Blanket impls constrained by different associated types falsely flagged as conflicting

[boringcactus]

I tried this code (aggressively simplified from Diesel, with which I originally encountered this bug):

trait Backend {
    type RawValue;
}

trait FromSql<B: Backend> {}

impl<B: Backend<RawValue=()>> FromSql<B> for String {}
impl<B: Backend<RawValue=u8>> FromSql<B> for String {}

I expected to see this happen: this code compiles, because by definition nothing can implement both Backend<RawValue=()> and Backend<RawValue=u8>.

Instead, this happened: rustc gives error[E0119]: conflicting implementations of trait 'FromSql<_>' for type 'std::string::String'

Meta

This happens across stable/beta/nightly on the Rust playground.

[jonas-schievink]

Looks like a duplicate of #20400