c = s[i] on a string warns 'unresolved type in codegen, defaulting to int' #3479

Binding one byte of a string to a local warns, and the local is typed by a default:

main() {
    s = "A"
    c = s[0]
    println(c)
}

warning: unresolved type in codegen, defaulting to int
  --> b.ae:3:5
3 |     c = s[0]

It then prints 65. A string parameter does the same (c = text[i]).

Expected: no warning. s[i] reads one byte (language reference, strings), so c gets the type that read has, as c = xs[0] does for an array. The default happens to give the right value here, but a typecheck that leaves a local's type to codegen is the failure, and the warning fails make check-examples, which builds every example with warnings as errors.

Found writing the Rosetta Code tranche of #1191 (PR #3477): Caesar cipher (c = text[i] and then match c { 65..=90 -> ... }) waits for it. Filed for the next batch.