use self::private::Sealed;
use std::error::Error;
use std::panic::UnwindSafe;

pub trait AsDynError<'a>: Sealed {
    fn as_dyn_error(&self) -> &(dyn Error + 'a);
}

impl<'a, T: Error + 'a> AsDynError<'a> for T {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + Send + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + Send + Sync + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + Send + Sync + UnwindSafe + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

mod private {
    use super::*;

    pub trait Sealed {}
    impl<'a, T: Error + 'a> Sealed for T {}
    impl<'a> Sealed for dyn Error + 'a {}
    impl<'a> Sealed for dyn Error + Send + 'a {}
    impl<'a> Sealed for dyn Error + Send + Sync + 'a {}
    impl<'a> Sealed for dyn Error + Send + Sync + UnwindSafe + 'a {}
}

use std::error::Error;
use std::panic::UnwindSafe;

pub trait AsDynError<'a> {
    fn as_dyn_error(&self) -> &(dyn Error + 'a);
}

impl<'a, T: Error + 'a> AsDynError<'a> for T {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + Send + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + Send + Sync + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

impl<'a> AsDynError<'a> for dyn Error + Send + Sync + UnwindSafe + 'a {
    #[inline]
    fn as_dyn_error(&self) -> &(dyn Error + 'a) {
        self
    }
}

--- 
+++ 
@@ -1,7 +1,8 @@
+use self::private::Sealed;
 use std::error::Error;
 use std::panic::UnwindSafe;
 
-pub trait AsDynError<'a> {
+pub trait AsDynError<'a>: Sealed {
     fn as_dyn_error(&self) -> &(dyn Error + 'a);
 }
 
@@ -39,3 +40,14 @@
         self
     }
 }
+
+mod private {
+    use super::*;
+
+    pub trait Sealed {}
+    impl<'a, T: Error + 'a> Sealed for T {}
+    impl<'a> Sealed for dyn Error + 'a {}
+    impl<'a> Sealed for dyn Error + Send + 'a {}
+    impl<'a> Sealed for dyn Error + Send + Sync + 'a {}
+    impl<'a> Sealed for dyn Error + Send + Sync + UnwindSafe + 'a {}
+}
Add Sealed supertrait to private AsDynError trait
