7.1 regression: comparing a recursive `-?` mapped type is quadratic in the number of properties and hits TS2589 (since typescript-go#4820)

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
48/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
go, typescript
Área
compilers, performance

Línea de trabajo

Start by reading the variance computation change in microsoft/typescript-go#4820, then reproduce the issue with the supplied one-file repro and tsc -p . --singleThreaded --extendedDiagnostics. Compare instantiation counts for the last good and first bad builds, focusing on the recursive Builder interface, its -? mapped type, and changing S | K argument. Done means the comparison stays near linear in property count and the 500-property repro succeeds without TS2589.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Bug
🔎 Search Terms

variance, TS2589, excessively deep, instantiation count, mapped type, -?, quadratic, 7.1 regression

🕗 Version & Regression Information
  • Last good: 7.1.0-dev.20260805.1 (gitHead 12318e599d)
  • First bad: 7.1.0-dev.20260806.1 (gitHead 86cc4767d4)
  • That range is one commit: microsoft/typescript-go#4820, "Order variance computation by associated type symbol".
  • Still failing on 7.1.0-beta and 7.1.0-dev.20261024.1.
  • 7.0.2 accepts the code.

Same commit as #64891 and #64930, but this one is about performance: the comparison finishes with a correct result
for small types, it just costs quadratically more instantiations, and on real types it hits the 5M cap.

⏯ Playground Link

The 7.1 builds aren't available in the Playground. CLI repro: one file in an empty directory, tsc -p . --singleThreaded --extendedDiagnostics.

tsconfig.json

{ "compilerOptions": { "strict": true, "noEmit": true, "types": [] }, "files": ["repro.ts"] }
💻 Code
type D = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
type Entity = Record<`f${'0' | '1' | '2' | '3' | '4'}${D}${D}`, string> // 500 properties

interface Method<T, K extends keyof T, S> {
	(): Builder<T, S | K>
}
type Builder<T, S = never> = { [K in keyof T]-?: Method<T, K, S> }

declare const builder: Builder<Entity>
export const widened: Builder<Entity & object> = builder
🙁 Actual behavior
repro.ts(10,14): error TS2589: Type instantiation is excessively deep and possibly infinite.
Instantiations: 5003892   Check time: 79 s
🙂 Expected behavior

No error, as in 7.0.2 and 7.1.0-dev.20260805.1: about 9k instantiations, 0.04 s check time.

Additional information about the issue

With 100 properties (Record<`f${D}${D}`, string>) there is no error, but the count goes from 5,105 to 256,939
instantiations. It grows with the square of the property count: about 66k at 50 properties, 1M at 200 and the 5M cap
near 460.

build 100 props: instantiations 500 props
7.0.2 5,105 9,105, ok
7.1.0-dev.20260805.1 (12318e599d) 5,105 9,105, ok
7.1.0-dev.20260806.1 (86cc4767d4) 256,939 TS2589
7.1.0-dev.20261024.1 258,143 TS2589

All three of these are needed for the quadratic count; removing any one brings it back to linear:

  • the -? modifier on the homomorphic mapped type;
  • Method as a generic interface that references Builder again (with an inline () => Builder<T, S | K> both
    7.0.2 and 7.1 are quadratic, so the regression is in how the interface's variance gets computed);
  • a type argument that actually changes, here S | K (with Builder<T, S> it stays linear).

The target also has to be a different but equivalent instantiation (Entity & object); a structurally identical
mapped copy of Entity does not trigger it.

Real-world impact: this is a reduced form of a fluent "selection builder" type from our library, whose
entity types resolve through UnionToIntersection<E> & object, so every selector is compared against
Builder<E & object>. On a ~2,100-file app under 7.1:

  • TS2589 appears on ordinary selector call sites, and the callbacks' destructured parameters cascade into TS7031/TS7006.
  • The depth error leaves the checker's caches in a bad state, so a later file that checks cleanly on its own reports
    TS2339 (members missing on the builder's result type). The error count depends on --checkers and on which files
    share a checker (0 to 77).
  • The project checks --singleThreaded in 25 s / 2.6 GB on 7.1.0-dev.20260805.1; on 7.1.0-dev.20260930.4 the
    same run was OOM-killed at an 18 GB cap, and with default checkers it reached 24 GB before being killed.
Lenguaje dominante
Go
Estrellas
112k
Forks
15.4k
Merge medio
3 d 1 h
PR fusionados (30 d)
136

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