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GeezerVison is showing github.com — Improve recursion depth checks by ahejlsberg · Pull Request #46599 · microsoft/TypeScript

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@ahejlsberg

@ahejlsberg Anders Hejlsberg (ahejlsberg) commented Oct 29, 2021 •
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This PR improves relationship checking for recursive types in a number of ways:

  • When checking for potentially recursive instantiations of the same type, we now only consider types with higher type IDs. Because type IDs are monotonically increasing, newer type instantiations have higher type IDs than older type instantiations--and in a recursive type that generates an infinite series of type instantiations in some pattern, we therefore continue to see increasing type IDs. One particular advantage of this change is that manually nested type instantiations of the form Foo<Foo<Foo<Foo<Foo<T>>>>> are not counted as recursive instantiations (because we instantiate from innermost to outermost).
  • We now consider a type to be "deeply nested" when we have seen three distinct instantiations of the type with increasing type IDs. Previously the limit was five, but without the increasing type ID requirement. This change means we'll do less work to detect and stop infinitely generated types, but will check manually nested types to any depth. Previously we couldn't distinguish between the two.
  • We improve our method of computing and checking "broadest equivalent IDs" that was introduced in Bail when comparing a specialized form of an already ongoing comparison #42727.

The change to computing equivalent IDs improves type checking performance by 2-5%. Our performance test suites don't contain examples of infinitely generated types, so the depth check changes show no effect there. However, several Definitely Typed packages see dramatic improvement. For example, redux-immutable, react-lazylog, and yup all see at least a 50% reduction in check time.

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DanielRosenwasser, tonivj5, tjjfvi, mkantor, okmttdhr, electron-space, kiprasmel, imranbarbhuiya, lukehatcher, and nickofthyme reacted with rocket emoji
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Anders Hejlsberg (ahejlsberg) added 2 commits October 29, 2021 16:40
@ahejlsberg
Decrease recursion depth limit to 3 + smarter check for recursion
ddc106b
@ahejlsberg
Accept new baselines
86185ad
@typescript-bot TypeScript Bot (typescript-bot) assigned Anders Hejlsberg (ahejlsberg) Oct 29, 2021
@typescript-bot TypeScript Bot (typescript-bot) added Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Oct 29, 2021
@ahejlsberg Anders Hejlsberg (ahejlsberg) added Experiment A fork with an experimental idea which might not make it into master and removed Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Oct 29, 2021
@typescript-bot TypeScript Bot (typescript-bot) added Author: Team For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Oct 29, 2021
@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Oct 29, 2021

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TypeScript Bot (@typescript-bot) perf test faster
TypeScript Bot (@typescript-bot) test this
TypeScript Bot (@typescript-bot) user test inline
TypeScript Bot (@typescript-bot) run dt
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TypeScript Bot (typescript-bot) commented Oct 29, 2021 •
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at 86185ad. You can monitor the build here.
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TypeScript Bot (typescript-bot) commented Oct 29, 2021 •
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 86185ad. You can monitor the build here.
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the abridged perf test suite on this PR at 86185ad. You can monitor the build here.

Update: The results are in!
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@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Oct 29, 2021

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TypeScript Bot (@typescript-bot) user test this inline
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the inline community code test suite on this PR at 86185ad. You can monitor the build here.

Update: The results are in!
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Great news! no new errors were found between main..refs/pull/46599/merge
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Anders Hejlsberg (@ahejlsberg)
The results of the perf run you requested are in!

Here they are:

