From 2f9ac7b8234b7fa38d6f95203119bbbe0e1fd73e Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Mon, 7 Jul 2025 12:24:24 -0400
Subject: [PATCH] Disallow the `@Test` attribute on operator declarations.
 (#1205)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

This PR prevents `@Test` from being applied to an operator declaration
such as:

```swift
@Test(arguments: ...)
static func +(lhs: A, rhs: B) { ... }
```

Now, the following error will be emitted by the compiler:

> 🛑 Attribute 'Test' cannot be applied to an operator

Previously, applying `@Test` to an operator produced undefined/unstable
effects.

Resolves #1204.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.
---
 .../Additions/FunctionDeclSyntaxAdditions.swift        | 10 ++++++++++
 Sources/TestingMacros/Support/DiagnosticMessage.swift  |  6 +++++-
 Sources/TestingMacros/TestDeclarationMacro.swift       |  2 +-
 .../TestingMacrosTests/TestDeclarationMacroTests.swift |  2 ++
 4 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift b/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift
index 9b9378283..9b55bc157 100644
--- a/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift
+++ b/Sources/TestingMacros/Support/Additions/FunctionDeclSyntaxAdditions.swift
@@ -34,6 +34,16 @@ extension FunctionDeclSyntax {
       .contains(.keyword(.nonisolated))
   }
 
+  /// Whether or not this function declares an operator.
+  var isOperator: Bool {
+    switch name.tokenKind {
+    case .binaryOperator, .prefixOperator, .postfixOperator:
+      true
+    default:
+      false
+    }
+  }
+
   /// The name of this function including parentheses, parameter labels, and
   /// colons.
   var completeName: DeclReferenceExprSyntax {
diff --git a/Sources/TestingMacros/Support/DiagnosticMessage.swift b/Sources/TestingMacros/Support/DiagnosticMessage.swift
index b7103bcc6..dd35b118d 100644
--- a/Sources/TestingMacros/Support/DiagnosticMessage.swift
+++ b/Sources/TestingMacros/Support/DiagnosticMessage.swift
@@ -93,7 +93,11 @@ struct DiagnosticMessage: SwiftDiagnostics.DiagnosticMessage {
     let result: (value: String, article: String)
     switch node.kind {
     case .functionDecl:
-      result = ("function", "a")
+      if node.cast(FunctionDeclSyntax.self).isOperator {
+        result = ("operator", "an")
+      } else {
+        result = ("function", "a")
+      }
     case .classDecl:
       result = ("class", "a")
     case .structDecl:
diff --git a/Sources/TestingMacros/TestDeclarationMacro.swift b/Sources/TestingMacros/TestDeclarationMacro.swift
index 50ac690d2..ef156edd6 100644
--- a/Sources/TestingMacros/TestDeclarationMacro.swift
+++ b/Sources/TestingMacros/TestDeclarationMacro.swift
@@ -61,7 +61,7 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
     }
 
     // The @Test attribute is only supported on function declarations.
-    guard let function = declaration.as(FunctionDeclSyntax.self) else {
+    guard let function = declaration.as(FunctionDeclSyntax.self), !function.isOperator else {
       diagnostics.append(.attributeNotSupported(testAttribute, on: declaration))
       return false
     }
diff --git a/Tests/TestingMacrosTests/TestDeclarationMacroTests.swift b/Tests/TestingMacrosTests/TestDeclarationMacroTests.swift
index 6c04eb9eb..c53783b6a 100644
--- a/Tests/TestingMacrosTests/TestDeclarationMacroTests.swift
+++ b/Tests/TestingMacrosTests/TestDeclarationMacroTests.swift
@@ -67,6 +67,8 @@ struct TestDeclarationMacroTests {
         "Attribute 'Test' cannot be applied to a structure",
       "@Test enum E {}":
         "Attribute 'Test' cannot be applied to an enumeration",
+      "@Test func +() {}":
+        "Attribute 'Test' cannot be applied to an operator",
 
       // Availability
       "@available(*, unavailable) @Suite struct S {}":