relax bounds on iterator adapter Default impls

library/core/src/iter/adapters/chain.rs

@@ -283,11 +283,7 @@ where
 }
 
 #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
-impl<A, B> Default for Chain<A, B>
+impl<A: Default, B: Default> Default for Chain<A, B> {
-where
-    A: Iterator + Default,
-    B: Iterator + Default,
-{
     /// Creates a `Chain` from the default values for `A` and `B`.
     ///
     /// ```

library/core/src/iter/adapters/cloned.rs

@@ -155,11 +155,7 @@ where
 }
 
 #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
-impl<'a, I, T: 'a> Default for Cloned<I>
+impl<I: Default> Default for Cloned<I> {
-where
-    I: Default + Iterator<Item = &'a T>,
-    T: Clone,
-{
     /// Creates a `Cloned` iterator from the default value of `I`
     /// ```
     /// # use core::slice;

library/core/src/iter/adapters/copied.rs

@@ -242,11 +242,7 @@ where
 }
 
 #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
-impl<'a, I, T: 'a> Default for Copied<I>
+impl<I: Default> Default for Copied<I> {
-where
-    I: Default + Iterator<Item = &'a T>,
-    T: Copy,
-{
     /// Creates a `Copied` iterator from the default value of `I`
     /// ```
     /// # use core::slice;

library/core/src/iter/adapters/enumerate.rs

@@ -266,10 +266,7 @@ where
 unsafe impl<I: InPlaceIterable> InPlaceIterable for Enumerate<I> {}
 
 #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
-impl<I> Default for Enumerate<I>
+impl<I: Default> Default for Enumerate<I> {
-where
-    I: Iterator + Default,
-{
     /// Creates an `Enumerate` iterator from the default value of `I`
     /// ```
     /// # use core::slice;

library/core/src/iter/adapters/flatten.rs

@@ -305,8 +305,7 @@ where
 #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
 impl<I> Default for Flatten<I>
 where
-    I: Iterator + Default,
+    I: Default + Iterator<Item: IntoIterator>,
-    <I as Iterator>::Item: IntoIterator,
 {
     /// Creates a `Flatten` iterator from the default value of `I`.
     ///

library/core/src/iter/adapters/rev.rs

@@ -137,10 +137,7 @@ impl<I> FusedIterator for Rev<I> where I: FusedIterator + DoubleEndedIterator {}
 unsafe impl<I> TrustedLen for Rev<I> where I: TrustedLen + DoubleEndedIterator {}
 
 #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
-impl<I> Default for Rev<I>
+impl<I: Default> Default for Rev<I> {
-where
-    I: Default + Iterator,
-{
     /// Creates a `Rev` iterator from the default value of `I`
     /// ```
     /// # use core::slice;