10s (- 0.72%)	14.08s	14.26s
Compiler-Unions - node (v12.1.0, x64)
Memory used	191,425k (± 0.04%)	191,331k (± 0.07%)	-94k (- 0.05%)	190,906k	191,583k
Parse Time	0.78s (± 0.67%)	0.78s (± 0.93%)	+0.00s (+ 0.13%)	0.77s	0.80s
Bind Time	0.53s (± 0.89%)	0.53s (± 0.71%)	-0.01s (- 0.94%)	0.52s	0.53s
Check Time	7.40s (± 0.55%)	7.41s (± 0.71%)	+0.00s (+ 0.07%)	7.32s	7.56s
Emit Time	2.46s (± 0.85%)	2.46s (± 1.46%)	+0.00s (+ 0.12%)	2.39s	2.55s
Total Time	11.17s (± 0.48%)	11.17s (± 0.72%)	+0.00s (+ 0.01%)	11.02s	11.36s
Monaco - node (v12.1.0, x64)
Memory used	325,197k (± 0.02%)	325,207k (± 0.02%)	+11k (+ 0.00%)	325,051k	325,336k
Parse Time	1.48s (± 0.91%)	1.46s (± 0.55%)	-0.02s (- 1.15%)	1.45s	1.49s
Bind Time	0.73s (± 0.85%)	0.73s (± 1.23%)	-0.00s (- 0.41%)	0.72s	0.76s
Check Time	5.32s (± 0.81%)	5.33s (± 0.38%)	+0.01s (+ 0.23%)	5.28s	5.39s
Emit Time	3.20s (± 0.72%)	3.18s (± 0.71%)	-0.02s (- 0.69%)	3.14s	3.23s
Total Time	10.73s (± 0.57%)	10.70s (± 0.34%)	-0.03s (- 0.25%)	10.63s	10.78s
TFS - node (v12.1.0, x64)
Memory used	289,506k (± 0.02%)	289,475k (± 0.02%)	-31k (- 0.01%)	289,342k	289,574k
Parse Time	1.22s (± 0.82%)	1.21s (± 0.87%)	-0.01s (- 0.90%)	1.20s	1.24s
Bind Time	0.70s (± 0.86%)	0.69s (± 1.06%)	-0.01s (- 1.01%)	0.68s	0.71s
Check Time	4.92s (± 0.26%)	4.90s (± 0.56%)	-0.02s (- 0.39%)	4.84s	4.96s
Emit Time	3.39s (± 0.99%)	3.37s (± 0.65%)	-0.02s (- 0.53%)	3.33s	3.42s
Total Time	10.23s (± 0.41%)	10.17s (± 0.47%)	-0.06s (- 0.57%)	10.08s	10.30s
material-ui - node (v12.1.0, x64)
Memory used	449,723k (± 0.06%)	448,617k (± 0.06%)	-1,106k (- 0.25%)	447,464k	448,866k
Parse Time	1.79s (± 0.91%)	1.78s (± 0.72%)	-0.01s (- 0.45%)	1.76s	1.82s
Bind Time	0.65s (± 1.03%)	0.64s (± 0.76%)	-0.00s (- 0.46%)	0.63s	0.65s
Check Time	13.01s (± 0.85%)	12.97s (± 0.73%)	-0.04s (- 0.28%)	12.83s	13.15s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.45s (± 0.73%)	15.40s (± 0.64%)	-0.05s (- 0.30%)	15.23s	15.58s
Angular - node (v14.15.1, x64)
Memory used	330,294k (± 0.01%)	330,303k (± 0.01%)	+9k (+ 0.00%)	330,268k	330,340k
Parse Time	1.96s (± 0.48%)	1.94s (± 0.48%)	-0.02s (- 1.27%)	1.91s	1.95s
Bind Time	0.87s (± 0.54%)	0.86s (± 0.39%)	-0.01s (- 0.80%)	0.86s	0.87s