Comparison Report - main..46599

Metric main 46599 Delta Best Worst
Angular - node (v14.15.1, x64)
Memory used 330,560k (± 0.01%) 330,575k (± 0.01%) +16k (+ 0.00%) 330,531k 330,652k
Parse Time 1.96s (± 0.83%) 1.95s (± 0.31%) -0.02s (- 0.97%) 1.93s 1.96s
Bind Time 0.86s (± 0.43%) 0.86s (± 0.72%) +0.00s (+ 0.35%) 0.85s 0.87s
Check Time 5.35s (± 0.39%) 5.39s (± 0.43%) +0.04s (+ 0.80%) 5.34s 5.46s
Emit Time 6.17s (± 0.49%) 6.13s (± 0.49%) -0.05s (- 0.73%) 6.08s 6.22s
Total Time 14.34s (± 0.41%) 14.33s (± 0.31%) -0.02s (- 0.10%) 14.22s 14.45s
Compiler-Unions - node (v14.15.1, x64)
Memory used 192,706k (± 0.49%) 192,977k (± 0.37%) +271k (+ 0.14%) 190,100k 193,398k
Parse Time 0.81s (± 0.71%) 0.81s (± 0.83%) -0.00s (- 0.37%) 0.80s 0.82s
Bind Time 0.55s (± 0.60%) 0.55s (± 0.86%) +0.00s (+ 0.55%) 0.55s 0.57s
Check Time 7.55s (± 0.36%) 7.57s (± 0.36%) +0.02s (+ 0.21%) 7.53s 7.65s
Emit Time 2.47s (± 0.64%) 2.45s (± 0.69%) -0.02s (- 0.61%) 2.42s 2.50s
Total Time 11.39s (± 0.23%) 11.38s (± 0.27%) -0.00s (- 0.02%) 11.32s 11.48s
Monaco - node (v14.15.1, x64)
Memory used 324,079k (± 0.00%) 324,079k (± 0.00%) -1k (- 0.00%) 324,054k 324,115k
Parse Time 1.50s (± 0.63%) 1.50s (± 0.70%) -0.00s (- 0.13%) 1.49s 1.53s
Bind Time 0.75s (± 0.77%) 0.75s (± 0.63%) -0.00s (- 0.00%) 0.75s 0.77s
Check Time 5.31s (± 0.47%) 5.31s (± 0.39%) +0.01s (+ 0.11%) 5.26s 5.35s
Emit Time 3.24s (± 0.77%) 3.23s (± 0.53%) -0.01s (- 0.43%) 3.20s 3.27s
Total Time 10.81s (± 0.44%) 10.80s (± 0.28%) -0.01s (- 0.10%) 10.72s 10.87s
TFS - node (v14.15.1, x64)
Memory used 288,454k (± 0.00%) 288,457k (± 0.01%) +3k (+ 0.00%) 288,417k 288,521k
Parse Time 1.24s (± 0.87%) 1.23s (± 0.56%) -0.01s (- 1.21%) 1.21s 1.24s
Bind Time 0.73s (± 0.71%) 0.73s (± 0.61%) -0.00s (- 0.41%) 0.72s 0.74s
Check Time 4.92s (± 0.44%) 4.92s (± 0.39%) 0.00s ( 0.00%) 4.89s 4.99s
Emit Time 3.50s (± 0.68%) 3.46s (± 1.41%) -0.04s (- 1.03%) 3.29s 3.56s
Total Time 10.39s (± 0.45%) 10.34s (± 0.52%) -0.05s (- 0.51%) 10.16s 10.43s
material-ui - node (v14.15.1, x64)
Memory used 448,633k (± 0.00%) 448,650k (± 0.01%) +17k (+ 0.00%) 448,579k 448,730k
Parse Time 1.82s (± 0.22%) 1.82s (± 0.61%) +0.00s (+ 0.11%) 1.81s 1.86s
Bind Time 0.68s (± 0.66%) 0.68s (± 0.59%) +0.00s (+ 0.44%) 0.67s 0.69s
Check Time 13.04s (± 0.34%) 13.08s (± 0.55%) +0.04s (+ 0.31%) 12.97s 13.29s
Emit Time 0.00s (± 0.00%) 0.00s (± 0.00%) 0.00s ( NaN%) 0.00s 0.00s
Total Time 15.54s (± 0.26%) 15.58s (± 0.43%) +0.05s (+ 0.30%) 15.47s 15.78s
xstate - node (v14.15.1, x64)
Memory used 534,289k (± 0.00%) 532,734k (± 0.00%) -1,555k (- 0.29%) 532,673k 532,798k
Parse Time 2.54s (± 0.24%) 2.55s (± 0.53%) +0.01s (+ 0.20%) 2.51s 2.58s
Bind Time 1.15s (± 0.82%) 1.15s (± 0.78%) -0.01s (- 0.78%) 1.13s 1.17s
Check Time 1.55s (± 0.47%) 1.50s (± 0.45%) -0.04s (- 2.78%) 1.49s 1.52s
Emit Time 0.07s (± 0.00%) 0.07s (± 0.00%) 0.00s ( 0.00%) 0.07s 0.07s
Total Time 5.32s (± 0.32%) 5.27s (± 0.44%) -0.04s (- 0.85%) 5.22s 5.33s
System
Machine Namets-ci-ubuntu
Platformlinux 4.4.0-210-generic
Architecturex64
Available Memory16 GB
Available Memory9 GB
CPUs4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
Hosts
  • node (v14.15.1, x64)
Scenarios
  • Angular - node (v14.15.1, x64)
  • Compiler-Unions - node (v14.15.1, x64)
  • Monaco - node (v14.15.1, x64)
  • TFS - node (v14.15.1, x64)
  • material-ui - node (v14.15.1, x64)
  • xstate - node (v14.15.1, x64)
Benchmark Name Iterations
Current 46599 10
Baseline main 10

