[Mateusz Burzyński (Andarist)]

inspired by tsrs's: oxc-project/tsrs#715

[typescript-automation]

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🟡 Changes recommended

The shortcut computes uncached containment results, introducing recursive work that a cache-only guard would avoid.

1 open issue

• Avoid recursive type-argument traversal for uncached containment checks · New

What changed in this PR

This PR aims to reduce compiler reference-narrowing work by skipping union traversal when a union cannot contain type variables, inspired by the linked tsrs optimization.

Changes:

• Routes the union-constraint check through a new helper.
• Adds a containment shortcut and documents why non-generic aliases remain eligible.

File	Description
tsc/​internal/​checker/​checker.go	Adds the union-containment shortcut to reference narrowing.

_{Review effort: Balanced}

[Copilot]

🔵 Needs a closer look

An unresolved narrowing regression and unvalidated hot-path performance tradeoff require further validation and human review.

No open issues 🎉

1 previously missed issue

In code that hasn't changed since last review

Preserve narrowing for NoInfer unions with substituted constraints

tsc/​internal/​checker/​checker.go:32305

For type U = NoInfer<{ kind: "a"; a: number } | { kind: "b"; b: number }> | { kind: "c" }, this guard skips required constraint substitution: couldContainTypeVariables(U) is false for the non-generic top-level alias, but the NoInfer constituent has a union base constraint. Consequently, for a parameter u: U, if (u.kind === "a") u.a retains the wrapped union instead of narrowing to the a branch. Preserve the walk when such substitutions may be present, or cache the exact predicate, and add a compiler regression for this case.

1 resolved since last review

• Avoid recursive type-argument traversal for uncached containment checks

_{Review effort: Balanced}