error[E0391]: cycle detected when computing the bounds for type parameter `T`
  --> $DIR/issue-21177.rs:6:21
   |
LL | fn foo<T: Trait<A = T::B>>() { }
   |                     ^^^^
   |
   = note: ...which immediately requires computing the bounds for type parameter `T` again
note: cycle used when computing explicit predicates of `foo`
  --> $DIR/issue-21177.rs:6:21
   |
LL | fn foo<T: Trait<A = T::B>>() { }
   |                     ^^^^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0391`.