Recursive conditional types by ahejlsberg · Pull Request #40002 · microsoft/TypeScript · GitHub

With this PR we officially support recursive conditional types. For example:

// Awaiting promises

type Awaited<T> =
    T extends null | undefined ? T :
    T extends PromiseLike<infer U> ? Awaited<U> :
    T;

type P1 = Awaited<Promise<string>>;  // string
type P2 = Awaited<Promise<Promise<string>>>;  // string
type P3 = Awaited<Promise<string | Promise<Promise<number> | undefined>>;  // string | number | undefined

A bit of context... When conditional types were first introduced, we explicitly restricted them to be non-recursive, i.e. we made it an error for a conditional type to directly or indirectly reference itself. This restriction was put in place primarily as a safeguard against runaway infinite recursion which the compiler didn't handle well at the time. However, it turns out it was still possible to construct recursive conditionally resolved types by combining object types (which have deferred resolution of property types) and conditional types using indexed access types (see #14833 for the core idea). In spite of being cumbersome and non-intuitive, this trick has become commonplace in several libraries. Consequently, over time we have "hardened" the compiler against infinite recursion with depth limiters in relationships, type inference, type instantiation, constraint computation, and so on. We're now at a point where it seems reasonable support an intuitive way of writing recursive conditional types.

Some more examples:

// Flattening arrays

type Flatten<T extends readonly unknown[]> = T extends unknown[] ? _Flatten<T>[] : readonly _Flatten<T>[];
type _Flatten<T> = T extends readonly (infer U)[] ? _Flatten<U> : T;

type InfiniteArray<T> = InfiniteArray<T>[];

type A1 = Flatten<string[][][]>;  // string[]
type A2 = Flatten<string[][] | readonly (number[] | boolean[][])[]>;  // string[] | readonly (number | boolean)[]
type A3 = Flatten<InfiniteArray<string>>;
type A4 = A3[0];  // Infinite depth error

// Repeating tuples

type TupleOf<T, N extends number> = N extends N ? number extends N ? T[] : _TupleOf<T, N, []> : never;
type _TupleOf<T, N extends number, R extends unknown[]> = R['length'] extends N ? R : _TupleOf<T, N, [T, ...R]>;

type T1 = TupleOf<string, 3>;  // [string, string, string]
type T2 = TupleOf<number, 0 | 2 | 4>;  // [] | [number, number] | [number, number, number, number]
type T3 = TupleOf<number, number>;  // number[]
type T4 = TupleOf<number, 100>;  // Depth error

Note that this PR doesn't change the recursion depth limits that are already in place. For example, an error is reported on T4 above because its resolution exceeds the limit of 50 nested type instantiations.

The type inference streamlining contained in the PR fixes several issues with inference to recursive types. For example:

interface Box<T> { value: T };
type RecBox<T> = T | Box<RecBox<T>>;

declare function unbox<T>(box: RecBox<T>): T

type T1 = Box<string>;
type T2 = Box<T1>;
type T3 = Box<T2>;
type T4 = Box<T3>;
type T5 = Box<T4>;
type T6 = Box<T5>;

declare let b1: Box<Box<Box<Box<Box<Box<string>>>>>>;
declare let b2: T6;

unbox(b1);  // string
unbox(b2);  // string (previously T6)
unbox({ value: { value: { value: 5 }}});  // number (previously { value: { value: number }})

Previously, only the unbox(b1) call produced the expected type inference.

Fixes #26223.
Fixes #26980.
Fixes #37801.

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

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized community code test suite on this PR at 7c4d923. 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 7c4d923. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the extended test suite on this PR at 7c4d923. You can monitor the build here.

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the perf test suite on this PR at 7c4d923. You can monitor the build here.

Update: The results are in!

The user suite test run you requested has finished and failed. I've opened a PR with the baseline diff from master.

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

Here they are:

Comparison Report - master..40002

Metric	master	40002	Delta	Best	Worst
Angular - node (v10.16.3, x64)
Memory used	343,622k (± 0.02%)	343,145k (± 0.03%)	-477k (- 0.14%)	342,941k	343,375k
Parse Time	2.02s (± 0.70%)	2.00s (± 0.59%)	-0.02s (- 0.74%)	1.98s	2.03s
Bind Time	0.82s (± 0.82%)	0.82s (± 0.98%)	+0.00s (+ 0.12%)	0.81s	0.84s
Check Time	4.77s (± 0.90%)	4.78s (± 0.62%)	+0.01s (+ 0.21%)	4.70s	4.83s
Emit Time	5.19s (± 0.87%)	5.21s (± 0.71%)	+0.01s (+ 0.29%)	5.14s	5.34s
Total Time	12.80s (± 0.71%)	12.81s (± 0.52%)	+0.01s (+ 0.08%)	12.70s	13.02s
Monaco - node (v10.16.3, x64)
Memory used	339,127k (± 0.02%)	339,168k (± 0.02%)	+41k (+ 0.01%)	339,004k	339,281k
Parse Time	1.59s (± 0.84%)	1.57s (± 0.49%)	-0.03s (- 1.88%)	1.55s	1.58s
Bind Time	0.72s (± 0.66%)	0.71s (± 0.67%)	-0.00s (- 0.56%)	0.70s	0.72s
Check Time	4.94s (± 0.62%)	4.89s (± 0.52%)	-0.05s (- 1.01%)	4.80s	4.92s
Emit Time	2.77s (± 1.05%)	2.74s (± 0.76%)	-0.03s (- 1.05%)	2.71s	2.78s
Total Time	10.02s (± 0.58%)	9.90s (± 0.30%)	-0.12s (- 1.16%)	9.83s	9.95s
TFS - node (v10.16.3, x64)
Memory used	0k	0k	0k ( NaN%)	0k	0k
Parse Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Bind Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Check Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Emit Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Total Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
material-ui - node (v10.16.3, x64)
Memory used	458,780k (± 0.01%)	460,209k (± 0.01%)	+1,428k (+ 0.31%)	460,011k	460,312k
Parse Time	2.05s (± 0.46%)	2.05s (± 0.52%)	-0.01s (- 0.39%)	2.03s	2.07s
Bind Time	0.66s (± 1.46%)	0.65s (± 1.25%)	-0.01s (- 1.66%)	0.64s	0.67s
Check Time	12.97s (± 0.66%)	13.21s (± 0.51%)	+0.24s (+ 1.84%)	13.08s	13.41s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	15.68s (± 0.54%)	15.90s (± 0.47%)	+0.22s (+ 1.40%)	15.75s	16.13s
Angular - node (v12.1.0, x64)
Memory used	320,898k (± 0.02%)	320,378k (± 0.02%)	-520k (- 0.16%)	320,232k	320,453k
Parse Time	2.01s (± 0.86%)	1.99s (± 0.73%)	-0.02s (- 0.90%)	1.95s	2.03s
Bind Time	0.80s (± 1.03%)	0.80s (± 0.74%)	+0.00s (+ 0.00%)	0.79s	0.82s
Check Time	4.66s (± 0.49%)	4.65s (± 0.64%)	-0.01s (- 0.24%)	4.59s	4.70s
Emit Time	5.38s (± 0.50%)	5.35s (± 0.49%)	-0.03s (- 0.61%)	5.29s	5.40s
Total Time	12.85s (± 0.41%)	12.79s (± 0.44%)	-0.06s (- 0.46%)	12.69s	12.90s
Monaco - node (v12.1.0, x64)
Memory used	321,569k (± 0.03%)	321,560k (± 0.03%)	-10k (- 0.00%)	321,395k	321,734k
Parse Time	1.55s (± 0.89%)	1.54s (± 0.66%)	-0.01s (- 0.65%)	1.51s	1.56s
Bind Time	0.69s (± 1.05%)	0.69s (± 0.83%)	-0.00s (- 0.29%)	0.68s	0.70s
Check Time	4.74s (± 0.32%)	4.69s (± 0.46%)	-0.05s (- 0.99%)	4.66s	4.76s
Emit Time	2.82s (± 0.50%)	2.79s (± 0.86%)	-0.02s (- 0.85%)	2.75s	2.86s
Total Time	9.79s (± 0.23%)	9.71s (± 0.36%)	-0.08s (- 0.84%)	9.63s	9.79s
TFS - node (v12.1.0, x64)
Memory used	0k	0k	0k ( NaN%)	0k	0k
Parse Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Bind Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Check Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Emit Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Total Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
material-ui - node (v12.1.0, x64)
Memory used	437,163k (± 0.01%)	438,348k (± 0.07%)	+1,185k (+ 0.27%)	437,478k	438,652k
Parse Time	2.04s (± 0.49%)	2.02s (± 0.64%)	-0.03s (- 1.27%)	1.97s	2.03s
Bind Time	0.64s (± 0.57%)	0.62s (± 0.58%)	-0.01s (- 1.89%)	0.62s	0.63s
Check Time	11.82s (± 0.48%)	11.83s (± 1.13%)	+0.01s (+ 0.06%)	11.66s	12.28s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	14.50s (± 0.34%)	14.47s (± 0.95%)	-0.03s (- 0.21%)	14.31s	14.93s
