Fixed mapped types not being considered as homomorphic with substitution constraints

[Mateusz Burzyński (Andarist)]

fixes microsoft/TypeScript#63132

[Copilot]

Pull request overview

This PR fixes a TypeScript checker behavior in the Go port so that mapped types remain homomorphic even when their keyof T constraint flows through a substitution type (matching the regression described in microsoft/TypeScript#63132). It also adds a targeted compiler regression test and commits the corresponding reference baselines.

Changes:

• Unwrap substitution types when detecting a homomorphic mapped type variable, so keyof T extends ... ? { [K in keyof T]: ... } : ... continues to be recognized as homomorphic.
• Add a new compiler test case reproducing the scenario from the linked upstream issue.
• Add committed .types and .symbols reference baselines for the new test.

Reviewed changes

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

File	Description
internal/checker/checker.go	Adjusts homomorphic mapped type detection to use the “actual” (unsubstituted) type variable.
testdata/tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts	New regression test covering homomorphic behavior through conditional wrapping.
testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.types	Reference type baseline for the new test.
testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.symbols	Reference symbol baseline for the new test.

[Jake Bailey (jakebailey)]

TypeScript Bot (@typescript-bot) test it

[typescript-automation]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
test top400	✅ Started	👀 Results
perf test this faster	✅ Started	👀 Results

[typescript-automation]

