tests/ui/typeck/issue-81885.rs

const TEST4: fn() -> _ = 42;
                  //~^ ERROR the placeholder `_` is not allowed within types on item signatures for functions
                  //~| ERROR the placeholder `_` is not allowed within types on item signatures for constant items

fn main() {
    const TEST5: fn() -> _ = 42;
                      //~^ ERROR the placeholder `_` is not allowed within types on item signatures for functions
                      //~| ERROR the placeholder `_` is not allowed within types on item signatures for constant items
}