ADDITIONAL SYSTEM INFORMATION :
java version "25" 2025-09-16 LTS
Java(TM) SE Runtime Environment (build 25+37-LTS-3491)
Java HotSpot(TM) 64-Bit Server VM (build 25+37-LTS-3491, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :

When using the Java java.lang.classfile API to construct and parse class files that contain branch instructions (such as ifge with labels), a bug surfaces in which the labels (used as branch targets) may remain uninflated after parsing with ClassFile.of().parse(byteCode). As a result, any subsequent processing or copying of the parsed CodeModel such as emitting a new class file using with(cde) can trigger the following error:

---------- BEGIN SOURCE ----------
import java.lang.classfile.*;
import java.lang.constant.*;

class ClassFileBug {
    public static void main(String[] args) {
        // (1) Build a trivial class with a branch:
        // equivalent to:
        // class TestClass {
        // static int clamp(int x) {
        // if (x < 0) return 0;
        // return x;
        // }
        // }
        ClassDesc className = ClassDesc.of("TestClass");
        String methodName = "clamp";
        MethodTypeDesc methodType = MethodTypeDesc.of(ConstantDescs.CD_int, ConstantDescs.CD_int);
        int methodFlags = ClassFile.ACC_STATIC;
        
        byte[] byteCode = ClassFile.of(ClassFile.StackMapsOption.DROP_STACK_MAPS).build(className, clb -> {
            clb.withMethodBody(methodName, methodType, methodFlags, cdb -> {
                var label = cdb.newLabel();
                // if (x < 0)
                cdb.iload(0);
                cdb.ifge(label);
                // return 0
                cdb.iconst_0();
                cdb.ireturn();
                // return x
                cdb.labelBinding(label);
                cdb.iload(0);
                cdb.ireturn();
            });
        });
        
        // (2) Load the CodeModel back in
        ClassModel classModel = ClassFile.of().parse(byteCode);
        CodeModel methodCode = classModel.methods().getFirst().code().get();
        
        // (3) Try to use the code in a new class; it blows up:
        // "Detected branch target out of bytecode range at bytecode offset 1 of method clamp(int)
        // 0000: 1a 9c ff fe 03 ac 1a ac"
        ClassFile.of().build(className, clb -> {
            clb.withMethodBody(methodName, methodType, methodFlags, cdb -> {
                //methodCode.toDebugString(); // ##### <-- uncomment this line and the exception disappears #####
                for (CodeElement cde : methodCode) {
                    cdb.with(cde);
                }
            });
        });
        
        System.out.println("Success");
    }
}
---------- END SOURCE ----------


This issue does not occur if the debugging method toDebugString() is called on the CodeModel before reuse, suggesting that the process of inflating labels is incorrectly deferred until such a call.