Angular - node (v8.9.0, x64)
Memory used	340,207k (± 0.02%)	339,812k (± 0.02%)	-395k (- 0.12%)	339,619k	339,967k
Parse Time	2.55s (± 0.68%)	2.53s (± 0.42%)	-0.01s (- 0.47%)	2.51s	2.56s
Bind Time	0.86s (± 0.52%)	0.85s (± 0.68%)	-0.01s (- 0.93%)	0.84s	0.86s
Check Time	5.42s (± 0.45%)	5.40s (± 0.59%)	-0.02s (- 0.44%)	5.34s	5.46s
Emit Time	5.98s (± 1.57%)	5.92s (± 0.85%)	-0.07s (- 1.12%)	5.76s	5.99s
Total Time	14.81s (± 0.73%)	14.70s (± 0.53%)	-0.12s (- 0.80%)	14.46s	14.81s
Monaco - node (v8.9.0, x64)
Memory used	340,558k (± 0.02%)	340,543k (± 0.02%)	-15k (- 0.00%)	340,392k	340,687k
Parse Time	1.88s (± 0.43%)	1.87s (± 0.69%)	-0.01s (- 0.37%)	1.85s	1.91s
Bind Time	0.89s (± 0.69%)	0.89s (± 0.73%)	-0.00s (- 0.34%)	0.88s	0.91s
Check Time	5.46s (± 0.57%)	5.40s (± 0.38%)	-0.06s (- 1.08%)	5.36s	5.44s
Emit Time	3.24s (± 1.02%)	3.24s (± 0.98%)	-0.00s (- 0.09%)	3.18s	3.32s
Total Time	11.46s (± 0.46%)	11.40s (± 0.45%)	-0.07s (- 0.58%)	11.28s	11.52s
TFS - node (v8.9.0, x64)
Memory used	0k	0k	0k ( NaN%)	0k	0k
Parse Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Bind Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Check Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Emit Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Total Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
material-ui - node (v8.9.0, x64)
Memory used	463,031k (± 0.01%)	464,543k (± 0.01%)	+1,512k (+ 0.33%)	464,410k	464,658k
Parse Time	2.41s (± 0.71%)	2.39s (± 0.57%)	-0.02s (- 1.00%)	2.35s	2.42s
Bind Time	0.79s (± 1.61%)	0.79s (± 2.04%)	-0.01s (- 0.76%)	0.76s	0.83s
Check Time	17.26s (± 1.01%)	17.75s (± 0.83%)	+0.49s (+ 2.86%)	17.45s	18.11s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	20.46s (± 0.88%)	20.93s (± 0.70%)	+0.47s (+ 2.28%)	20.65s	21.27s
Angular - node (v8.9.0, x86)
Memory used	195,206k (± 0.04%)	195,040k (± 0.03%)	-166k (- 0.08%)	194,913k	195,193k
Parse Time	2.46s (± 0.43%)	2.46s (± 0.88%)	-0.00s (- 0.12%)	2.42s	2.50s
Bind Time	0.99s (± 1.06%)	0.99s (± 1.34%)	+0.00s (+ 0.40%)	0.96s	1.02s
Check Time	4.87s (± 0.75%)	4.86s (± 0.72%)	-0.01s (- 0.25%)	4.79s	4.93s
Emit Time	5.97s (± 1.52%)	5.95s (± 1.30%)	-0.02s (- 0.39%)	5.72s	6.10s
Total Time	14.30s (± 0.76%)	14.27s (± 0.74%)	-0.03s (- 0.23%)	14.04s	14.51s
Monaco - node (v8.9.0, x86)
Memory used	193,549k (± 0.03%)	193,538k (± 0.02%)	-11k (- 0.01%)	193,481k	193,629k
Parse Time	1.92s (± 0.83%)	1.92s (± 1.46%)	+0.01s (+ 0.42%)	1.87s	1.99s
Bind Time	0.70s (± 0.43%)	0.71s (± 1.28%)	+0.01s (+ 1.58%)	0.70s	0.74s
Check Time	5.50s (± 1.49%)	5.57s (± 0.35%)	+0.07s (+ 1.22%)	5.51s	5.61s
Emit Time	2.71s (± 3.10%)	2.66s (± 0.98%)	-0.04s (- 1.55%)	2.63s	2.76s
Total Time	10.82s (± 0.66%)	10.86s (± 0.37%)	+0.04s (+ 0.36%)	10.78s	10.97s
TFS - node (v8.9.0, x86)
Memory used	0k	0k	0k ( NaN%)	0k	0k
Parse Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Bind Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Check Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Emit Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
Total Time	0.00s	0.00s	0.00s ( NaN%)	0.00s	0.00s
material-ui - node (v8.9.0, x86)
Memory used	262,269k (± 0.02%)	263,018k (± 0.02%)	+749k (+ 0.29%)	262,896k	263,212k
Parse Time	2.46s (± 0.61%)	2.43s (± 0.41%)	-0.03s (- 1.06%)	2.41s	2.46s
Bind Time	0.68s (± 1.38%)	0.68s (± 1.70%)	-0.00s (- 0.44%)	0.66s	0.71s
Check Time	15.74s (± 0.86%)	16.25s (± 0.53%)	+0.51s (+ 3.25%)	16.03s	16.43s
Emit Time	0.00s (± 0.00%)	0.00s (± 0.00%)	0.00s ( NaN%)	0.00s	0.00s
Total Time	18.88s (± 0.68%)	19.36s (± 0.44%)	+0.48s (+ 2.55%)	19.20s	19.54s

