[AutoDiff] Guard let optional unwrapping is not differentiable

[loonatick-src]

Description

Closely related to #75280: the reproducer provided there now results in a compilation error instead of crash in Swift 6.0.3:

import _Differentiation
@differentiable(reverse) func a<F, A>(_ f: Optional<F>, c: @differentiable(reverse) (F) -> A) -> Optional<A> where F: Differentiable, A: Differentiable
{
    guard let f else {return nil}
    return c(f)
}

<source>:2:2: error: function is not differentiable
1 | import _Differentiation
2 | @differentiable(reverse) func a<F, A>(_ f: Optional<F>, c: @differentiable(reverse) (F) -> A) -> Optional<A> where F: Differentiable, A: Differentiable
  |  |                            `- note: when differentiating this function definition
  |  `- error: function is not differentiable
3 | {
4 |     guard let f else {return nil}
  |                              `- note: expression is not differentiable
5 |     return c(f)
6 | }
Compiler returned: 1

But, the following semantically equivalent function compiles successfully.

import _Differentiation
@differentiable(reverse) func a<F, A>(_ f: Optional<F>, c: @differentiable(reverse) (F) -> A) -> Optional<A> where F: Differentiable, A: Differentiable
{
    guard let f else {
        let a: Optional<A> = nil
        return a
    }
    return c(f)
}

Compiler explorer link

Reproduction

// main.swift
import _Differentiation
@differentiable(reverse) func a<F, A>(_ f: Optional<F>, c: @differentiable(reverse) (F) -> A) -> Optional<A> where F: Differentiable, A: Differentiable
{
    guard let f else {return nil}
    return c(f)
}

Using Swift version 6.0.3, run swiftc main.swift.

Expected behavior

Both should compile successfully

Environment

Tested on the following platforms.

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

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx14.0

Additional information

No response

[loonatick-src]

@JaapWijnen

[JaapWijnen]

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