Change:
This code generates an ICE with this message. Replacing by makes the code to compile and run fine, which is strange because ! does not implements . By removing the specialization the compiler refuses to compile the code with the error:
The ICE has been fixed in the latest nightly, marked as E-needstest
Thanks Indeed, the example compiles fine in nightly ( 2019-11-12), however, replacing the main function by these two generates the same ICE.
ah, thanks for the update! I confirmed it ()
Triage: the ICE doesn't occur anymore with the latest nightly, marking as E-needs-test.

Discussion:
<ins> // check-pass #![feature(impl_trait_in_bindings)] #![allow(incomplete_features)] struct A<'a>(&'a ()); trait Trait<T> {} impl<T> Trait<T> for () {} pub fn foo<'a>() { let _x: impl Trait<A<'a>> = (); } fn main() {} </ins>

<ins> #![feature(never_type, specialization)] #![allow(incomplete_features)] use std::iter::{self, Empty}; trait Trait { type Out: Iterator<Item = u32>; fn f(&self) -> Option<Self::Out>; } impl<T> Trait for T { default type Out = !; //~ ERROR: `!` is not an iterator default fn f(&self) -> Option<Self::Out> { None } } struct X; impl Trait for X { type Out = Empty<u32>; fn f(&self) -> Option<Self::Out> { Some(iter::empty()) } } fn f<T: Trait>(a: T) { if let Some(iter) = a.f() { println!("Some"); for x in iter { println!("x = {}", x); } } } pub fn main() { f(10); } </ins>

<ins> error[E0277]: `!` is not an iterator --> $DIR/issue-51506.rs:13:5 | LL |     type Out: Iterator<Item = u32>; |     ------------------------------- required by `Trait::Out` ... LL |     default type Out = !; |     ^^^^^^^^^^^^^^^^^^^^^ `!` is not an iterator | = help: the trait `std::iter::Iterator` is not implemented for `!` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`. </ins>

<ins> #![crate_type = "lib"] #![feature(specialization)] #![feature(unsize, coerce_unsized)] #![allow(incomplete_features)] use std::ops::CoerceUnsized; pub struct SmartassPtr<A: Smartass+?Sized>(A::Data); pub trait Smartass { type Data; type Data2: CoerceUnsized<*const [u8]>; } pub trait MaybeObjectSafe {} impl MaybeObjectSafe for () {} impl<T> Smartass for T { type Data = <Self as Smartass>::Data2; default type Data2 = (); //~^ ERROR: the trait bound `(): std::ops::CoerceUnsized<*const [u8]>` is not satisfied } impl Smartass for () { type Data2 = *const [u8; 1]; } impl Smartass for dyn MaybeObjectSafe { type Data = *const [u8]; type Data2 = *const [u8; 0]; } impl<U: Smartass+?Sized, T: Smartass+?Sized> CoerceUnsized<SmartassPtr<T>> for SmartassPtr<U> where <U as Smartass>::Data: std::ops::CoerceUnsized<<T as Smartass>::Data> {} pub fn conv(s: SmartassPtr<()>) -> SmartassPtr<dyn MaybeObjectSafe> { s } </ins>

<ins> error[E0277]: the trait bound `(): std::ops::CoerceUnsized<*const [u8]>` is not satisfied --> $DIR/issue-44861.rs:21:5 | LL |     type Data2: CoerceUnsized<*const [u8]>; |     --------------------------------------- required by `Smartass::Data2` ... LL |     default type Data2 = (); |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::ops::CoerceUnsized<*const [u8]>` is not implemented for `()` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`. </ins>

<ins> #![feature(specialization)] #![allow(incomplete_features)] struct MyStruct {} trait MyTrait { type MyType: Default; } impl MyTrait for i32 { default type MyType = MyStruct; //~^ ERROR: the trait bound `MyStruct: std::default::Default` is not satisfied } fn main() { let _x: <i32 as MyTrait>::MyType = <i32 as MyTrait>::MyType::default(); } </ins>

<ins> error[E0277]: the trait bound `MyStruct: std::default::Default` is not satisfied --> $DIR/issue-59435.rs:11:5 | LL |     type MyType: Default; |     --------------------- required by `MyTrait::MyType` ... LL |     default type MyType = MyStruct; |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::default::Default` is not implemented for `MyStruct` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`. </ins>