MacroSystem should qualify nested types when expanding extension macros

Resolves rdar://119850970.

Author: bnbarham

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -320,10 +320,25 @@ private func expandExtensionMacro(
   indentationWidth: Trivia
 ) throws -> CodeBlockItemListSyntax? {
   let extendedType: TypeSyntax
-  if let identified = attachedTo.asProtocol(NamedDeclSyntax.self) {
-    extendedType = "\(identified.name.trimmed)"
-  } else if let ext = attachedTo.as(ExtensionDeclSyntax.self) {
+  if let ext = attachedTo.as(ExtensionDeclSyntax.self) {
     extendedType = "\(ext.extendedType.trimmed)"
+  } else if let identified = attachedTo.asProtocol(NamedDeclSyntax.self) {
+    var types: [TokenSyntax] = []
+    types.append(identified.name.trimmed)
+
+    var possibleParent = attachedTo.parent
+    while let parent = possibleParent {
+      possibleParent = parent.parent
+      if let parent = parent.asProtocol(NamedDeclSyntax.self) {
+        types.append(parent.name.trimmed)
+      }
+    }
+
+    var baseType: TypeSyntax = TypeSyntax(IdentifierTypeSyntax(name: types.popLast()!))
+    for type in types.reversed() {
+      baseType = TypeSyntax(MemberTypeSyntax(baseType: baseType, name: type))
+    }
+    extendedType = baseType
   } else {
     return nil
   }

Tests/SwiftSyntaxMacroExpansionTest/ExtensionMacroTests.swift

@@ -99,7 +99,7 @@ final class ExtensionMacroTests: XCTestCase {
           }
         }
 
-        extension MyType: Sendable {
+        extension Wrapper.MyType: Sendable {
         }
         """,
       macros: ["AddSendableExtension": SendableExtensionMacro.self],