"TypeError: Cannot read property 'flags' of undefined" when returning a non Promise custom type from an async function

[robotlovesyou]

I'm fairly sure this is unrelated to #5389

Returning a non-Promise custom type from an async function causes the compiler to crash.

Given this tsconfig

{
    "compilerOptions": {
        "experimentalAsyncFunctions": true,
        "target": "ES6"
    }
}

and this code

type SomeType = {
    someValue: string
}

async function someFunction(): SomeType {
    return {
        someValue: ''
    }
}

tsc will crash with the following

TypeError: Cannot read property 'flags' of undefined
    at isKnownProperty (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:14041:25)
    at hasExcessProperties (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:14063:26)
    at isRelatedTo (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:13955:25)
    at checkTypeRelatedTo (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:13903:26)
    at checkTypeAssignableTo (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:13886:20)
    at checkReturnStatement (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:19460:29)
    at checkSourceElement (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:20461:28)
    at Object.forEach (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:90:30)
    at checkBlock (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:18973:16)
    at checkSourceElement (/Users/andrewsmith/.nvm/versions/node/v0.12.6/lib/node_modules/typescript/lib/tsc.js:20440:28)

tsc -v returns message TS6029: Version 1.6.2

[DanielRosenwasser]

Might be a duplicate of #5115. Looks like this has been fixed for release-1.7 and master. You can try out our recent nightly builds by installing typescript@next on npm. Thanks for filing this @robotlovesyou.

[robotlovesyou]

Oops. Sorry. Only checked open issues. I'll test against a @next release before filing next time.