Check Time	5.37s (± 0.44%)	5.37s (± 0.37%)	-0.00s (- 0.07%)	5.32s	5.40s
Emit Time	6.19s (± 0.98%)	6.19s (± 0.63%)	-0.01s (- 0.10%)	6.08s	6.26s
Total Time	14.39s (± 0.54%)	14.35s (± 0.37%)	-0.04s (- 0.29%)	14.18s	14.43s
Compiler-Unions - node (v14.15.1, x64)
Memory used	192,554k (± 0.49%)	192,879k (± 0.37%)	+325k (+ 0.17%)	189,971k	193,279k
Parse Time	0.81s (± 0.72%)	0.81s (± 1.17%)	+0.01s (+ 0.87%)	0.80s	0.84s
Bind Time	0.56s (± 0.53%)	0.55s (± 0.62%)	-0.01s (- 0.90%)	0.55s	0.56s
Check Time	7.60s (± 0.38%)	7.56s (± 0.47%)	-0.05s (- 0.63%)	7.48s	7.65s
Emit Time	2.45s (± 0.92%)	2.44s (± 0.82%)	-0.01s (- 0.57%)	2.41s	2.50s
Total Time	11.42s (± 0.33%)	11.36s (± 0.35%)	-0.06s (- 0.49%)	11.29s	11.46s
Monaco - node (v14.15.1, x64)
Memory used	323,999k (± 0.01%)	323,990k (± 0.01%)	-9k (- 0.00%)	323,951k	324,030k
Parse Time	1.51s (± 0.88%)	1.51s (± 0.72%)	-0.01s (- 0.46%)	1.49s	1.53s
Bind Time	0.76s (± 0.48%)	0.76s (± 0.48%)	0.00s ( 0.00%)	0.75s	0.76s
Check Time	5.33s (± 0.50%)	5.30s (± 0.42%)	-0.03s (- 0.53%)	5.27s	5.38s
Emit Time	3.23s (± 0.47%)	3.23s (± 0.68%)	-0.00s (- 0.03%)	3.18s	3.27s
Total Time	10.83s (± 0.36%)	10.79s (± 0.24%)	-0.04s (- 0.37%)	10.73s	10.87s
TFS - node (v14.15.1, x64)
Memory used	288,354k (± 0.01%)	288,339k (± 0.01%)	-15k (- 0.01%)	288,308k	288,377k
Parse Time	1.24s (± 0.63%)	1.23s (± 0.53%)	-0.01s (- 0.89%)	1.21s	1.24s
Bind Time	0.74s (± 0.46%)	0.73s (± 0.71%)	-0.01s (- 1.09%)	0.72s	0.74s
Check Time	4.94s (± 0.31%)	4.91s (± 0.48%)	-0.03s (- 0.57%)	4.87s	4.97s
Emit Time	3.49s (± 0.35%)	3.49s (± 0.82%)	+0.01s (+ 0.17%)	3.43s	3.55s
Total Time	10.40s (± 0.24%)	10.36s (± 0.31%)	-0.04s (- 0.38%)	10.28s	10.45s
material-ui - node (v14.15.1, x64)
Memory used	447,964k (± 0.06%)	446,820k (± 0.08%)	-1,143k (- 0.26%)	445,818k	447,128k
Parse Time	1.83s (± 0.42%)	1.82s (± 0.49%)	-0.01s (- 0.33%)	1.80s	1.85s
Bind Time	0.68s (± 0.70%)	0.68s (± 0.44%)	+0.00s (+ 0.15%)	0.67s	0.68s
Check Time	13.19s (± 0.94%)	13.10s (± 0.46%)	-0.09s (- 0.68%)	12.97s	13.25s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.70s (± 0.82%)	15.61s (± 0.38%)	-0.09s (- 0.59%)	15.48s	15.75s

