error[E0277]: the trait bound `fn() -> impl T {foo}: T` is not satisfied
  --> $DIR/fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:17:5
   |
LL | fn bar(f: impl T<O=()>) {}
   | ----------------------- required by `bar`
...
LL |     bar(foo);
   |     ^^^ the trait `T` is not implemented for `fn() -> impl T {foo}`
   |
   = help: use parentheses to call the function: `foo()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.