similar case:

Arg this destroyed this case:

@dsyme:

One of the issues is:

// f x
| SynExpr.App (hpa,_,func,arg,mFuncAndArg) ->
    TcExprThen cenv overallTy env tpenv func ((DelayedApp (hpa, arg, mFuncAndArg)):: delayed)

we want to keep the context for the result type check, but erase for x and for whaterver happens inside of f.

I think what we actually want to do is not set a context here at all. We just want to make the very last UnifyTypes call in that else expression aware of the situation

something like:

            let tempty = NewInferenceType ()
            let e3',tpenv = TcExprThatCanBeCtorBody cenv tempty env tpenv e3 

            let env = { env with eContextInfo = ContextInfo.ElseBranch }                 
            UnifyTypes cenv env e3.Range overallTy tempty

instead of:

            let env = { env with eContextInfo = ContextInfo.ElseBranch } 
            let e3',tpenv = TcExprThatCanBeCtorBody cenv overallTy env tpenv e3 

looks like #1827 (comment) is not enough since it forces us for type hints: cb3fa24

and there are coming more.

@dsyme is there another way to let the context only work for the return type check?

That sort of implementation looks much more robust (once it's green)

Ok this strategy didn't work, but now I think we can do this:

            let env = { env with eContextInfo = ContextInfo.ElseBranchResult e3.Range }
            let e3',tpenv = TcExprThatCanBeCtorBody cenv overallTy env tpenv e3 
            e3',SequencePointAtTarget,tpenv

and at the error reporting site we check if the type error comes from the right range. that's basically narrowing the context to a minimum.

This is how it felt:

But I think this is now the correct strategy and my manual checks are all working.

This is fixed as well 👍

this is ready for review. I had to disable the nice async error message for now since the range where the exception happens is inside the "return" part. So that can't be found right now. I leave that for F# 4.2

But this is now removign all the false positive warnings and should be merged to FCS as soon as possible since Ionide and Xamarin Studio? (cc @Krzysztof-Cieslak @nosami) are already using it.

All green. Yay. Finally