System

Machine Name	ts-ci-ubuntu
Platform	linux 4.4.0-210-generic
Architecture	x64
Available Memory	16 GB
Available Memory	6 GB
CPUs	4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz

Hosts

node (v10.16.3, x64)
node (v12.1.0, x64)
node (v14.15.1, x64)

Scenarios

Angular - node (v10.16.3, x64)
Angular - node (v12.1.0, x64)
Angular - node (v14.15.1, x64)
Compiler-Unions - node (v10.16.3, x64)
Compiler-Unions - node (v12.1.0, x64)
Compiler-Unions - node (v14.15.1, x64)
Monaco - node (v10.16.3, x64)
Monaco - node (v12.1.0, x64)
Monaco - node (v14.15.1, x64)
TFS - node (v10.16.3, x64)
TFS - node (v12.1.0, x64)
TFS - node (v14.15.1, x64)
material-ui - node (v10.16.3, x64)
material-ui - node (v12.1.0, x64)
material-ui - node (v14.15.1, x64)

Benchmark	Name	Iterations
Current	46429	10
Baseline	main	10

Developer Information:

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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at 75ab347. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized community code test suite on this PR at 75ab347. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the tarball bundle task on this PR at 75ab347. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 75ab347. You can monitor the build here.

This is impossible in the general case

Agreed, and so be it. But it does seem like there are classes of cases where we ought to do better in conditional type construction. Both of the test cases that revert back to failing with this PR seem quite trivial--for example, checking if the check and extends types are identical, or if the extends type is a union that contains the check type, would cause us to resolve the conditional type to the true case.

Hey Anders Hejlsberg (@ahejlsberg), 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/113306/artifacts?artifactName=tgz&fileId=A263304AC84860808E706B7F5590B0AA3616661B50F7CE35CED050B21FF1B0C802&fileName=/typescript-4.5.0-insiders.20211019.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@4.5.0-pr-46429-17".;

I think I can say with a high degree of certainty that whatever is going on in Andrew Casey (@amcasey)'s example is unrelated. I removed all of the logic having to do with assignability to conditional types (i.e. both #30639 and this PR), and the example still blows up with an instantiation depth error.

Please, no more plot twists, my heart can't handle it anymore.

Yeah, if the real world code is fast then I’m happy. Just wanted to make sure it was checked against the latest changes.

is there an example of a type that is no longer allowed as of this PR? the typescript 4.5 blog post mentions this PR

TypeScript no longer allows types to be assignable to conditional types that use infer, or that are distributive. Doing so previously often ended up causing major performance issues. For more information, see the specific change on GitHub.

but i can't seem to find a minimal example of what this means exactly

DetachHead good news (??), we found one: #47127 (comment)

DetachHead good news (??), we found one: #47127 (comment)

This is a BAD news for me. I deeply dependency this feature. Because of this breaking change, I cant update TypeScript, it's so bad for me, could you add a compiler option to support it as a migrate plan?

…osoft#46429) * Simplify relationship check for conditional type on target side * Accept new baselines * Better support for non-distribution-dependent types * Accept new API baselines * Accept new baselines

Anders Hejlsberg (ahejlsberg) added 2 commits October 19, 2021 09:21

Simplify relationship check for conditional type on target side

9ea685b

Accept new baselines

75ab347

TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Oct 19, 2021

TypeScript Bot (typescript-bot) added the Author: Team label Oct 19, 2021

TypeScript Bot (typescript-bot) assigned Andrew Branch (andrewbranch) Oct 19, 2021

TypeScript Bot (typescript-bot) added the For Milestone Bug PRs that fix a bug with a specific milestone label Oct 19, 2021

Anders Hejlsberg (ahejlsberg) requested review from Daniel Rosenwasser (DanielRosenwasser), Andrew Branch (andrewbranch), Armando Aguirre (armanio123) and Wesley Wigham (weswigham) October 19, 2021 16:48

Daniel Rosenwasser (DanielRosenwasser) reviewed Oct 19, 2021

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Daniel Rosenwasser (DanielRosenwasser) approved these changes Oct 19, 2021

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Andrew Branch (andrewbranch) approved these changes Oct 21, 2021

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Anders Hejlsberg (ahejlsberg) merged commit f2f35ae into main Oct 22, 2021