Developer Information:

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@ahejlsberg
Always set last type id
52e10d3
@ahejlsberg

Anders Hejlsberg (ahejlsberg) commented Oct 30, 2021

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TypeScript Bot (@typescript-bot) perf test faster
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TypeScript Bot (@typescript-bot) run dt
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TypeScript Bot (typescript-bot) commented Oct 30, 2021 •
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized Definitely Typed test suite on this PR at 52e10d3. You can monitor the build here.
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the abridged perf test suite on this PR at 52e10d3. You can monitor the build here.

Update: The results are in!
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Heya Anders Hejlsberg (@ahejlsberg), I've started to run the inline community code test suite on this PR at 52e10d3. You can monitor the build here.

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

Here they are:

Comparison Report - main..46599

Metric main 46599 Delta Best Worst
Angular - node (v14.15.1, x64)
Memory used 330,574k (± 0.01%) 330,580k (± 0.01%) +6k (+ 0.00%) 330,539k 330,619k
Parse Time 1.95s (± 0.57%) 1.95s (± 0.61%) -0.00s (- 0.05%) 1.93s 1.98s
Bind Time 0.86s (± 0.67%) 0.86s (± 0.52%) -0.00s (- 0.23%) 0.85s 0.87s
Check Time 5.35s (± 0.28%) 5.36s (± 0.43%) +0.01s (+ 0.26%) 5.32s 5.42s
Emit Time 6.13s (± 0.39%) 6.15s (± 0.62%) +0.02s (+ 0.39%) 6.10s 6.28s
Total Time 14.29s (± 0.21%) 14.33s (± 0.44%) +0.04s (+ 0.26%) 14.23s 14.52s
Compiler-Unions - node (v14.15.1, x64)
Memory used 192,659k (± 0.49%) 192,659k (± 0.49%) -1k (- 0.00%) 190,091k 193,381k
Parse Time 0.81s (± 0.68%) 0.80s (± 0.45%) -0.01s (- 0.99%) 0.80s 0.81s
Bind Time 0.55s (± 0.67%) 0.55s (± 0.90%) 0.00s ( 0.00%) 0.55s 0.57s
Check Time 7.51s (± 0.33%) 7.52s (± 0.23%) +0.01s (+ 0.13%) 7.48s 7.56s
Emit Time 2.46s (± 0.78%) 2.46s (± 0.79%) -0.00s (- 0.04%) 2.42s 2.49s
Total Time 11.34s (± 0.24%) 11.35s (± 0.28%) +0.00s (+ 0.00%) 11.27s 11.41s
Monaco - node (v14.15.1, x64)
Memory used 324,067k (± 0.01%) 324,079k (± 0.01%) +12k (+ 0.00%) 324,037k 324,104k
Parse Time 1.50s (± 0.50%) 1.50s (± 0.44%) -0.00s (- 0.20%) 1.48s 1.51s
Bind Time 0.76s (± 0.48%) 0.76s (± 1.19%) +0.00s (+ 0.40%) 0.75s 0.79s
Check Time 5.33s (± 0.42%) 5.33s (± 0.36%) +0.00s (+ 0.08%) 5.30s 5.38s
Emit Time 3.25s (± 0.63%) 3.23s (± 0.44%) -0.02s (- 0.74%) 3.20s 3.26s
Total Time 10.83s (± 0.40%) 10.82s (± 0.30%) -0.02s (- 0.14%) 10.74s 10.89s
TFS - node (v14.15.1, x64)
Memory used 288,464k (± 0.01%) 288,466k (± 0.01%) +2k (+ 0.00%) 288,411k 288,524k
Parse Time 1.24s (± 0.55%) 1.23s (± 0.62%) -0.01s (- 0.81%) 1.21s 1.25s
Bind Time 0.73s (± 0.64%) 0.73s (± 0.71%) -0.00s (- 0.14%) 0.72s 0.74s
Check Time 4.93s (± 0.26%) 4.91s (± 0.41%) -0.01s (- 0.30%) 4.88s 4.96s
