"expected fn item, found a different fn item" for [(f, "f"), (g, "g")] even though [f, g] infers fine

I tried this code:

fn a(_: &str) {}
fn b(_: &str) {}
fn main() {
    for (f, s) in [(a, "a"), (b, "b")] {
        f(s);
    }
    // workaround
    for (f, s) in [a, b].into_iter().zip(["a", "b"]) {
        f(s);
    }
}

I expected to see this happen: both work

Instead, this happened: latter works, former errors with

error[E0308]: mismatched types
 --> a.rs:4:31
  |
4 |     for (f, s) in [(a, "a"), (b, "b")] {
  |                               ^ expected fn item, found a different fn item
  |
  = note: expected fn item `for<'a> fn(&'a _) {a}`
             found fn item `for<'a> fn(&'a _) {b}`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.

Meta

rustc --version --verbose:

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Start by compiling the minimal reproducer with rustc 1.87.0 and compare the tuple-array case with the zip workaround. Investigate how the compiler treats distinct function items during array type inference; done means the reported case is handled consistently without breaking the working workaround.