coercions do not reach into aggregates - converging coercions

[gnzlbg]

This (playground):

fn foo() {}
fn bar() {}
fn main() {
    let _ = [(foo), (bar)]; // OK
    let _ = [(foo,), (bar,)]; // FAILS
}

fails to compile with the following error:

error[E0308]: mismatched types
 --> src/main.rs:5:23
  |
5 |     let _ = [(foo,), (bar,)]; // FAILS
  |                       ^^^ expected fn item, found a different fn item
  |
  = note: expected type `fn() {foo}`
             found type `fn() {bar}`

[gnzlbg]

cc @eddyb

[hanna-kruppe]

Aside: as @eddyb's working example shows, my description of coercions "not reaching into aggregates" is not actually correct.