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	40002	10
Baseline	master	10

This is pretty much the inference equivalent of recursiveTypeRelatedTo at this point.

Huh, and this doesn't affect the user baselines or DT?

No. Previously conditional types couldn't be recursive, so they weren't included here. But for that same reason we have no tests that could be affected by this.

Awesome to finally see this! 😀

Anders Hejlsberg (@ahejlsberg) maybe something worthwhile is to build another PR on top of this to see if changing the definition of FlatArray would impact real-world code.

The tests revealed OOMs in a few projects due to the switch to use isDeeplyNestedType for recursion tracking in type inference (which permits up to five levels of recursion). With the latest commits I have reverted to the previous scheme of terminating after just one level of recursion, but with the added twist that we track both the source and target sides (similarly to recursiveTypeRelatedTo) and terminate only when both have a circularity.

Intuitively, in inference we want to terminate when we encounter a duplicate attempt to infer from source and target types with the same origin, so getRecursionIdentity needs to get us as close as possible to the AST node that caused the type instantiation. Because most type instantiations are interned and shared (and thus have no reference to their originating AST node), this isn't always possible. For example, when inferring from Box2<Box2<string>> to Box1<Box1<T>>, where Box1 and Box2 are unique but structurally identical types, we end up with the same recursion identity for each Box1 and Box2 reference, and therefore terminate inference prematurely. This is a known problem (i.e. not new to this PR) and something we should continue to think about. A brute force way to work around the problem is to allow multiple levels of recursion, but that only works up to some level of nested and, as illustrated by the test failures, generates way too much work in general.

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

Heya Anders Hejlsberg (@ahejlsberg), I've started to run the parallelized community code test suite on this PR at fed0e8c. You can monitor the build here.

Anders Hejlsberg (@ahejlsberg) According to your example in the OP, this also fixes #26223 😊

Test runs all look clean. Slight regression in check time for material-ui, but it's worth it for the added precision in type inference.

Anders Hejlsberg (@ahejlsberg) Ron Buckton (@rbuckton)

Does this change mean we no longer need hacks like awaited keyword to handle the recursive nature of Promise?

Can we get a playground for this PR? I'd like to play around with the new options this gives us.

T6 (@tjjfvi) you can choose "Nightly" version

type T4 = TupleOf<number, 100>;  // Depth error

why the max number is 43 ? not 48 ? not 98 ?

Anders Hejlsberg (ahejlsberg) added 7 commits August 7, 2020 20:26

Support recursive conditional types

419e8d6

Accept new API baselines

beab30a

Accept new baselines

c24cacc

Simplify recursive type tracking in type inference

5a45585

Accept new baselines

4f8dc24

Add tests

dd64bf8

Accept new baselines

7c4d923

TypeScript Bot (typescript-bot) assigned Wesley Wigham (weswigham) Aug 11, 2020

TypeScript Bot (typescript-bot) added the For Milestone Bug PRs that fix a bug with a specific milestone label Aug 11, 2020

Anders Hejlsberg (ahejlsberg) requested review from Daniel Rosenwasser (DanielRosenwasser), Ryan Cavanaugh (RyanCavanaugh) and Wesley Wigham (weswigham) August 11, 2020 17:21

Anders Hejlsberg (ahejlsberg) added this to the TypeScript 4.1.0 milestone Aug 11, 2020

TypeScript Bot (typescript-bot) mentioned this pull request Aug 11, 2020

🤖 User test baselines have changed for recursiveConditionalTypes typescript-bot/TypeScript#71

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Wesley Wigham (weswigham) approved these changes Aug 11, 2020

