make projection bounds with const bounds satisfy const

compiler/rustc_middle/src/traits/select.rs

@@ -119,8 +119,9 @@ pub enum SelectionCandidate<'tcx> {
 
     /// This is a trait matching with a projected type as `Self`, and we found
     /// an applicable bound in the trait definition. The `usize` is an index
-    /// into the list returned by `tcx.item_bounds`.
-    ProjectionCandidate(usize),
+    /// into the list returned by `tcx.item_bounds`. The constness is the
+    /// constness of the bound in the trait.
+    ProjectionCandidate(usize, ty::BoundConstness),
 
     /// Implementation of a `Fn`-family trait by one of the anonymous types
     /// generated for an `||` expression.