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Consider the following code snippet given some class SampleClass { }:

ClassReader reader = new ClassReader(SampleClass.class.getName());
ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_MAXS);
reader.accept(new ClassVisitor(Opcodes.ASM6, writer) {
  @Override
  public MethodVisitor visitMethod(int access, 
        String name, 
        String descriptor, 
        String signature, 
        String[] exceptions) {
    return super.visitMethod(access,
      name,
      "(Ljava/lang/Void;)V", // original: "()V"
      signature,
      exceptions);
  }
}, 0);

ClassReader result = new ClassReader(writer.toByteArray());
result.accept(new TraceClassVisitor(new PrintWriter(System.out)), 0);

The resulting class defines MAXLOCALS = 1 despite the constructor being widened to define one argument such that two slots are required for both the this argument and the added Void are not available what results in a verification error.

The problem does not occur if the class writer does not copy the constant pool.