[Marc Rousavy (mrousavy)]

The native checker hangs while checking a function that returns a namespace imported from a CommonJS module whose export is undefined.

// target.cjs
module.exports = undefined
// bridge.mjs
import * as ns from './target.cjs'; export function read() { return ns }

// main.ts
import { read } from './bridge.mjs'; read();

resolveAliasWithDeprecationCheck reaches a non-nil declaration-free intrinsic undefined symbol. Its declaration-bearing branch neither advances nor exits for this target. Require declarations before continuing that branch; the existing else terminates the walk and retains the resolved synthetic namespace target. Declared alias/deprecation behavior and both identity exits remain covered.

This changes one production function and adds three compiler fixtures, one declared-alias/deprecation/type-error control and six generated type/symbol baselines. The namespace baselines retain { default: undefined; }. The original cycle and an unused-namespace control are included alongside the reduction.

AI assistance was used to investigate and prepare this patch and its tests. No human personal-review or shepherding claim is made. This draft is offered for maintainer review or adaptation; the repository-wide gates below remain unrun.

Tested on exact main 056d69edb807d4cd4351059f084a7f191c6aea0f, Go1.27.4, Node24.21.0 and pinned npm11.21.1:

go -C ./tsc test -count=1 -run=TestLocal/aliasToDeclarationFreeUndefined -v ./internal/testrunner
go -C ./tsc test -count=1 -run=TestDeclaredAliasDeprecationAndTypeError -v ./internal/checker

All three maintained fixtures and 27 leaf checks pass with their six generated reference baselines. The declared control retains TS2322 and four deprecation suggestions. Scoped pinned dprint/gofumpt checks pass for the two Go files; testdata follows the repository's formatting exclusion.

Removing only the production guard makes the reduction and original cycle time out, with native stacks in the alias deprecation walk. Restoring it gives a fresh passing focused run. The unused control completes its checks. No timeout or oracle was relaxed for the candidate. Initial missing-baseline, assertion int/int32 and formatting failures were corrected before the final passing runs; the generated baselines were accepted through the normal harness task.

Remaining gates:

• Associated issue and Backlog triage.
• Whole-repository validation, lint, format and tidy.
• Full test suite and distribution checks.
• Contributor License Agreement handling.

No new published fixed package or broad language-server/API qualification is claimed.

[Marc Rousavy (mrousavy)]

@microsoft-github-policy-service agree

[typescript-automation]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Copilot]

🟢 Approval recommended

No active review issues are reported for the fix to the checker hang during CommonJS namespace resolution.

No open issues 🎉

What changed in this PR

Fixes a native TypeScript checker hang when resolving a namespace imported from a CommonJS module that exports undefined.

Changes:

• Stops alias deprecation walks at declaration-free targets while preserving the resolved namespace.
• Adds reduced, cyclic-import, and unused-namespace compiler fixtures with type and symbol baselines.
• Adds a checker control test for declared-alias deprecation suggestions and TS2322 diagnostics.

File	Description
tsc/​internal/​checker/​checker.go	Terminates alias walks when the target has no declarations.
tsc/​internal/​checker/​alias_terminal_test.go	Checks declared-alias deprecations and an assignment type error.
tsc/​testdata/​tests/​cases/​compiler/​aliasToDeclarationFreeUndefined.ts	Adds the minimal hang regression.
tsc/​testdata/​tests/​cases/​compiler/​aliasToDeclarationFreeUndefinedCycle.ts	Covers the original cyclic-import scenario.
tsc/​testdata/​tests/​cases/​compiler/​aliasToDeclarationFreeUndefinedUnused.ts	Adds an unused-namespace control.
tsc/​testdata/​baselines/​reference/​compiler/​aliasToDeclarationFreeUndefined.types	Records the reduced case’s namespace and return types.
tsc/​testdata/​baselines/​reference/​compiler/​aliasToDeclarationFreeUndefined.symbols	Records symbol resolution for the reduced case.
tsc/​testdata/​baselines/​reference/​compiler/​aliasToDeclarationFreeUndefinedCycle.types	Records inferred types across the import cycle.
tsc/​testdata/​baselines/​reference/​compiler/​aliasToDeclarationFreeUndefinedCycle.symbols	Records symbol resolution across the import cycle.
tsc/​testdata/​baselines/​reference/​compiler/​aliasToDeclarationFreeUndefinedUnused.types	Records types for the unused-namespace control.
tsc/​testdata/​baselines/​reference/​compiler/​aliasToDeclarationFreeUndefinedUnused.symbols	Records symbols for the unused-namespace control.

_{Review effort: Balanced}