Jake Bailey (@jakebailey)
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

Metric	baseline	pr	Delta	Best	Worst	p-value
Compiler-Unions - native
Errors	41	41	~	~	~	p=1.000 n=12
Symbols	115,480	115,480	~	~	~	p=1.000 n=12
Types	96,945	96,945	~	~	~	p=1.000 n=12
Memory Used	149,032k (± 0.61%)	149,199k (± 0.37%)	~	147,892k	151,190k	p=0.590 n=12
Memory Allocs	2,272,044 (± 0.00%)	2,271,952 (± 0.00%)	~	2,271,785	2,272,168	p=0.198 n=12
Config Time	0.000s	0.000s	~	~	~	p=1.000 n=12
Parse Time	0.040s (± 5.52%)	0.041s (± 3.56%)	~	0.038s	0.044s	p=0.170 n=12
Bind Time	0.014s (±12.11%)	0.012s (±10.62%)	~	0.011s	0.018s	p=0.231 n=12
Check Time	0.488s (± 0.81%)	0.492s (± 0.87%)	~	0.481s	0.507s	p=0.297 n=12
Emit Time	0.272s (± 1.73%)	0.268s (± 1.58%)	~	0.255s	0.281s	p=0.192 n=12
Total Time	0.821s (± 0.53%)	0.820s (± 0.69%)	~	0.809s	0.838s	p=0.679 n=12
angular-1 - native
Errors	3	3	~	~	~	p=1.000 n=12
Symbols	847,058 (± 0.08%)	847,226 (± 0.07%)	~	845,783	848,492	p=0.551 n=12
Types	249,913 (± 0.00%)	249,911 (± 0.00%)	~	249,910	249,915	p=0.054 n=12
Memory Used	793,328k (± 0.06%)	793,307k (± 0.09%)	~	792,309k	796,267k	p=0.799 n=12
Memory Allocs	13,038,905 (± 0.05%)	13,035,618 (± 0.02%)	~	13,031,041	13,041,920	p=0.443 n=12
Config Time	0.016s (± 1.13%)	0.016s (± 1.13%)	~	0.016s	0.017s	p=1.000 n=12
Parse Time	0.278s (± 2.80%)	0.280s (± 2.40%)	~	0.259s	0.293s	p=0.639 n=12
Bind Time	0.063s (± 7.75%)	0.062s (± 7.23%)	~	0.058s	0.083s	p=0.971 n=12
Check Time	0s	0s	~	~	~	p=1.000 n=12
Emit Time	1.641s (± 1.05%)	1.631s (± 0.76%)	~	1.612s	1.685s	p=0.368 n=12
Total Time	2.015s (± 1.19%)	2.007s (± 0.87%)	~	1.972s	2.077s	p=0.887 n=12
mui-docs - native
Errors	11,274 (± 0.05%)	11,280 (± 0.02%)	~	11,275	11,282	p=0.099 n=12
Symbols	4,602,406	4,602,406	~	~	~	p=1.000 n=12
Types	1,639,118	1,639,118	~	~	~	p=1.000 n=12
Memory Used	5,614,823k (± 0.02%)	5,614,123k (± 0.03%)	~	5,610,308k	5,619,648k	p=0.347 n=12
Memory Allocs	53,528,405 (± 0.03%)	53,522,370 (± 0.04%)	~	53,479,002	53,577,370	p=0.443 n=12
Config Time	0.016s (± 1.14%)	0.016s (± 1.81%)	~	0.015s	0.016s	p=0.590 n=12
Parse Time	0.558s (± 1.91%)	0.549s (± 2.19%)	~	0.516s	0.582s	p=0.355 n=12
Bind Time	0.002s	0.002s	~	~	~	p=1.000 n=12
Check Time	17.200s (± 0.37%)	17.224s (± 0.43%)	~	17.043s	17.355s	p=0.418 n=12
Emit Time	0.457s (± 2.73%)	0.454s (± 2.78%)	~	0.436s	0.479s	p=0.968 n=12
Total Time	18.967s (± 0.38%)	18.994s (± 0.46%)	~	18.788s	19.149s	p=0.418 n=12
strada-build-src - native
Errors	0	0	~	~	~	p=1.000 n=12
Symbols	1,394,411	1,394,411	~	~	~	p=1.000 n=12
Types	443,627	443,627	~	~	~	p=1.000 n=12
Memory Used	1,649,226k (± 0.27%)	1,649,935k (± 0.35%)	~	1,631,111k	1,662,554k	p=0.887 n=12
Memory Allocs	97,227,912 (± 0.04%)	97,204,615 (± 0.04%)	~	97,110,379	97,310,198	p=0.347 n=12
Config Time	0.004s (±16.26%)	0.003s (±19.95%)	~	0.002s	0.006s	p=0.654 n=12
Parse Time	0.193s (± 2.72%)	0.201s (± 2.25%)	🔻+0.009s (+ 4.46%)	0.185s	0.209s	p=0.023 n=12
Bind Time	0.000s	0.000s	~	~	~	p=1.000 n=12
Check Time	1.782s (± 0.46%)	1.780s (± 0.41%)	~	1.762s	1.796s	p=0.744 n=12
Emit Time	0.280s (± 5.76%)	0.282s (± 6.50%)	~	0.243s	0.332s	p=1.000 n=12
Total Time	24.522s (± 0.79%)	24.652s (± 0.87%)	~	24.314s	25.463s	p=0.551 n=12
strada-compiler - native
Errors	0	0	~	~	~	p=1.000 n=12
Symbols	337,618	337,618	~	~	~	p=1.000 n=12
Types	198,941	198,941	~	~	~	p=1.000 n=12
Memory Used	319,686k (± 0.07%)	319,609k (± 0.02%)	~	319,436k	319,809k	p=0.843 n=12
