Async arrow functions generate broken code when targeting es5

[nevir]

TypeScript Version: Tested on 1.8.9 and Version 1.9.0-dev.20160327

Code

let foo = async (_arg) => {};

Compiles to the following (when {"target": "es5"}):

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
    return new (P || (P = Promise))(function (resolve, reject) {
        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
        function rejected(value) { try { step(generator.throw(value)); } catch (e) { reject(e); } }
        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
        step((generator = generator.apply(thisArg, _arguments)).next());
    });
};
var foo = function (_arg) __awaiter(this, void 0, void 0, function* () { });

[Arnavion]

The compiler also gives an error:

foo.ts(1,11): error TS1057: An async function or method must have a valid awaitable return type.

It's by design that the compiler may emit some output (which may be garbage) even if there's an error. If you don't want the compiler to emit on error then use noEmitOnError: true

Edit: And note that the error is somewhat misleading since the real reason is that async functions aren't supported when targeting ES5.

[yortus]

I also reported this a month ago (#7334).

It's not specific to async arrow functions, it's when the async function is an expression (e.g. RHS of assignment) rather than a declaration.

@Arnavion that error TS1057 is due to an internal check that relies on the existence of Promise (which is not in ES5 by default). #7334 defines Promise, which unmasks the real problem - that no compiler errors are raised for async function expressions in ES5 even though the generated code is unusable.