error: generic args in patterns require the turbofish syntax
  --> $DIR/issue-22647.rs:2:15
   |
LL |     let caller<F> = |f: F|
   |               ^
   |
help: use `::<...>` instead of `<...>` to specify lifetime, type, or const arguments
   |
LL |     let caller::<F> = |f: F|
   |               ++

error: aborting due to 1 previous error