Memory Allocs	4,676,745 (± 0.01%)	4,676,702 (± 0.01%)	~	4,675,500	4,678,242	p=0.887 n=12
Config Time	0.001s	0.001s	~	~	~	p=1.000 n=12
Parse Time	0.118s (± 4.84%)	0.114s (± 3.26%)	~	0.105s	0.124s	p=0.202 n=12
Bind Time	0.000s	0.000s	~	~	~	p=1.000 n=12
Check Time	1.034s (± 0.36%)	1.029s (± 0.35%)	-0.005s (- 0.48%)	1.023s	1.042s	p=0.029 n=12
Emit Time	0.125s (±12.74%)	0.147s (± 7.16%)	~	0.101s	0.163s	p=0.097 n=12
Total Time	1.336s (± 1.14%)	1.340s (± 0.72%)	~	1.310s	1.361s	p=0.469 n=12
ts-pre-modules - native
Errors	87	87	~	~	~	p=1.000 n=12
Symbols	305,341	305,341	~	~	~	p=1.000 n=12
Types	182,038	182,038	~	~	~	p=1.000 n=12
Memory Used	277,362k (± 0.02%)	277,351k (± 0.02%)	~	277,185k	277,487k	p=0.854 n=12
Memory Allocs	1,644,397 (± 0.01%)	1,644,392 (± 0.01%)	~	1,644,096	1,644,770	p=0.843 n=12
Config Time	0.000s (±113.79%)	0.000s	~	~	~	p=0.217 n=12
Parse Time	0.099s (± 4.56%)	0.100s (± 3.38%)	~	0.092s	0.109s	p=0.722 n=12
Bind Time	0.036s (±11.42%)	0.037s (±12.24%)	~	0.028s	0.050s	p=0.722 n=12
Check Time	0.817s (± 0.69%)	0.816s (± 0.77%)	~	0.803s	0.834s	p=0.831 n=12
Emit Time	0.000s	0.000s	~	~	~	p=1.000 n=12
Total Time	0.967s (± 0.69%)	0.968s (± 0.71%)	~	0.950s	0.986s	p=0.560 n=12
vscode - native
Errors	0	0	~	~	~	p=1.000 n=12
Symbols	7,930,740	7,930,740	~	~	~	p=1.000 n=12
Types	2,712,367	2,712,367	~	~	~	p=1.000 n=12
Memory Used	5,313,282k (± 0.01%)	5,313,892k (± 0.02%)	~	5,312,075k	5,317,562k	p=0.590 n=12
Memory Allocs	41,082,165 (± 0.01%)	41,084,873 (± 0.01%)	~	41,077,510	41,110,174	p=0.590 n=12
Config Time	0.061s (± 0.53%)	0.061s (± 0.53%)	~	0.061s	0.062s	p=1.000 n=12
Parse Time	1.622s (± 1.57%)	1.616s (± 2.51%)	~	1.477s	1.700s	p=1.000 n=12
Bind Time	0.335s (± 0.37%)	0.376s (±17.69%)	🔻+0.041s (+12.33%)	0.333s	0.699s	p=0.030 n=12
Check Time	9.647s (± 0.59%)	9.580s (± 1.43%)	~	9.054s	9.700s	p=0.989 n=12
Emit Time	3.051s (± 4.61%)	3.273s (±11.86%)	🔻+0.222s (+ 7.26%)	2.978s	4.797s	p=0.034 n=12
Total Time	14.794s (± 0.54%)	14.989s (± 1.91%)	~	14.750s	16.195s	p=0.110 n=12
webpack - native
Errors	2	2	~	~	~	p=1.000 n=12
Symbols	1,380	1,380	~	~	~	p=1.000 n=12
Types	340	340	~	~	~	p=1.000 n=12
Memory Used	199,009k (± 0.06%)	199,071k (± 0.06%)	~	198,714k	199,343k	p=0.443 n=12
Memory Allocs	843,155 (± 0.08%)	843,268 (± 0.14%)	~	841,207	848,076	p=0.932 n=12
Config Time	0.008s	0.008s	~	~	~	p=1.000 n=12
Parse Time	0.210s (± 2.89%)	0.209s (± 2.89%)	~	0.190s	0.227s	p=0.522 n=12
Bind Time	0s	0s	~	~	~	p=1.000 n=12
Check Time	0s	0s	~	~	~	p=1.000 n=12
Emit Time	0.000s	0.000s	~	~	~	p=1.000 n=12
Total Time	0.219s (± 2.77%)	0.217s (± 2.81%)	~	0.198s	0.235s	p=0.540 n=12
xstate-main - native
Errors	0	0	~	~	~	p=1.000 n=12
Symbols	1,020,005	1,020,005	~	~	~	p=1.000 n=12
Types	376,891	376,891	~	~	~	p=1.000 n=12
Memory Used	603,497k (± 0.02%)	603,549k (± 0.01%)	~	603,402k	603,735k	p=0.378 n=12
Memory Allocs	4,751,663 (± 0.03%)	4,751,274 (± 0.03%)	~	4,748,046	4,754,055	p=0.799 n=12
Config Time	0.003s	0.003s (± 6.23%)	~	0.002s	0.003s	p=1.000 n=12
Parse Time	0.143s (± 2.12%)	0.146s (± 3.87%)	~	0.131s	0.156s	p=0.520 n=12
Bind Time	0.050s (±15.33%)	0.045s (±17.58%)	~	0.033s	0.069s	p=0.245 n=12
Check Time	1.028s (± 0.56%)	1.015s (± 0.79%)	-0.013s (- 1.25%)	0.994s	1.037s	p=0.006 n=12
Emit Time	0.000s	0.000s	~	~	~	p=1.000 n=12
Total Time	1.236s (± 0.73%)	1.218s (± 0.95%)	-0.018s (- 1.42%)	1.191s	1.244s	p=0.025 n=12