View reviewed changes

Anders Hejlsberg (ahejlsberg) added 3 commits August 13, 2020 13:07

Revise recursion tracking in type inference

6c11cf4

Revise tests

5b45f42

Accept new baselines

fed0e8c

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Aug 14, 2020

Design Meeting Notes, 8/14/2020 #40052

Closed

Anders Hejlsberg (ahejlsberg) added 2 commits August 15, 2020 17:50

Add more tests

199d568

Accept new baselines

324b3fb

Joe Calzaretta (jcalz) mentioned this pull request Aug 16, 2020

Feature request: lift the circular constraint for conditional types #26980

Closed

4 tasks

Anders Hejlsberg (ahejlsberg) merged commit cd30534 into master Aug 16, 2020

Anders Hejlsberg (ahejlsberg) deleted the recursiveConditionalTypes branch August 16, 2020 01:26

Daniel Rosenwasser (DanielRosenwasser) mentioned this pull request Aug 18, 2020

TypeScript 4.1 Iteration Plan #40124

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Simon Chan (yume-chan) mentioned this pull request Aug 21, 2020

Promise<Promise<T>> cannot exist in JS #27711

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pierre (millsp) mentioned this pull request Aug 23, 2020

Support for TypeScript 4.0 millsp/ts-toolbelt#140

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Anders Hejlsberg (ahejlsberg) mentioned this pull request Aug 26, 2020

Partially disable inference recursion tracking changes #40256

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Eli Barzilay (elibarzilay) mentioned this pull request Aug 27, 2020

Properly type Lazy.js Sequence.chunk method DefinitelyTyped/DefinitelyTyped#46699

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10 tasks

Dan Greene (dgreene1) mentioned this pull request Sep 3, 2020

Refactoring to utilize new recursive conditional feature piotrwitek/utility-types#151

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Hyeseong Kim (cometkim) mentioned this pull request Sep 8, 2020

WIP: feat(core): Add utilities for natural numbers cometkim/cometjs#10

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Karol Majewski (karol-majewski) mentioned this pull request Sep 9, 2020

Make the translation function fully type-safe i18next/i18next#1504

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Dan Vanderkam (danvk) mentioned this pull request Sep 14, 2020

Implement addition in the type system (TS 4.1) type-challenges/type-challenges#185

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Grzegorz Bielski (grzegorz-bielski) mentioned this pull request Sep 18, 2020

Unexpected action order with synchronous epics grzegorz-bielski/rxsv#21

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AllanZhengYP mentioned this pull request Oct 12, 2020

[util-dynamodb]: use recursive conditional type to support better type inferrence. aws/aws-sdk-js-v3#1564

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Ryan Cavanaugh (RyanCavanaugh) mentioned this pull request Oct 16, 2020

[4.1.0-beta] Incorrect method overload selected #41099

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Piotr Witek (piotrwitek) mentioned this pull request Jan 6, 2021

Add TupleOf type piotrwitek/utility-types#161

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github-actions Bot mentioned this pull request Jul 14, 2021

Translate 1 file to ko - TypeScript 4.1.md microsoft/TypeScript-Website-Localizations#94

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webcompat bot (webcompat-bot) mentioned this pull request Jan 10, 2022

github.com - see bug description webcompat/web-bugs#97894

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Mateusz Burzyński (Andarist) mentioned this pull request Feb 13, 2023

Add an extra test case for an aliased mapped type inference #52742

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Jake Bailey (jakebailey) mentioned this pull request Feb 22, 2023

PromiseLike.then returns PromiseLike<Promise<T>> instead of PromiseLike<T> #17862

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

                           }
-                          function invokeOnce(source: Type, target: Type, action: (source: Type, target: Type) => void) {
+                          function invokeWithDepthLimit(source: Type, target: Type, action: (source: Type, target: Type) => void) {

+                          }
+                          if (type.flags & TypeFlags.Conditional) {
+                              // The root object represents the origin of the conditional type
+                              return (type as ConditionalType).root;

Recursive conditional types by ahejlsberg · Pull Request #40002 · microsoft/TypeScript · GitHub

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Recursive conditional types - #40002

Recursive conditional types#40002Anders Hejlsberg (ahejlsberg) merged 12 commits intomastermicrosoft/TypeScript:masterfrom recursiveConditionalTypesmicrosoft/TypeScript:recursiveConditionalTypesCopy head branch name to clipboard

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Recursive conditional types#40002
Anders Hejlsberg (ahejlsberg) merged 12 commits into
masterfrom
recursiveConditionalTypes

Anders Hejlsberg (ahejlsberg) commented Aug 11, 2020 •
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