check Projection supertrait bounds when confirming dyn candidate

Author: compiler-errors

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -468,7 +468,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .predicates
             .into_iter()
         {
-            if let ty::PredicateKind::Trait(..) = super_trait.kind().skip_binder() {
+            if let ty::PredicateKind::Trait(..) | ty::PredicateKind::Projection(..) =
+                super_trait.kind().skip_binder()
+            {
                 let normalized_super_trait = normalize_with_depth_to(
                     self,
                     obligation.param_env,

src/test/ui/traits/object/enforce-supertrait-projection.rs

@@ -0,0 +1,24 @@
+trait SuperTrait {
+    type A;
+    type B;
+}
+
+trait Trait: SuperTrait<A = <Self as SuperTrait>::B> {}
+
+fn transmute<A, B>(x: A) -> B {
+    foo::<A, B, dyn Trait<A = A, B = B>>(x)
+    //~^ ERROR type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
+}
+
+fn foo<A, B, T: ?Sized>(x: T::A) -> B
+where
+    T: Trait<B = B>,
+{
+    x
+}
+
+static X: u8 = 0;
+fn main() {
+    let x = transmute::<&u8, &[u8; 1_000_000]>(&X);
+    println!("{:?}", x[100_000]);
+}

src/test/ui/traits/object/enforce-supertrait-projection.stderr

@@ -0,0 +1,26 @@
+error[E0271]: type mismatch resolving `<dyn Trait<A = A, B = B> as SuperTrait>::A == B`
+  --> $DIR/enforce-supertrait-projection.rs:9:5
+   |
+LL | fn transmute<A, B>(x: A) -> B {
+   |              -  - expected type parameter
+   |              |
+   |              found type parameter
+LL |     foo::<A, B, dyn Trait<A = A, B = B>>(x)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected type parameter `B`, found type parameter `A`
+   |
+   = note: expected type parameter `B`
+              found type parameter `A`
+   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
+note: required by a bound in `foo`
+  --> $DIR/enforce-supertrait-projection.rs:15:8
+   |
+LL | fn foo<A, B, T: ?Sized>(x: T::A) -> B
+   |    --- required by a bound in this
+LL | where
+LL |     T: Trait<B = B>,
+   |        ^^^^^^^^^^^^ required by this bound in `foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.