Confusing error: cannot infer type for type u8

[is8ac]

rustc gives confusing error, suggesting that I give b an explicit type when b already has an explicit type.

Replacing 0u16.foo(b) with <u16 as Foo<[(); 3]>>::foo(0u16, b) fixes it.

This may be a duplicate of #69123

Source

pub trait Element<S> {
    type Array;
}

impl<T> Element<()> for T {
    type Array = T;
}

impl<T: Element<S>, S> Element<[S; 3]> for T {
    type Array = [T::Array; 3];
}

trait Foo<I>
where
    u8: Element<I>,
{
    fn foo(self, x: <u8 as Element<I>>::Array);
}

impl<I> Foo<I> for u16
where
    u8: Element<I>,
{
    fn foo(self, _: <u8 as Element<I>>::Array) {}
}

fn main() {
    let b: [u8; 3] = [0u8; 3];

    0u16.foo(b);
    //<u16 as Foo<[(); 3]>>::foo(0u16, b);
}

Log

$ rustc src/bin/type_param_demo.rs 
error[E0284]: type annotations needed for `[u8; 3]`
  --> src/bin/type_param_demo.rs:30:10
   |
28 |     let b: [u8; 3] = [0u8; 3];
   |         - consider giving `b` the explicit type `[u8; 3]`, where the type parameter `u8` is specified
29 | 
30 |     0u16.foo(b);
   |          ^^^ cannot infer type for type `u8`
   |
   = note: cannot resolve `<u8 as Element<_>>::Array == [u8; 3]`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0284`.

Meta

Same issue occurs in both latest and nightly and stable.

$ rustc --version --verbose
rustc 1.43.0-nightly (18c275b42 2020-03-02)
binary: rustc
commit-hash: 18c275b423f9f13c0e404ae3804967d2ab66337c
commit-date: 2020-03-02
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

$ rustc --version --verbose
rustc 1.41.1 (f3e1a954d 2020-02-24)
binary: rustc
commit-hash: f3e1a954d2ead4e2fc197c7da7d71e6c61bad196
commit-date: 2020-02-24
host: x86_64-unknown-linux-gnu
release: 1.41.1
LLVM version: 9.0