Suggestion: disallow synthetic imports for ES modules

TypeScript Version: 3.6.3

Search Terms: allowSyntheticDefaultImports esModuleInterop babel synthetic es modules default exports imports commonjs

Code

allowSyntheticDefaultImports is great when you're importing a CommonJS module, where/when Babel/TS will make sure there is a default export/import via their "interop" layers (e.g. esModuleInterop).

// foo.js
exports.foo = () => {}

// foo.d.ts
declare const _default: { foo: () => {} }
export = _default

// main.ts
import Foo from './foo';
Foo.foo(); // all good!

However, it's possible to shoot yourself in the foot by trying to import a non-existent default from an ES module.

// foo.js
export const foo = () => {}

// foo.d.ts
export declare const foo: () => {}

// main.ts
import Foo from './foo'; // no error, but should be `* as Foo`!
// runtime error!!
// TypeError: Cannot read property 'foo' of undefined
Foo.foo();

Ideally it would not be possible to use synthetic default imports with an ES module, so runtime errors such as the one above would not happen.

Start with the TypeScript 3.6.3 repro showing allowSyntheticDefaultImports accepting a default import from an ES module, and compare it with the CommonJS example. Done means the invalid ES-module default import is rejected while the intended CommonJS synthetic-default case remains supported.