Keep returned (and yielded) literal types as const when their types using `const` type variables #56859

Andarist · 2023-12-23T13:51:42Z

…sing `const` type variables

typescript-bot · 2023-12-25T12:00:55Z

The TypeScript team hasn't accepted the linked issue #53813. If you can get it accepted, this PR will have a better chance of being reviewed.

sandersn

The change seems sensible but we should probably review it at a design meeting.

src/compiler/checker.ts

…-vs-const-type-variables

sandersn · 2024-01-05T19:27:09Z

My notes from the design meeting:

This improves the apparent consistency so much that it's worth taking. The implementation needs to be right of course.

To check that:

run all test suites,
test multiply-nested cases
test overloads
test async functions, generators, async generators

Also decide whether this is the right place to implement it; could be that it's better to put it in getWidenedType. (@weswigham brought this concern up but seemed to be believe that its current place is better.)

sandersn · 2024-01-05T19:27:39Z

@typescript-bot test top100
@typescript-bot user test this
@typescript-bot run dt
@typescript-bot perf test this
@typescript-bot pack this

typescript-bot · 2024-01-05T19:27:42Z

Heya @sandersn, I've started to run the tarball bundle task on this PR at 78831c2. You can monitor the build here.

typescript-bot · 2024-01-05T19:27:42Z

Heya @sandersn, I've started to run the diff-based top-repos suite on this PR at 78831c2. You can monitor the build here.

Update: The results are in!

typescript-bot · 2024-01-05T19:27:42Z

Heya @sandersn, I've started to run the parallelized Definitely Typed test suite on this PR at 78831c2. You can monitor the build here.

Update: The results are in!

typescript-bot · 2024-01-05T19:27:42Z

Heya @sandersn, I've started to run the regular perf test suite on this PR at 78831c2. You can monitor the build here.

Update: The results are in!

typescript-bot · 2024-01-05T19:27:42Z

Heya @sandersn, I've started to run the diff-based user code test suite on this PR at 78831c2. You can monitor the build here.

Update: The results are in!

typescript-bot · 2024-01-05T19:29:35Z

Hey @sandersn, 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/159283/artifacts?artifactName=tgz&fileId=CCBB4994699BAF8077FFA5F10F4D9FE2A4243C14A9540A067A3112B12804D78702&fileName=/typescript-5.4.0-insiders.20240105.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/[email protected]".;

typescript-bot · 2024-01-05T19:47:07Z

@sandersn
The results of the perf run you requested are in!

Here they are:

