Behavior difference: import * as x of an export = callable loses call signatures without esModuleInterop (TS2349)

[greg-flux]

Behavior difference: import * as x of an export = callable loses its call signatures without esModuleInterop (TS2349)

🔎 Search terms

import star, export equals, export =, namespace import, not callable, TS2349, esModuleInterop, allowSyntheticDefaultImports, synthetic default, getSpreadType, resolveESModuleSymbol

🕗 Version & Regression Information

• Reproduces on 7.0.0-dev.20260707.2 (and every earlier nightly tried).
• Behavior differs from tsc (TypeScript 5.7).

⏯ Playground / Repro

tsc accepts, tsgo reports TS2349. No esModuleInterop.

mod.d.ts

declare const thing: {
  (x: string): number;
  prop: boolean;
};
export = thing;

use.ts

import * as thing from './mod';
const a: number = thing('hello');   // tsgo: TS2349 — This expression is not callable
const b: boolean = thing.prop;

tsconfig.json

{
  "compilerOptions": {
    "module": "commonjs",
    "allowSyntheticDefaultImports": true,
    "strict": false,
    "noEmit": true
  },
  "include": ["use.ts"]
}

🙁 Actual behavior (tsgo)

use.ts(2,19): error TS2349: This expression is not callable.
  Type '{ prop: boolean; default: { (x: string): number; prop: boolean; } }' has no call signatures.

tsgo synthesises a namespace type { prop; default } for the import * as
binding — it hoists the export = value to .default, spreads its properties,
and drops the call/construct signatures.

🙂 Expected behavior (tsc)

tsc produces no error — thing keeps its call signature and is callable. Only
when esModuleInterop: true (or an ESM→CJS reference) does tsc emit the same
TS2349. Real-world impact: any import * as request from 'supertest' /
import * as archiver from 'archiver' followed by request(...) / archiver(...)
fails under tsgo but is accepted by tsc — hundreds of errors across a normal
NestJS/e2e test suite that runs module: commonjs without esModuleInterop.

🌳 Root cause

resolveESModuleSymbol (internal/checker/checker.go) gates the synthetic-default
wrap on hasSignatures(typ) || hasDefault || isEsmCjsRef. tsc's
resolveESModuleSymbol gates it on getESModuleInterop(compilerOptions) || isEsmCjsRef
and only then checks for signatures/default. tsgo dropped the esModuleInterop
guard, so a callable export = is wrapped (and its signatures spread away by
getSpreadType) even when interop is off.

There is also no getESModuleInterop equivalent on CompilerOptions in tsgo, so
the implicit-true cases (module: node16/18/20/nodenext/preserve) aren't accounted
for at this call site.

✅ Suggested fix

Add CompilerOptions.GetESModuleInterop() mirroring tsc (explicit option, else
true for node16/18/20/nodenext/preserve), and gate the wrap on
GetESModuleInterop() || isEsmCjsRef with the hasSignatures || hasDefault check
moved inside. Verified against the local conformance suite (TestLocal) — passes,
with one cosmetic symbol-baseline shift (globalArrayAugmentation…:
foo.items→items, which is the un-wrapped, tsc-correct form). PR to follow.

[jakebailey]

without esModuleInterop

This is an unsupported config; we default to true, no?