Parameter type cannot be inferred from contextual type in pack expansion on a closure associated with a result builder

[ccerino-adobe]

Description

When defining a closure that leverages parameter packs, the compiler can't infer the parameter type from the values in the closure.

Reproduction

Below is a code snippet from the attached example project.

@resultBuilder
enum ShowcaseContentBuilder {
    public static func buildBlock<each Content: View, ColumnType: ShowcaseColumnRepresentable>(_ content: repeat ShowcaseRow<ColumnType, each Content>) -> (repeat ShowcaseRow<ColumnType, each Content>) {
        (repeat each content)
    }
}

Showcase(title: "Showcase", columns: IconTestAsset.allCases) {
  ShowcaseRow(name: "Test 1") { icon in
    Image(systemName: icon.rawValue)
  }
  ShowcaseRow(name: "Test 2") { icon in
    Image(systemName: icon.rawValue)
  }
}

Expected behavior

The above code should be able to to infer icon is of type IconTestAsset, but the code fails to compile with the following errors:

• Generic parameter 'each EntryContent' could not be inferred
• Cannot infer type of closure parameter 'icon' without a type annotation

Environment

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-macosx15.0

Additional information

I joined a 1on1 lab with folks from the Swift team during WWDC and they suggested this is a dupe of #78426, but they encouraged me to file a new issue and share my example code

ResultBuilderTest.zip

[xedin]

@ccerino-adobe I think this actually behaves correctly and we misunderstood the problem during the lab.

These types cannot be inferred because they are contextual dependent and result builder transforms body of ShowCase trailing closure into something like this:

Showcase(title: "Showcase", columns: IconTestAsset.allCases) {
  $__builder0 = ShowcaseRow(name: "Test 1") { icon in
    Image(systemName: icon.rawValue)
  }
  $__builder1 = ShowcaseRow(name: "Test 2") { icon in
    Image(systemName: icon.rawValue)
  }

  return buildBlock($__builder0, $__builder1)
}

Where each $__builder variable is type-checked independently from return.

This is why icon cannot be inferred because it depends on the result type of the parameter type of Showcase and it works just fine when you remove the result builder because it's type-checked as a single-expression in this case and both SnowcaseRow instances are directly connected to (repeat ShowcaseRow<ColumnType, each EntryContent>).

[xedin]

The example would start failing if you introduce a variable in the body or make it some other way a multi-statement closure.

[xedin]

Does this explanation make sense to you?