error[E0276]: impl has stricter requirements than trait
  --> $DIR/trait-bound-on-type-parameter.rs:15:13
   |
LL |   fn b<C,D>(&self, x: C) -> C;
   |   ---------------------------- definition of `b` from trait
...
LL |     fn b<F: Sync, G>(&self, _x: F) -> F { panic!() }
   |             ^^^^ impl has extra requirement `F: Sync`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0276`.