MatDialog `open` method `R` generic is too loose

[jelbourn]

Because the R generic on MatDialog.open isn't constrained by arguments to the function, it's effectively always any. We should remove the method generic and change the return generic to {}, which will force people to explicitly state which type it is when consuming it.

[crisbeto]

The reason R defaults to any was to avoid breaking changes at the time we introduced the argument.

[rkirov]

The issue is not with 'any', but with allowing R to be unsafely inferred to be anything. For example, the following always compiles.

let ref: MatDialogRef<T, SomeMadeUpType> = open(...);

It looks like any other code where the function call returned some type, but the developer choose to write it out for clarity.

By forcing the developer to write let ref = open(...) as MatDialogRef<T, SomeMadeUpType>; it becomes clear that this is an unsafe cast.