Unhelpful error with "overlapping" never applicable impls

[matthewjasper]

These two implementations are overlapping (it's allowed to write where for<'a> M: B<'a> even when M doesn't implent B<'a>) but the error doesn't say this.

trait A { /* ... */ }
trait B<'a> {}

struct M;

impl A for M where for<'a> M: B<'a> { /* ... */ }
impl A for M where for<'a> M: B<'a> { /* ... */ }

fn main() {}

Compiler output:

   Compiling playground v0.0.1 (file:///playground)
error[E0283]: type annotations required: cannot resolve `M: A`
 --> src/main.rs:8:6
  |
8 | impl A for M where for<'a> M: B<'a> {}
  |      ^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

cc #48214 which will make this issue easier to come across.