Ternary operator/if-expression not differentiable

[loonatick-src]

Description

Closely related to #54195: the following throws a compilation error on Swift 6.0.3.

import _Differentiation

@differentiable(reverse)
func TF_966(_ x: Float, _ bool: Bool) -> Float {
  let tuple = (x, 1)
  let result = bool ? tuple : tuple
  return result.0
}

<source>:3:2: error: function is not differentiable
1 | import _Differentiation
2 | 
3 | @differentiable(reverse)
  |  `- error: function is not differentiable
4 | func TF_966(_ x: Float, _ bool: Bool) -> Float {
  |      `- note: when differentiating this function definition
5 |   let tuple = (x, 1)
6 |   let result = bool ? tuple : tuple
  |                       `- note: expression is not differentiable
7 |   return result.0
8 | }

But, the following semantically equivalent function compiles successfully.

import _Differentiation

@differentiable(reverse)
func TF_966(_ x: Float, _ bool: Bool) -> Float {
  let tuple = (x, 1)
  return bool ? tuple.0 : tuple.0
}

Compiler explorer link

Reproduction

In a single file

// main.swift
import _Differentiation

@differentiable(reverse)
func TF_966(_ x: Float, _ bool: Bool) -> Float {
  let tuple = (x, 1)
  let result = bool ? tuple : tuple
  return result.0
}

Run swiftc main.swift.

Expected behavior

Should compile successfully.

Environment

Swift version 6.0.3 (swift-6.0.3-RELEASE)
Target: x86_64-unknown-linux-gnu

Additional information

No response

[asl]

The code might look equivalent, but in reality they are not. In particular, here:

  ...
  let result = bool ? tuple : tuple
  return result.0

A single active tuple member makes the whole tuple active. And we are having a non-differentiable tuple initializer, hence the error.