comments

Author: lcnr

compiler/rustc_infer/src/infer/generalize.rs

@@ -153,10 +153,11 @@ struct Generalizer<'me, 'tcx, D> {
 
     cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
 
-    /// This is set once we're generalizing the arguments of an alias. In case
-    /// we encounter an occurs check failure we generalize the alias to an
-    /// inference variable instead of erroring. This is necessary to avoid
-    /// incorrect errors when relating `?0` with `<?0 as Trait>::Assoc`.
+    /// This is set once we're generalizing the arguments of an alias.
+    /// 
+    /// This is necessary to correctly handle
+    /// `<T as Bar<<?0 as Foo>::Assoc>::Assoc == ?0`. This equality can
+    /// hold by either normalizing the outer or the inner associated type.
     in_alias: bool,
 
     /// See the field `needs_wf` in `Generalization`.
@@ -330,6 +331,12 @@ where
             }
 
             ty::Alias(kind, data) => {
+                // An occurs check failure inside of an alias does not mean
+                // that the types definitely don't unify. We may be able
+                // to normalize the alias after all.
+                //
+                // We handle this by lazily equating the alias and generalizing
+                // it to an inference variable.
                 let is_nested_alias = mem::replace(&mut self.in_alias, true);
                 let result = match self.relate(data, data) {
                     Ok(data) => Ok(Ty::new_alias(self.tcx(), kind, data)),