An if used as a value does not parse when its condition ends in a call: s = if f(x) { a } else { b } #3478

An if used as a value does not parse when its condition ends in a call:

yes(x: int) -> bool -> x > 0

main() {
    s = if yes(1) { "yes" } else { "no" }
    println(s)
}

error[E0100]: Expected LEFT_BRACE, got ELSE
  --> a.ae:4:29
4 |     s = if yes(1) { "yes" } else { "no" }

Expected: it prints yes. The language reference and examples/01-language/04-control-flow.ae say if c { a } else { b } is a value whatever c is.

The same condition works as an if statement (if yes(1) { s = "yes" }), and in an if expression when parenthesized (if (yes(1)) { ... }), so it looks like the expression form's condition is parsed with the trailing-block rule: yes(1) { "yes" } taken as a call passed a block. A condition with a call that is not last parses too (if yes(1) && x > 0 { ... } else { ... } is fine; if 1 < 2 && yes(1) { 1 } else { 2 } fails the same way).

Found writing the Rosetta Code tranche of #1191 (PR #3477): Roman numerals/Decode (total += if i + 1 < n && v < worth(roman, i + 1) { -v } else { v }) and Leap year (kind = if leap(year) { "a leap year" } else { "not a leap year" }) wait for it, written without the parentheses. Filed for the next batch.