[Macros] Add test case for #67989 / rdar://114105615.

The bug described by this report has been fixed on main. Add a test case
to ensure that it doesn't regress.

Author: DougGregor

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -1343,6 +1343,25 @@ public struct EquatableViaMembersMacro: ExtensionMacro {
   }
 }
 
+public struct FooExtensionMacro: ExtensionMacro {
+  public static func expansion(of node: AttributeSyntax, attachedTo declaration: some DeclGroupSyntax, providingExtensionsOf type: some TypeSyntaxProtocol, conformingTo protocols: [TypeSyntax], in context: some MacroExpansionContext) throws -> [ExtensionDeclSyntax] {
+    let decl: DeclSyntax =
+    """
+    extension Foo {
+      var foo: String { "foo" }
+      func printFoo() {
+        print(foo)
+      }
+    }
+    """
+    guard let extensionDecl = decl.as(ExtensionDeclSyntax.self) else {
+      return []
+    }
+
+    return [extensionDecl]
+  }
+}
+
 public struct ConformanceViaExtensionMacro: ExtensionMacro {
   public static func expansion(
     of node: AttributeSyntax,

test/Macros/macro_expand_extensions.swift

@@ -216,3 +216,22 @@ func testMakeMeHashable(mmh: MakeMeHashable, dict: [MakeMeHashable: Int]) {
 
 @ImpliesHashable
 public struct MakeMeHashable { }
+
+
+// https://github.com/apple/swift/issues/67989 - member added via extension
+// to a protocol is not visible
+
+@attached(extension, names: named(foo), named(printFoo))
+public macro FooExtension() = #externalMacro(module: "MacroDefinition", type: "FooExtensionMacro")
+
+@FooExtension
+protocol Foo {
+  var foo: String { get }
+}
+
+extension String: Foo { }
+
+func testStringFoo(s: String) {
+  "Test".printFoo()
+  s.printFoo()
+}