Anders Hejlsberg (ahejlsberg) deleted the fix44851 branch October 22, 2021 00:35

Anders Hejlsberg (ahejlsberg) mentioned this pull request Oct 22, 2021

Defer computation of isDistributionDependent to avoid circularities #46474

Merged

Nathan Shively-Sanders (sandersn) mentioned this pull request Oct 22, 2021

[NewErrors] 4.5.0-dev.20211017 vs 4.4.4 #46404

Closed

Max Ruman (mmvsk) mentioned this pull request Oct 24, 2021

TS 4.5 type instantiation is excessively deep and possibly infinite, with seemingly legal type #46500

Closed

JSin mentioned this pull request Nov 8, 2021

Slow IntelliSense with react-hook-form resolvers on commands completionInfo, completionEntryDetails, and, encodedSemanticClassifications-full #46726

Closed

sinclairzx81 mentioned this pull request Nov 18, 2021

TS 4.5: Type instantiation is excessively deep and possibly infinite. sinclairzx81/typebox#120

Closed

Andrei Alecu (andreialecu) mentioned this pull request Nov 18, 2021

Mongoose slows down performance of VSCode (TSServer) by a lot Automattic/mongoose#10349

Closed

Ryan Cavanaugh (RyanCavanaugh) mentioned this pull request Dec 13, 2021

conditional generic Type assignable Error #47127

Closed

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Jan 20, 2022

Experiment short-circuiting of union distribution #47511

Closed

5 tasks

Mateusz Burzyński (Andarist) mentioned this pull request Mar 12, 2022

Explicit type annotation required in TS 4.5 #47384

Open

This was referenced Apr 18, 2022

Generic Extended Types not inferring Correctly #48741

Closed

Type 'T' is not assignable to type 'T extends Sub ? Sub : T'. #48746

Closed

whzx5byb mentioned this pull request Sep 30, 2022

InstanceType<(typeof this)["somefield"]> type issue #51013

Closed

wbt mentioned this pull request Nov 11, 2022

Can't assign type though it is on both branches of conditional type #51488

Closed

Benjamin Bray (benrbray) mentioned this pull request Feb 28, 2023

[chore] address breaking changes to conditional types in typescript@4.5 benrbray/noteworthy#30

Open

Mateusz Burzyński (Andarist) mentioned this pull request Apr 26, 2023

Add extra tests for recursive conditional types #54030

Merged

Kael (KaelWD) mentioned this pull request May 16, 2023

[Bug Report][3.2.4] Cryptic TS error when passing generic values to v-autocomplete/v-select vuetifyjs/vuetify#17383

Closed

Ryan Cavanaugh (RyanCavanaugh) mentioned this pull request Jan 23, 2024

Function fields in class with conditional optionality of parameters #57139

Closed

Joe Calzaretta (jcalz) mentioned this pull request Aug 29, 2024

Add missing relationship allowing a type to be assignable to a conditional when assignable to both branches #30639

Merged

Joe Calzaretta (jcalz) mentioned this pull request Sep 26, 2024

Type 'null' is not assignable to type 'T extends T ? null : Omit<T, keyof T>' #60076

Closed

Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 21, 2025

                     var x: T | null = Math.random() > 0.5 ? null : t;
                     onlyNullablePlease(x); // should work
+                                       ~
+              !!! error TS2345: Argument of type 'T | null' is not assignable to parameter of type 'null extends T | null ? any : never'.

Simplify relationship check for conditional type on target side by ahejlsberg · Pull Request #46429 · microsoft/TypeScript · GitHub

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Simplify relationship check for conditional type on target side - #46429

Simplify relationship check for conditional type on target side#46429Anders Hejlsberg (ahejlsberg) merged 5 commits intomainmicrosoft/TypeScript:mainfrom fix44851microsoft/TypeScript:fix44851Copy head branch name to clipboard

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Reviewers

Simplify relationship check for conditional type on target side#46429
Anders Hejlsberg (ahejlsberg) merged 5 commits into
mainfrom
fix44851

Anders Hejlsberg (ahejlsberg) commented Oct 19, 2021 •
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