Nested declaration macro diagnoses generic as incorrect type

[stephencelis]

Description

I encountered very bizarre behavior when trying to create a declaration macro that takes a generic closure. The closure seems to be incorrectly identifying one generic type as another.

Steps to reproduce

Download the following project and run the DeclarationMacroBugClient target:

DeclarationMacroBug.zip

You'll see that the closure is evaluating a parameter as if it is the wrong generic type.

Source

Macro plugin source:

import SwiftCompilerPlugin
import SwiftDiagnostics
import SwiftOperators
import SwiftSyntax
import SwiftSyntaxBuilder
import SwiftSyntaxMacros

public enum ReduceMacro: DeclarationMacro {
  public static func expansion<N: FreestandingMacroExpansionSyntax, C: MacroExpansionContext>(
    of node: N,
    in context: C
  ) throws -> [DeclSyntax] {
    guard let trailingClosure = node.trailingClosure
    else {
      fatalError()
    }
    return [
      """
      let body = Reduce<State, Action> \(trailingClosure)
      """
    ]
  }
}

@main
struct MacrosPlugin: CompilerPlugin {
  let providingMacros: [Macro.Type] = [
    ReduceMacro.self,
  ]
}

Library source:

public struct Reduce<State, Action> {
  public let reducer: (inout State, Action) -> Effect<Action>

  public init(_ reducer: @escaping (inout State, Action) -> Effect<Action>) {
    self.reducer = reducer
  }
}

public struct Effect<Action> {
  public static var none: Self { Self() }
}

@freestanding(declaration, names: named(body))
public macro Reduce<State, Action>(
  _ reducer: (inout State, Action) -> Effect<Action>
) = #externalMacro(
  module: "DeclarationMacroBugMacros", type: "ReduceMacro"
)

Client source:

import DeclarationMacroBug

struct State {
  var count = 0
}

enum Action {
  case decrementButtonTapped
  case incrementButtonTapped
}

struct Feature {
  #Reduce<State, Action> { state, action in
    switch action {
    case .decrementButtonTapped:
      state.count -= 1
      return .none
    case .incrementButtonTapped:
      state.count += 1
      return .none
    }
  }
}

Expected behavior

I expect action to be treated as an Action, not State.

Environment

• Swift compiler version info

  swift-driver version: 1.87.2 Apple Swift version 5.9 (swiftlang-5.9.2.1.6     clang-1500.1.0.1.1)
  Target: arm64-apple-macosx14.0
  

• Xcode version info

  Xcode 15.1
  Build version 15C5028h