Conflicting trait implementation despite type restriction

Asked Aug 29 '23 at 08:29

Active Aug 29 '23 at 08:36

Viewed 23 times

Consider the following:

struct Foo(f32);

impl<T> std::convert::From<T> for Foo
where f32: std::convert::From<T>
{
    fn from(value : T) -> Self {
        Self(value.into())
    }
}

fn main() {
    let a: Foo = 42.into();
}

This of course doesn't work because

the trait bound f32: From<i32> is not satisfied

So I'm trying to add another trait implementation with less constraints:

impl std::convert::From<i32> for Foo
{
    fn from(value : i32) -> Self {
        Self(value as f32)
    }
}

Which then gives me

conflicting implementations of trait From<i32> for type Foo

What's the issue here, shouldn't the generic instantiation be discard for i32 because the type requirement failed, thus allowing another explicit implementation for From<i32>?

edited Aug 29 '23 at 08:35

Frederiko Ribeiro

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asked Aug 29 '23 at 08:29

Jaffa

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_Currently_ `From for f32` is not implemented, but std may implement it at any time. – Chayim Friedman Aug 29 '23 at 08:34
3

Also, always look at the whole error from `cargo check`: ```note: upstream crates may add a new impl of trait `std::convert::From` for type `f32` in future versions```. – Chayim Friedman Aug 29 '23 at 08:36

Conflicting trait implementation despite type restriction

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