Behavior difference: declaration emit synthesizes extensionless `import()` specifier under `allowImportingTsExtensions`, invalid in nodenext

[dario-causaly]

When a project uses allowImportingTsExtensions-style relative imports (import { x } from './helper.ts'), tsgo's declaration emit prints synthesized import() type references to package subpaths without their file extension. Under --module nodenext the extensionless specifier does not resolve, so the emitted .d.ts is invalid. Emit succeeds with no diagnostic.

Reproduced with typescript@7.0.2 and nightly @typescript/native-preview@7.0.0-dev.20260707.2.

Steps to reproduce

Self-contained — no npm install; the dependency is materialized by the script (a plain CJS package with no exports map, imported via an explicit-extension subpath, like fp-ts among others):

mkdir repro && cd repro && mkdir -p src node_modules/mylib/lib

cat > package.json <<'EOF'
{ "name": "repro", "type": "module" }
EOF

cat > tsconfig.json <<'EOF'
{
  "compilerOptions": {
    "module": "nodenext",
    "declaration": true,
    "emitDeclarationOnly": true,
    "allowImportingTsExtensions": true,
    "rootDir": "src",
    "outDir": "out"
  },
  "include": ["src"]
}
EOF

cat > node_modules/mylib/package.json <<'EOF'
{ "name": "mylib", "version": "1.0.0", "main": "./lib/index.js" }
EOF

cat > node_modules/mylib/lib/Box.js <<'EOF'
exports.box = (value) => ({ value });
EOF

cat > node_modules/mylib/lib/Box.d.ts <<'EOF'
export interface Box<T> { readonly value: T }
export declare function box<T>(value: T): Box<T>;
EOF

cat > src/helper.ts <<'EOF'
import * as B from 'mylib/lib/Box.js';
export const inner = B.box('hello');
EOF

cat > src/mod.ts <<'EOF'
import { inner } from './helper.ts';
export const out = inner;
EOF

Then compile with each compiler and inspect out/mod.d.ts, as shown in the two sections below.

Behavior with typescript@6.0

npx -y -p typescript@6.0.2 tsc -p tsconfig.json
cat out/mod.d.ts

Emits a resolvable specifier:

export declare const out: import("mylib/lib/Box.js").Box<string>;

Behavior with tsgo

npx -y -p typescript@7.0.2 tsc -p tsconfig.json
cat out/mod.d.ts

The extension is dropped:

export declare const out: import("mylib/lib/Box").Box<string>;

The trigger is the .ts-extension relative import

One-character bisect: change src/mod.ts to import './helper.js' instead of './helper.ts' and tsgo emits the extension correctly. Generics, type aliases, and rewriteRelativeImportExtensions were all eliminated as factors.

The emitted file is invalid

cat > tsconfig.verify.json <<'EOF'
{
  "compilerOptions": { "module": "nodenext", "noEmit": true, "skipLibCheck": false },
  "include": ["out/mod.d.ts"]
}
EOF
npx -y -p typescript@7.0.2 tsc -p tsconfig.verify.json

out/mod.d.ts(1,34): error TS2307: Cannot find module 'mylib/lib/Box' or its corresponding type declarations.

(typescript@6.0.2 rejects it identically)

In a monorepo with skipLibCheck: true this is silent — the broken reference degrades to an error type and surfaces as unrelated-looking inference failures ('x' is of type 'unknown') in downstream packages.

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Disclosure: this bug was found, minimized, and verified interactively by a human (me) working with Claude Code; the write-up was drafted by the model and reviewed by me. All commands and outputs above were executed and confirmed against the stated versions.