Type inference incorrect for "union of functions" types

[FremyCompany]

TypeScript Version: 3.1.2
Search Terms: function inference

Code

interface Prop<T> {
	(): T
	(v:T): void
}
function cachedCast<I,O>(
    input$: ()=>I, 
    convertInput: (i:I)=>O
): ()=>O { /*…*/return null as any/*…*/ }

let title$ : Prop<string>
let uppercase_title$ = cachedCast(title$, title=>title.toUpperCase()); // Error: `void.toUpperCase` is undefined

Expected behavior:

The title variable should be of type string

Actual behavior:

The title variable ends up of type void.

Playground Link: https://www.typescriptlang.org/play/index.html#src=interface%20Prop%3CT%3E%20%7B%0D%0A%09()%3A%20T%0D%0A%09(v%3AT)%3A%20void%0D%0A%7D%0D%0Afunction%20cachedCast%3CI%2CO%3E(%0D%0A%20%20%20%20input%24%3A%20()%3D%3EI%2C%20%0D%0A%20%20%20%20convertInput%3A%20(i%3AI)%3D%3EO%0D%0A)%3A%20()%3D%3EO%20%7B%20%2F*%E2%80%A6*%2Freturn%20null%20as%20any%2F*%E2%80%A6*%2F%20%7D%0D%0A%0D%0Alet%20title%24%20%3A%20Prop%3Cstring%3E%0D%0Alet%20uppercase_title%24%20%3D%20cachedCast(title%24%2C%20title%3D%3Etitle.toUpperCase())%3B%20%2F%2F%20Error%3A%20%60void.toUpperCase%60%20is%20undefined

Related Issues: I think this might be the same problem as issue #27027 but it is different in the sense that here I define an interface, not a function overload (though I guess it might be the same thing under the hood?)

I have the impression that this used to work at some point, but I might be wrong. Or maybe it used to be inferred as any and I never noticed?

[ghost]

Yeah, duplicate of #27027