Compiler

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Angular - node (v18.15.0, x64)}
_{Memory used}	_{295,476k (± 0.01%)}	_{295,467k (± 0.01%)}	_~	_295,424k	_295,517k	_{p=0.810 n=6}
_{Parse Time}	_{2.65s (± 0.19%)}	_{2.65s (± 0.40%)}	_~	_2.63s	_2.66s	_{p=0.794 n=6}
_{Bind Time}	_{0.82s (± 0.50%)}	_{0.82s (± 0.50%)}	_~	_0.82s	_0.83s	_{p=1.000 n=6}
_{Check Time}	_{8.17s (± 0.26%)}	_{8.16s (± 0.10%)}	_~	_8.15s	_8.17s	_{p=0.565 n=6}
_{Emit Time}	_{7.11s (± 0.21%)}	_{7.11s (± 0.29%)}	_~	_7.08s	_7.14s	_{p=0.745 n=6}
_{Total Time}	_{18.75s (± 0.12%)}	_{18.73s (± 0.12%)}	_~	_18.72s	_18.78s	_{p=0.216 n=6}
_{Compiler-Unions - node (v18.15.0, x64)}
_{Memory used}	_{191,523k (± 0.02%)}	_{192,472k (± 1.23%)}	_~	_191,414k	_197,303k	_{p=0.810 n=6}
_{Parse Time}	_{1.36s (± 0.77%)}	_{1.35s (± 1.69%)}	_~	_1.31s	_1.38s	_{p=0.802 n=6}
_{Bind Time}	_{0.72s (± 0.00%)}	_{0.72s (± 0.00%)}	_~	_0.72s	_0.72s	_{p=1.000 n=6}
_{Check Time}	_{9.25s (± 0.28%)}	_{9.25s (± 0.26%)}	_~	_9.21s	_9.27s	_{p=0.935 n=6}
_{Emit Time}	_{2.62s (± 0.24%)}	_{2.61s (± 0.45%)}	_~	_2.60s	_2.63s	_{p=0.068 n=6}
_{Total Time}	_{13.94s (± 0.22%)}	_{13.92s (± 0.38%)}	_~	_13.83s	_13.99s	_{p=0.810 n=6}
_{Monaco - node (v18.15.0, x64)}
_{Memory used}	_{347,397k (± 0.00%)}	_{347,419k (± 0.01%)}	_~	_347,369k	_347,453k	_{p=0.090 n=6}
_{Parse Time}	_{2.46s (± 0.31%)}	_{2.46s (± 0.54%)}	_~	_2.44s	_2.48s	_{p=0.548 n=6}
_{Bind Time}	_{0.93s (± 0.56%)}	_{0.92s (± 0.56%)}	_~	_0.92s	_0.93s	_{p=0.311 n=6}
_{Check Time}	_{6.88s (± 0.35%)}	_{6.90s (± 0.47%)}	_~	_6.85s	_6.93s	_{p=0.170 n=6}
_{Emit Time}	_{4.05s (± 0.37%)}	_{4.04s (± 0.34%)}	_~	_4.03s	_4.07s	_{p=0.242 n=6}
_{Total Time}	_{14.31s (± 0.26%)}	_{14.32s (± 0.29%)}	_~	_14.27s	_14.38s	_{p=0.683 n=6}
_{TFS - node (v18.15.0, x64)}
_{Memory used}	_{302,737k (± 0.01%)}	_{302,718k (± 0.00%)}	_{-19k (- 0.01%)}	_302,700k	_302,737k	_{p=0.045 n=6}
_{Parse Time}	_{2.01s (± 1.08%)}	_{2.00s (± 0.92%)}	_~	_1.98s	_2.02s	_{p=0.459 n=6}
_{Bind Time}	_{1.00s (± 1.03%)}	_{1.00s (± 1.09%)}	_~	_0.99s	_1.02s	_{p=0.546 n=6}
_{Check Time}	_{6.28s (± 0.55%)}	_{6.28s (± 0.19%)}	_~	_6.27s	_6.30s	_{p=0.568 n=6}
_{Emit Time}	_{3.59s (± 0.62%)}	_{3.57s (± 0.40%)}	_~	_3.55s	_3.59s	_{p=0.139 n=6}
_{Total Time}	_{12.88s (± 0.29%)}	_{12.85s (± 0.29%)}	_~	_12.81s	_12.92s	_{p=0.072 n=6}
_{material-ui - node (v18.15.0, x64)}
_{Memory used}	_{506,810k (± 0.00%)}	_{506,791k (± 0.00%)}	_~	_506,764k	_506,817k	_{p=0.230 n=6}
_{Parse Time}	_{2.59s (± 0.63%)}	_{2.58s (± 0.53%)}	_~	_2.57s	_2.61s	_{p=0.867 n=6}
_{Bind Time}	_{0.99s (± 0.00%)}	_{0.99s (± 0.76%)}	_~	_0.98s	_1.00s	_{p=0.598 n=6}
_{Check Time}	_{16.93s (± 0.40%)}	_{16.95s (± 0.31%)}	_~	_16.85s	_17.00s	_{p=0.368 n=6}
_{Emit Time}	_{0.00s (± 0.00%)}	_{0.00s (± 0.00%)}	_~	_0.00s	_0.00s	_{p=1.000 n=6}
_{Total Time}	_{20.51s (± 0.36%)}	_{20.52s (± 0.20%)}	_~	_20.45s	_20.57s	_{p=0.748 n=6}
_{xstate - node (v18.15.0, x64)}
_{Memory used}	_{512,865k (± 0.01%)}	_{512,795k (± 0.01%)}	_{-69k (- 0.01%)}	_512,730k	_512,865k	_{p=0.031 n=6}
_{Parse Time}	_{3.27s (± 0.19%)}	_{3.27s (± 0.27%)}	_~	_3.26s	_3.28s	_{p=1.000 n=6}
_{Bind Time}	_{1.54s (± 0.53%)}	_{1.54s (± 0.36%)}	_~	_1.53s	_1.54s	_{p=0.859 n=6}
_{Check Time}	_{2.82s (± 1.08%)}	_{2.81s (± 0.76%)}	_~	_2.77s	_2.83s	_{p=0.519 n=6}
_{Emit Time}	_{0.07s (± 5.69%)}	_{0.07s (± 0.00%)}	_~	_0.07s	_0.07s	_{p=0.405 n=6}
_{Total Time}	_{7.71s (± 0.44%)}	_{7.69s (± 0.40%)}	_~	_7.64s	_7.73s	_{p=0.630 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Angular - node (v18.15.0, x64)
Compiler-Unions - node (v18.15.0, x64)
Monaco - node (v18.15.0, x64)
TFS - node (v18.15.0, x64)
material-ui - node (v18.15.0, x64)
xstate - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