System info unknown

Hosts

• native

Scenarios

• Compiler-Unions - native
• angular-1 - native
• mui-docs - native
• strada-build-src - native
• strada-compiler - native
• ts-pre-modules - native
• vscode - native
• webpack - native
• xstate-main - native

Benchmark	Name	Iterations
Current	pr	12
Baseline	baseline	12

Developer Information:

Download Benchmarks

[typescript-automation]

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/4596/merge:

Something interesting changed - please have a look.

Details

sindresorhus/type-fest

tsconfig.json

• error TS2345: Argument of type '{ tuple: [number]; } | { tuple: [number, string]; foo: any; }' is not assignable to parameter of type '{ tuple: [number]; }'.
  • test-d/shared-union-fields-deep.ts#L99
• error TS2345: Argument of type '{ tuple: boolean[]; } | { tuple: [number, string]; }' is not assignable to parameter of type '{ tuple: [number | boolean, string | boolean]; }'.
  • test-d/shared-union-fields-deep.ts#L105
• error TS2345: Argument of type '{ tuple: [number, string]; } | { tuple: [number, string, boolean]; }' is not assignable to parameter of type '{ tuple: [number, string]; }'.
  • test-d/shared-union-fields-deep.ts#L109
• error TS2345: Argument of type '{ tuple: [number, string]; } | { tuple: []; }' is not assignable to parameter of type '{ tuple: []; }'.
  • test-d/shared-union-fields-deep.ts#L112
• error TS2345: Argument of type '{ tuple: [number, string, number]; } | { tuple: [number, boolean, ...string[]]; }' is not assignable to parameter of type '{ tuple: [number, string | boolean, string | number]; }'.
  • test-d/shared-union-fields-deep.ts#L115
• error TS2345: Argument of type '{ tuple: [number, string]; } | { tuple: [number, string, boolean]; } | { tuple: number[]; }' is not assignable to parameter of type '{ tuple: [number, string | number]; }'.
  • test-d/shared-union-fields-deep.ts#L118
• error TS2345: Argument of type '{ tuple: [number, ...string[]]; } | { tuple: boolean[]; }' is not assignable to parameter of type '{ tuple: [number | boolean, ...(string | boolean)[]]; }'.
  • test-d/shared-union-fields-deep.ts#L122
• error TS2345: Argument of type '{ tuple: [number, ...string[]]; } | { tuple: [number, string, ...boolean[]]; }' is not assignable to parameter of type '{ tuple: [number, string, ...(string | boolean)[]]; }'.
  • test-d/shared-union-fields-deep.ts#L125

[Jake Bailey (jakebailey)]

Hmmmm, this is a break?

[Mateusz Burzyński (Andarist)]

Jake Bailey (@jakebailey) this is kinda expected.

A distributive homomorphic type requires to be written in this form (roughly):

type M<T> = {
  [K in keyof T]: T[K]
}

A type-fest-based repro case can be seen here. As we can see, that exact form ([K in keyof T]: ...) is used there. Therefore, I think the expectation should be that this would be treated as a distributive homomorphic type.

But because of the issue fixed by this PR, that mapped type was not recognized as homomorphic and distributive. That happened because keyof T in [K in keyof T] became (invisibly!) a substitution type internally. With the fix here, it gets recognized properly~ so it started to distribute and thus it broke type-fest's implementation as it was accidentally leveraging the incorrect behavior.

It's common to change the code a little bit to enable/disable distributivity of conditional types and I think the same principle kinda applied to mapped types, given the distributivity (or the lack of it) is also an implied trait of any mapped type.

I just pushed out a test documenting this and showcasing 2 possible workarounds for the type-fest's case, see this commit. If this PR lands then I will go to type-fest and PR a fix for them.