Blanket impl resolution vs. trait bounds

[jcape]

I have run into similar errors to #50133, so that may share the same ultimate cause as mine.

I'm having an issue with the impl<T> From<T> for T blanket implementation causing conflicts with my own blanket implementation of a From trait. For example, if I have a type which implements AsRef<u8>, and From<T: AsRef<[u8]>>, that blanket implementation conflicts, because my implementation of the trait and the trait-bound-from obviously provide a different path for T::from(T). I think this is wrong, though in my specific case I have worked around it by doing From<&'bytes [u8]>, but wrong is wrong :-)

Some sample code:

struct MyType(Vec<u8>);

impl AsRef<[u8]> for MyType {
    fn as_ref(&self) -> &[u8] {
        self.0.as_ref()
    }
}

impl<B: AsRef<[u8]> From<B> for MyType {
    fn from(src: B) -> Self {
        Self(Vec::from(src))
    }
}

Resulting error:

error[E0119]: conflicting implementations of trait `std::convert::From<MyType>` for type `MyType`:
  --> src/main.rs:10:1
   |
10 | impl<B: AsRef<[u8]>> From<B> for MyType {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: conflicting implementation in crate `core`:
           - impl<T> std::convert::From<T> for T;

[jonas-schievink]

The second impl could be instantiated with B = MyType because of the first impl. This would collide with the blanket impl in core, shown in the error message. Specialization could possibly allow this, but right now this is intentionally forbidden.