tsserver

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-UnionsTSServer - node (v18.15.0, x64)}
_{Req 1 - updateOpen}	_{2,338ms (± 0.33%)}	_{2,346ms (± 0.84%)}	_~	_2,327ms	_2,378ms	_{p=0.748 n=6}
_{Req 2 - geterr}	_{5,393ms (± 1.11%)}	_{5,401ms (± 0.99%)}	_~	_5,366ms	_5,506ms	_{p=0.521 n=6}
_{Req 3 - references}	_{326ms (± 1.28%)}	_{325ms (± 1.05%)}	_~	_322ms	_331ms	_{p=0.747 n=6}
_{Req 4 - navto}	_{276ms (± 1.14%)}	_{277ms (± 1.01%)}	_~	_271ms	_278ms	_{p=0.652 n=6}
_{Req 5 - completionInfo count}	_{1,356 (± 0.00%)}	_{1,356 (± 0.00%)}	_~	_1,356	_1,356	_{p=1.000 n=6}
_{Req 5 - completionInfo}	_{86ms (± 5.64%)}	_{85ms (± 4.64%)}	_~	_83ms	_93ms	_{p=1.000 n=6}
_{CompilerTSServer - node (v18.15.0, x64)}
_{Req 1 - updateOpen}	_{2,483ms (± 0.93%)}	_{2,486ms (± 0.65%)}	_~	_2,461ms	_2,500ms	_{p=0.630 n=6}
_{Req 2 - geterr}	_{4,138ms (± 1.99%)}	_{4,190ms (± 1.91%)}	_~	_4,084ms	_4,254ms	_{p=0.298 n=6}
_{Req 3 - references}	_{338ms (± 1.82%)}	_{337ms (± 1.04%)}	_~	_333ms	_341ms	_{p=1.000 n=6}
_{Req 4 - navto}	_{285ms (± 0.88%)}	_{284ms (± 0.37%)}	_~	_282ms	_285ms	_{p=0.139 n=6}
_{Req 5 - completionInfo count}	_{1,518 (± 0.00%)}	_{1,518 (± 0.00%)}	_~	_1,518	_1,518	_{p=1.000 n=6}
_{Req 5 - completionInfo}	_{84ms (± 6.80%)}	_{85ms (± 6.44%)}	_~	_78ms	_90ms	_{p=0.466 n=6}
_{xstateTSServer - node (v18.15.0, x64)}
_{Req 1 - updateOpen}	_{2,604ms (± 0.88%)}	_{2,599ms (± 0.69%)}	_~	_2,575ms	_2,617ms	_{p=0.376 n=6}
_{Req 2 - geterr}	_{1,728ms (± 2.64%)}	_{1,721ms (± 1.55%)}	_~	_1,689ms	_1,762ms	_{p=0.689 n=6}
_{Req 3 - references}	_{115ms (± 9.20%)}	_{113ms (±10.21%)}	_~	_102ms	_123ms	_{p=1.000 n=6}
_{Req 4 - navto}	_{367ms (± 1.21%)}	_{365ms (± 0.83%)}	_~	_361ms	_369ms	_{p=0.361 n=6}
_{Req 5 - completionInfo count}	_{2,073 (± 0.00%)}	_{2,073 (± 0.00%)}	_~	_2,073	_2,073	_{p=1.000 n=6}
_{Req 5 - completionInfo}	_{310ms (± 1.88%)}	_{310ms (± 1.27%)}	_~	_303ms	_314ms	_{p=0.574 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

CompilerTSServer - node (v18.15.0, x64)
Compiler-UnionsTSServer - node (v18.15.0, x64)
xstateTSServer - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Startup

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{tsc-startup - node (v18.15.0, x64)}
_{Execution time}	_{153.24ms (± 0.21%)}	_{153.09ms (± 0.18%)}	_{-0.14ms (- 0.09%)}	_152.01ms	_155.15ms	_{p=0.000 n=600}
_{tsserver-startup - node (v18.15.0, x64)}
_{Execution time}	_{229.09ms (± 0.18%)}	_{228.88ms (± 0.16%)}	_{-0.21ms (- 0.09%)}	_227.63ms	_234.32ms	_{p=0.000 n=600}
_{tsserverlibrary-startup - node (v18.15.0, x64)}
_{Execution time}	_{230.73ms (± 0.21%)}	_{230.71ms (± 0.20%)}	_~	_228.64ms	_241.90ms	_{p=0.848 n=600}
_{typescript-startup - node (v18.15.0, x64)}
_{Execution time}	_{230.19ms (± 0.19%)}	_{230.09ms (± 0.18%)}	_~	_228.52ms	_233.27ms	_{p=0.075 n=600}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

tsc-startup - node (v18.15.0, x64)
tsserver-startup - node (v18.15.0, x64)
tsserverlibrary-startup - node (v18.15.0, x64)
typescript-startup - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

typescript-bot · 2024-01-05T20:05:50Z

@sandersn Here are the results of running the user test suite comparing main and refs/pull/56859/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Package install failed"

Otherwise...

