[ASTGen] Handle 'prefixed(_)' in macro introduced names

'_' is not a 'DeclReferenceExprSyntax', but a
'DiscardAssignmentExprSyntax'.

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -1246,12 +1246,19 @@ extension ASTGenVisitor {
         // E.g. 'named(foo())', use the callee to generate the name.
         arg = call.calledExpression
       }
-      guard let arg = arg.as(DeclReferenceExprSyntax.self) else {
-        // TODO: Diagnose.
-        return nil
-      }
+
+      if let arg = arg.as(DeclReferenceExprSyntax.self) {
         name = self.generateDeclNameRef(declReferenceExpr: arg).name
+      } else if arg.is(DiscardAssignmentExprSyntax.self) {
+        name = BridgedDeclNameRef.createParsed(.createIdentifier(self.ctx.getIdentifier("_")))
+      } else {
+        // TODO: Diagnose
+        fatalError("expected name")
+        //return nil
+      }
+
       if arguments.count >= 2 {
+        fatalError("unexpected arguments")
         // TODO: Diagnose.
       }
 

test/ASTGen/macros.swift

@@ -115,3 +115,34 @@ func f(a: Int, b: String) async throws -> String
 struct TestArbitrary {
   #bitwidthNumberedStructs("MyIntOne")
 }
+
+// Stored properties generated by a peer macro
+@attached(accessor)
+@attached(peer, names: prefixed(_))
+macro myPropertyWrapper() =
+    #externalMacro(module: "MacroDefinition", type: "PropertyWrapperMacro")
+
+struct MyWrapperThingy<T> {
+  var storage: T
+
+  var wrappedValue: T {
+    get {
+      print("Getting value \(storage)")
+      return storage
+    }
+
+    set {
+      print("Setting value \(newValue)")
+      storage = newValue
+    }
+  }
+}
+
+struct S3 {
+  @myPropertyWrapper
+  var x: Int = 0
+
+  init(x: Int) {
+    self._x = MyWrapperThingy(storage: x)
+  }
+}