Emit Time 3.47s (± 1.21%) 3.51s (± 0.64%) +0.04s (+ 1.07%) 3.47s 3.57s
Total Time 10.37s (± 0.35%) 10.38s (± 0.26%) +0.01s (+ 0.12%) 10.32s 10.43s
material-ui - node (v14.15.1, x64)
Memory used 448,495k (± 0.06%) 448,630k (± 0.00%) +135k (+ 0.03%) 448,572k 448,658k
Parse Time 1.83s (± 0.68%) 1.82s (± 0.49%) -0.01s (- 0.55%) 1.80s 1.84s
Bind Time 0.68s (± 0.77%) 0.67s (± 0.51%) -0.01s (- 0.88%) 0.67s 0.68s
Check Time 13.08s (± 0.52%) 13.05s (± 0.34%) -0.03s (- 0.26%) 12.98s 13.18s
Emit Time 0.00s (± 0.00%) 0.00s (± 0.00%) 0.00s ( NaN%) 0.00s 0.00s
Total Time 15.60s (± 0.47%) 15.55s (± 0.30%) -0.05s (- 0.31%) 15.48s 15.68s
xstate - node (v14.15.1, x64)
Memory used 534,282k (± 0.00%) 532,755k (± 0.01%) -1,527k (- 0.29%) 532,629k 532,836k
Parse Time 2.54s (± 0.49%) 2.55s (± 0.45%) +0.01s (+ 0.31%) 2.52s 2.58s
Bind Time 1.16s (± 1.19%) 1.15s (± 1.05%) -0.01s (- 0.78%) 1.12s 1.17s
Check Time 1.55s (± 0.30%) 1.51s (± 0.44%) -0.04s (- 2.39%) 1.50s 1.53s
Emit Time 0.07s (± 0.00%) 0.07s (± 0.00%) 0.00s ( 0.00%) 0.07s 0.07s
Total Time 5.32s (± 0.35%) 5.29s (± 0.36%) -0.04s (- 0.70%) 5.23s 5.32s
System
Machine Namets-ci-ubuntu
Platformlinux 4.4.0-210-generic
Architecturex64
Available Memory16 GB
Available Memory9 GB
CPUs4 × Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
Hosts
  • node (v14.15.1, x64)
Scenarios
  • Angular - node (v14.15.1, x64)
  • Compiler-Unions - node (v14.15.1, x64)
  • Monaco - node (v14.15.1, x64)
  • TFS - node (v14.15.1, x64)
  • material-ui - node (v14.15.1, x64)
  • xstate - node (v14.15.1, x64)
Benchmark Name Iterations
Current 46599 10
Baseline main 10

Developer Information:

Download Benchmark
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Great news! no new errors were found between main..refs/pull/46599/merge
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@ahejlsberg
Keep indexed access recursion depth check
5f37d89
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Anders Hejlsberg (ahejlsberg) commented Oct 31, 2021

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TypeScript Bot (@typescript-bot) perf test faster
TypeScript Bot (@typescript-bot) run dt
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weswigham
Wesley Wigham (weswigham) reviewed Nov 1, 2021
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Comment thread src/compiler/checker.ts
return getTypeReferenceId(source as TypeReference, typeParameters) + "," + getTypeReferenceId(target as TypeReference, typeParameters) + postFix;
}
return source.id + "," + target.id + postFix;
return isTypeReferenceWithGenericArguments(source) && isTypeReferenceWithGenericArguments(target) ?

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I've been wondering this for awhile and worked on a change for a bit, but can/should we expand this detailed id generation to generic type-alias'd types as well (and not just type references), so type alias relations can also benefit from the broadest-equivalent-id check?

