Error handling with generics and traits

I’m trying to use the “multiple error type into one using enum” approach to handle associated errors on types I use in my library, accessed only through generics and traits (in other words I do not know, or control what actual types are going to be used here).

The playground link with some comments is here:

The error I’m getting is: conflicting implementations of traitstd::convert::From<UnifiedError<_, >>for typeUnifiedError<, _>:

How can I make use of From for these generics/associated types such that ? would work? Do I have to use manual boilerplate and construct the error enum manually for each operation ala if func().is_ok() { return Err(MyError::fromSecondGeneric(...) } ??

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In the following trait implementation

// why is there a conflicting implementation in core???
impl<S1, S2> From<S1::Error> for UnifiedError<S1, S2>
    S1: SomeTrait,
    S2: SomeTrait,

What if we have a type S1 whose associated type Error is UnifiedError<U, V> ? In original snippet of code there is no-where such restriction is specified.

And it looks like we cannot do negative trait bound, atleast not yet.

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