Allow default type to be projected in the fully monomorphic case

src/librustc/traits/project.rs

@@ -38,28 +38,9 @@ use util::common::FN_OUTPUT_NAME;
 /// more or less conservative.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum Reveal {
-    /// At type-checking time, we refuse to project any associated
-    /// type that is marked `default`. Non-`default` ("final") types
-    /// are always projected. This is necessary in general for
-    /// soundness of specialization. However, we *could* allow
-    /// projections in fully-monomorphic cases. We choose not to,
-    /// because we prefer for `default type` to force the type
-    /// definition to be treated abstractly by any consumers of the
-    /// impl. Concretely, that means that the following example will
-    /// fail to compile:
-    ///
-    /// ```
-    /// trait Assoc {
-    ///     type Output;
-    /// }
-    ///
-    /// impl<T> Assoc for T {
-    ///     default type Output = bool;
-    /// }
-    ///
-    /// fn main() {
-    ///     let <() as Assoc>::Output = true;
-    /// }
+    /// At type-checking time, we avoid projecting the concrete type of
+    /// `impl Trait`. We do allow other monomorphic projections, such as
+    /// `default type` in the fully monomorphic case.
     UserFacing,
 
     /// At trans time, all monomorphic projections will succeed.
@@ -949,15 +930,13 @@ fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
                 // get a result which isn't correct for all monomorphizations.
                 let new_candidate = if !is_default {
                     Some(ProjectionTyCandidate::Select)
-                } else if obligation.param_env.reveal == Reveal::All {
+                } else {
                     assert!(!poly_trait_ref.needs_infer());
                     if !poly_trait_ref.needs_subst() {
                         Some(ProjectionTyCandidate::Select)
                     } else {
                         None
                     }
-                } else {
-                    None
                 };
 
                 candidate_set.vec.extend(new_candidate);

src/test/compile-fail/specialization/defaultimpl/specialization-default-projection.rs

@@ -32,10 +32,10 @@ fn generic<T>() -> <T as Foo>::Assoc {
 }
 
 fn monomorphic() -> () {
-    // Even though we know that `()` is not specialized in a
-    // downstream crate, typeck refuses to project here.
+    // Since we know that `()` is not specialized in a
+    // downstream crate, typeck allows projection here.
 
-    generic::<()>() //~ ERROR mismatched types
+    generic::<()>()
 }
 
 fn main() {

src/test/compile-fail/specialization/specialization-default-projection.rs

@@ -32,10 +32,10 @@ fn generic<T>() -> <T as Foo>::Assoc {
 }
 
 fn monomorphic() -> () {
-    // Even though we know that `()` is not specialized in a
-    // downstream crate, typeck refuses to project here.
+    // Since we know that `()` is not specialized in a
+    // downstream crate, typeck allows projection here.
 
-    generic::<()>() //~ ERROR mismatched types
+    generic::<()>()
 }
 
 fn main() {

src/test/compile-fail/ufcs-partially-resolved.rs

@@ -45,7 +45,7 @@ fn main() {
     <u8 as A>::N::NN; //~ ERROR cannot find associated type `N` in `A`
     let _: <u8 as Tr>::Y::NN; //~ ERROR ambiguous associated type
     let _: <u8 as E>::Y::NN; //~ ERROR expected associated type, found variant `E::Y`
-    <u8 as Tr>::Y::NN; //~ ERROR no associated item named `NN` found for type `<u8 as Tr>::Y`
+    <u8 as Tr>::Y::NN; //~ ERROR no associated item named `NN` found for type `u16`
     <u8 as E>::Y::NN; //~ ERROR expected associated type, found variant `E::Y`
 
     let _: <u8 as Tr::N>::NN; //~ ERROR cannot find associated type `NN` in `Tr::N`
@@ -62,5 +62,5 @@ fn main() {
     let _: <u8 as Dr>::Z; //~ ERROR expected associated type, found method `Dr::Z`
     <u8 as Dr>::X; //~ ERROR expected method or associated constant, found associated type `Dr::X`
     let _: <u8 as Dr>::Z::N; //~ ERROR expected associated type, found method `Dr::Z`
-    <u8 as Dr>::X::N; //~ ERROR no associated item named `N` found for type `<u8 as Dr>::X`
+    <u8 as Dr>::X::N; //~ ERROR no associated item named `N` found for type `u16`
 }