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Wesley Wigham (weswigham) approved these changes Nov 1, 2021
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I like the structure of this - the broadestEquivalentId check is ultimately an approximation; ideally we'd be able to note all relations that are strictly a specialization of an ongoing relation check and know to bail, but the id comparison is a pretty cheap way to check something close to that and hits the very common example of recursion during variance computations. Thus, any changes that make us pick up more equivalent ids in a cheap way is generally welcome. The depth detection changes are a bit more worrysome - doesn't the id-increasing check mean that repeated usages of the exact same type id no longer count as "deeply nested"? I guess the assumption there is that deep nesting that isn't generative in some way must form a cycle at some point which will be detected. Still - definitely has the potential to convert some "quietly works" code into "loudly breaks with a stack depth error" code.

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Anders Hejlsberg (ahejlsberg) commented Nov 1, 2021

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doesn't the id-increasing check mean that repeated usages of the exact same type id no longer count as "deeply nested"?

Actually no, we still include identical type IDs in the count. We only skip counting when we see a lower type ID. My comments aren't exactly clear on that, I'll revise.
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Wesley Wigham (weswigham) commented Nov 1, 2021

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Oh, I know, but we could have a check sequence like 1,2,1,1,1,1,... and previously we'd have marked it as deep (5x repeated 1), whereas because of the 2, now we won't.
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Anders Hejlsberg (ahejlsberg) commented Nov 1, 2021

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Actually, we will mark that as deep because we always compare against the previous type ID, not the highest type ID.
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Wesley Wigham (weswigham) commented Nov 1, 2021

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Oh, so I see. I guess that means 1,2,3,4,3,2,1 won't be marked as deep (the last 3,2,1 won't be counted), but 1,2,3,4,1,2,3 will (4 -> 1 won't count, but 1->2 and 2->3 will), even though it's just a reordering of the same types.
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Anders Hejlsberg (ahejlsberg) merged commit 373accf into main Nov 2, 2021
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Anders Hejlsberg (ahejlsberg) deleted the improveRecursionDepthChecks branch November 2, 2021 21:05
@DanielRosenwasser Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Nov 5, 2021
Design Meeting Notes, 11/5/2021 #46697
Closed
@ahejlsberg Anders Hejlsberg (ahejlsberg) mentioned this pull request Nov 20, 2021
[Build Performance] Including yup adds 3 seconds to lib type-checking #46856
Open
@andrewbranch Andrew Branch (andrewbranch) mentioned this pull request Nov 29, 2021
Performance regression in react-hook-form from 4.4 to 4.5 #46948
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Daniel Rosenwasser (DanielRosenwasser) commented Dec 1, 2021

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TypeScript Bot (@typescript-bot) cherry-pick this to release-4.5
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TypeScript Bot (typescript-bot) commented Dec 1, 2021 •
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Heya Daniel Rosenwasser (@DanielRosenwasser), I've started to run the task to cherry-pick this into release-4.5 on this PR at 9df07a8. You can monitor the build here.
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@typescript-bot TypeScript Bot (typescript-bot) mentioned this pull request Dec 1, 2021
🤖 Pick PR #46599 (Improve recursion depth checks) into release-4.5 #46974
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Hey Daniel Rosenwasser (@DanielRosenwasser), I've opened #46974 for you.
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TypeScript Bot (typescript-bot) pushed a commit to typescript-bot/TypeScript that referenced this pull request Dec 1, 2021
@ahejlsberg @typescript-bot
Cherry-pick PR microsoft#46599 into release-4.5
f26e0d1 
Component commits:
ddc106b Decrease recursion depth limit to 3 + smarter check for recursion

86185ad Accept new baselines

52e10d3 Always set last type id

5f37d89 Keep indexed access recursion depth check

9df07a8 Less expensive and corrected check for broadest equivalent keys
Andrew Branch (andrewbranch) pushed a commit to typescript-bot/TypeScript that referenced this pull request Dec 3, 2021
@ahejlsberg @andrewbranch
Cherry-pick PR microsoft#46599 into release-4.5
156d219 
Component commits:
ddc106b Decrease recursion depth limit to 3 + smarter check for recursion

86185ad Accept new baselines

52e10d3 Always set last type id

5f37d89 Keep indexed access recursion depth check

9df07a8 Less expensive and corrected check for broadest equivalent keys
Daniel Rosenwasser (DanielRosenwasser) pushed a commit that referenced this pull request Dec 7, 2021
TypeScript Bot @ahejlsberg
Cherry-pick PR #46599 into release-4.5 (#46974)
ae44862 
Component commits:
ddc106b Decrease recursion depth limit to 3 + smarter check for recursion

