Catch improper use of monadic return in async

**Submitted by William Blum on 10/17/2015 12:00:00 AM**
**11 votes on UserVoice prior to migration**

The following code should issue a type checking error since `return ()` should yield an `Async` and return 42 should yield an `Async`.

``` fsharp
let f c =
async {
return ()
printfn "You passed %A and I am returning 42" c
return 42
}
```

Additionally the following code should issue a warning since the return statement does not actually affect the flow of execution as the name suggests (The printf statement is actually executed).

``` fsharp
let g c =
async {
return ()
printfn "You passed %A and I am returning 42" c
return ()
}
```

**[Original UserVoice Submission](https://fslang.uservoice.com/forums/245727-f-language/suggestions/10256475)**
[Archived Uservoice Comments](../tree/master/archive/suggestion-10256475-catch-improper-use-of-monadic-return-in-async.md#comments)

Start with the two async examples in the issue and review the archived UserVoice comments for prior discussion. Determine the intended type-checking error for mixed return types and the warning for unreachable-looking returns; done means both diagnostics are produced consistently.

Written by the indexing model from the issue text.