Seemingly wrong arguments object when both es6 and commonjs options are present

TypeScript Version: 2.4.2

Code

export const foo = (bar: number) => {
    console.log(arguments)
}

Expected behavior:

The code either produces errors or prints the arguments object of foo consistently in all targets and module types

Actual behavior:

• When the module is built with tsc bug.ts command line, error TS2496: The 'arguments' object cannot be referenced in an arrow function is produced and no code is generated.
• With tsc -t es6 bug.ts it builds an es6 module and the code prints the arguments of foo at runtime
• With tsc -t es6 -m commonjs the generatesd commonjs module prints a strange object with 4 arguments, which seems to be an implementation detail of commonjs modules in node.js.

So I’d like at least to produce a warning in the third case, because I hit this problem in my program and the resulting bug was hard to locate.

[RyanCavanaugh]

We introduced the error rather than alias arguments because early versions of TS (created before the arrow function spec was finalized) did allow use of arguments in arrow functions, so changing to capturing the outer arguments value instead would have changed the behavior of the emitted code (extremely subtle and dangerous).

The plan was to introduce this error, wait “a while”, then see if anyone cared enough for us to bother implementing the aliasing (seems like the answer is ‘no’).

[vladima]

I would argue that TS should error here (assuming that initial source file is ES6 module). Per spec: ModuleDeclarationEnvironmentSetup sets lexical environment for the module to be NewModuleEnvironment parented by GlobalEnvironment and non of these will have bindings to arguments object (that is created only during FunctionDeclarationInstantiation). The fact that in cjs case instantiation of the module happens inside some function call and arguments is available is an implementation details which should not be applied to behavior described in spec.