Class generation error on an usage of "auto" with intersection types

[dzmauchy]

Describe the bug
the "auto" return type on a method with a intersection return type leads to a compilation error

To Reproduce
Steps to reproduce the behavior:

1. Create a simple class with a method with auto return type and an intersection actual type
2. Compile the java file
3. Get an error like "error: error while generating class Main
   (illegal signature attribute for type INT#1)"

Expected behavior
The compilation is successful

Screenshots

Desktop (please complete the following information):

• OS Type & Version: Manjaro Linux
• Java/JDK version: OpenJDK 25
• IDE version (IntelliJ IDEA or Android Studio): 2026.1
• Manifold version: 2026.1.6
• Manifold IntelliJ plugin version: 2026.1.7

Additional context
Gradle compilation with

tasks.withType<JavaCompile> {
  options.compilerArgs.add("-Xplugin:Manifold")
}

of a java file

import manifold.ext.rt.api.auto;

import java.util.List;

public class Main {

  public static auto x() {
    return List.of(1, "2");
  }

  static void main(String[] args) throws Exception {
    var method = Main.class.getMethod("x");
    System.out.println(method.getGenericReturnType());
  }
}

Stack trace

error: error while generating class Main
  (illegal signature attribute for type INT#1)
error: error while generating class Main

  where INT#1,INT#2 are intersection types:
    INT#1 extends Object,Serializable,Comparable<? extends INT#2>,Constable,ConstantDesc
    INT#2 extends Object,Serializable,Comparable<?>,Constable,ConstantDesc
1 error