Something interesting changed - please have a look.

Details

puppeteer

packages/browsers/test/src/tsconfig.json

[MISSING] error TS2307: Cannot find module '@pptr/testserver' or its corresponding type declarations.
- packages/browsers/test/src/utils.ts#L13 in packages/browsers/test/src/tsconfig.json

typescript-bot · 2024-01-05T20:10:37Z

Hey @sandersn, the results of running the DT tests are ready.
Everything looks the same!
You can check the log here.

typescript-bot · 2024-01-05T20:41:23Z

@sandersn Here are the results of running the top-repos suite comparing main and refs/pull/56859/merge:

Everything looks good!

sandersn · 2025-07-30T21:55:20Z

It looks like we decided in a design meeting that this is a good change, but it still needs more tests:

test multiply-nested cases
test overloads
test async functions, generators, async generators

@Andarist do you want to finish up this PR by adding those tests?

…-vs-const-type-variables

Copilot

Pull Request Overview

This PR fixes TypeScript's type inference for const type parameters by preserving literal types when functions with const type parameters return or yield literal values. Previously, const type parameters weren't properly maintaining literal types in return statements and yield expressions.

Key changes:

Fixed literal type preservation for return expressions in functions with const type parameters
Fixed literal type preservation for yield expressions in generator functions
Ensured const context is properly applied to returned and yielded values

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
`tests/cases/conformance/types/typeParameters/typeParameterLists/typeParameterConstModifiersReturnsAndYields.ts`	Test cases covering const type parameters with returns, yields, and generator functions
`tests/baselines/reference/typeParameterConstModifiersReturnsAndYields.types`	Expected type baseline showing literal types are preserved
`tests/baselines/reference/typeParameterConstModifiersReturnsAndYields.symbols`	Expected symbol baseline for the test cases
`src/compiler/checker.ts`	Core compiler fix adding const context checks for returns and yields

Comments suppressed due to low confidence (3)

Andarist · 2025-08-09T22:36:00Z

@sandersn I added some new test cases here. Please take a look and see if those are satisfactory.

Keep returned (and yielded) literal types as const when their types u…

6ad94f1

…sing `const` type variables

typescript-bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Dec 23, 2023

add an extra test case with reverse mapped types

f057c90

Andarist mentioned this pull request Dec 24, 2023

Type-level narrowing constraints (like as const) #53813

Open

5 tasks

sandersn requested a review from ahejlsberg January 3, 2024 14:32

typescript-bot removed the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Jan 3, 2024

sandersn assigned ahejlsberg Jan 3, 2024

typescript-bot added the For Backlog Bug PRs that fix a backlog bug label Jan 3, 2024

sandersn requested changes Jan 3, 2024

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src/compiler/checker.ts Outdated Show resolved Hide resolved

Andarist added 2 commits January 3, 2024 18:24

Merge remote-tracking branch 'origin/main' into fix/returned-literals…

f3e57d6

…-vs-const-type-variables

address review feedback

78831c2

sandersn assigned weswigham and unassigned ahejlsberg Jan 5, 2024

sandersn requested review from weswigham and removed request for ahejlsberg January 5, 2024 19:31

Andarist mentioned this pull request Jun 11, 2024

Design Meeting Notes, 1/5/2024 #56965

Closed

Merge remote-tracking branch 'origin/main' into fix/returned-literals…

b43558e

…-vs-const-type-variables

Copilot AI review requested due to automatic review settings August 9, 2025 21:42

Copilot AI reviewed Aug 9, 2025

View reviewed changes

add test

913d009

sandersn approved these changes Aug 11, 2025

View reviewed changes

github-project-automation bot moved this from Waiting on author to Needs merge in PR Backlog Aug 11, 2025

Keep returned (and yielded) literal types as const when their types using const type variables #56859

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Keep returned (and yielded) literal types as const when their types using const type variables #56859

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Compiler

tsserver

Startup

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puppeteer

packages/browsers/test/src/tsconfig.json

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Pull Request Overview

Reviewed Changes

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Keep returned (and yielded) literal types as const when their types using `const` type variables #56859

Keep returned (and yielded) literal types as const when their types using `const` type variables #56859

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