86185ad Accept new baselines

52e10d3 Always set last type id

5f37d89 Keep indexed access recursion depth check

9df07a8 Less expensive and corrected check for broadest equivalent keys

Co-authored-by: Anders Hejlsberg <[email protected]>
@DanielRosenwasser Daniel Rosenwasser (DanielRosenwasser) added this to the TypeScript 4.5.3 milestone Dec 7, 2021
@mkantor

Matt Kantor (mkantor) commented Dec 11, 2021

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I think this fixes #42070.
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@ostrowr Robbie Ostrow (ostrowr) mentioned this pull request Dec 14, 2021
Conditional types incorrect for complex types #35533
Closed
@mkantor Matt Kantor (mkantor) mentioned this pull request Dec 15, 2021
Nested array/object types 6+ deep can break typechecking for smaller nested types across the entire file #42070
Closed
@sync-by-unito sync-by-unito Bot mentioned this pull request Jan 1, 2022
build(deps-dev): bump typescript from 4.3.5 to 4.5.4 numbersprotocol/capture-cam#1150
Closed
Martin Probst (mprobst) pushed a commit to mprobst/TypeScript that referenced this pull request Jan 10, 2022
@ahejlsberg @mprobst
Improve recursion depth checks (microsoft#46599)
fc3a158 
* Decrease recursion depth limit to 3 + smarter check for recursion

* Accept new baselines

* Always set last type id

* Keep indexed access recursion depth check

* Less expensive and corrected check for broadest equivalent keys
@Jym77

Jean-Yves Moyen (Jym77) commented Feb 14, 2022

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Hi, I want to point a small regression that I believe has been introduced by this PR 😕 Alas I haven't been able to isolate a MNWE after 1 day trying to extract code from my project and reaching 200 lines in the non so minimal and still working example 😞

TLDR: This project builds in TS4.5.2 but doesn't in TS 4.5.3 (nor in 4.5.5).
Given the error message and the crashing on our controlled-recursive type, I do believe that this PR is the cause of it.

We are building recursive type but controlling the recursion through a type Depths = [-1, 0, 1, 2] (ensuring Depths[D] = D-1), then the type looks like (over simplified) type Nested<T, D extends number = 3> = D extends -1 ? T : Nested<T, Depths[D]>. The D parameter ensures that type recursion stops…

Of course, the actual type is much more crazy. It models an "interview" which is a sequence of questions (asked to an oracle), where each question expects a typed answer (T is a representation of this type (e.g. "boolean", "string[]"), and A is the actual corresponding type (e.g. boolean, Array<string>); the QUESTION parameter is meta-data mapping the questions' URI to their type). An interview may contains several questions in a row (hence the recursion), and the depth is used to make sure the type works.

The type itself does build in TS 4.5.3, but then the way we use it (through yet some other layers of complication) breaks…
I've tried to adapt the code to get it working in 4.5.5. From first attempts, it seems that giving some hints about the actual value of D helps 🤔 It feels like TS is now too eager in setting D=3 and thus struggles when nesting interviews…

I don't really consider this as a bug. Feels more like a very corner case… Plus haven't been able to get a MWNE to study… But I guess you might want to hear about it 😊
(and my secret hope is that you'll have a "aha!" moment seing the description and knowing the content of this PR much better than me 😀 )
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@andrewbranch Andrew Branch (andrewbranch) mentioned this pull request Sep 12, 2022
Compiler missing "deep" type errors - 4.5.5 regression #50670
Closed
@martinwepner martinwepner mentioned this pull request Feb 22, 2023
Undetected illegal assignment of nested array types #52912
Closed
@Andarist Mateusz Burzyński (Andarist) mentioned this pull request Apr 26, 2023
Add extra tests for recursive conditional types #54030
Merged
@ahrjarrett andrew jarrett (ahrjarrett) mentioned this pull request Aug 28, 2023
Assignability false positive for recursive mapped types, even when fully instantiated #55535
Closed
@microsoft Microsoft (microsoft) locked as resolved and limited conversation to collaborators Oct 21, 2025
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@weswigham Wesley Wigham (weswigham) weswigham approved these changes

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@ahejlsberg Anders Hejlsberg (ahejlsberg)

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@ahejlsberg @typescript-bot @weswigham @DanielRosenwasser @mkantor @Jym77
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