#[test]
fn vtable_string_boundary_cases() {
    use crate::backend::VM;
    use crate::mir::{
        BasicBlockId, ConstValue, EffectMask, FunctionSignature, MirFunction, MirInstruction,
        MirModule, MirType,
    };
    std::env::set_var("NYASH_ABI_VTABLE", "1");

    // Case 1: empty string length == 0
    let sig1 = FunctionSignature {
        name: "main".into(),
        params: vec![],
        return_type: MirType::Integer,
        effects: EffectMask::PURE,
    };
    let mut f1 = MirFunction::new(sig1, BasicBlockId::new(0));
    let bb1 = f1.entry_block;
    let s = f1.next_value_id();
    f1.get_block_mut(bb1)
        .unwrap()
        .add_instruction(MirInstruction::Const {
            dst: s,
            value: ConstValue::String("".into()),
        });
    let sb = f1.next_value_id();
    f1.get_block_mut(bb1)
        .unwrap()
        .add_instruction(MirInstruction::NewBox {
            dst: sb,
            box_type: "StringBox".into(),
            args: vec![s],
        });
    let ln = f1.next_value_id();
    f1.get_block_mut(bb1)
        .unwrap()
        .add_instruction(MirInstruction::BoxCall {
            dst: Some(ln),
            box_val: sb,
            method: "len".into(),
            args: vec![],
            method_id: Some(300),
            effects: EffectMask::PURE,
        });
    f1.get_block_mut(bb1)
        .unwrap()
        .add_instruction(MirInstruction::Return { value: Some(ln) });
    let mut m1 = MirModule::new("str_empty_len".into());
    m1.add_function(f1);
    let mut vm1 = VM::new();
    let out1 = vm1.execute_module(&m1).expect("vm exec");
    assert_eq!(out1.to_string_box().value, "0");

    // Case 2: indexOf not found returns -1
    let sig2 = FunctionSignature {
        name: "main".into(),
        params: vec![],
        return_type: MirType::Integer,
        effects: EffectMask::PURE,
    };
    let mut f2 = MirFunction::new(sig2, BasicBlockId::new(0));
    let bb2 = f2.entry_block;
    let s2 = f2.next_value_id();
    f2.get_block_mut(bb2)
        .unwrap()
        .add_instruction(MirInstruction::Const {
            dst: s2,
            value: ConstValue::String("abc".into()),
        });
    let sb2 = f2.next_value_id();
    f2.get_block_mut(bb2)
        .unwrap()
        .add_instruction(MirInstruction::NewBox {
            dst: sb2,
            box_type: "StringBox".into(),
            args: vec![s2],
        });
    let z = f2.next_value_id();
    f2.get_block_mut(bb2)
        .unwrap()
        .add_instruction(MirInstruction::Const {
            dst: z,
            value: ConstValue::String("z".into()),
        });
    let idx = f2.next_value_id();
    f2.get_block_mut(bb2)
        .unwrap()
        .add_instruction(MirInstruction::BoxCall {
            dst: Some(idx),
            box_val: sb2,
            method: "indexOf".into(),
            args: vec![z],
            method_id: Some(303),
            effects: EffectMask::PURE,
        });
    f2.get_block_mut(bb2)
        .unwrap()
        .add_instruction(MirInstruction::Return { value: Some(idx) });
    let mut m2 = MirModule::new("str_indexof_not_found".into());
    m2.add_function(f2);
    let mut vm2 = VM::new();
    let out2 = vm2.execute_module(&m2).expect("vm exec");
    assert_eq!(out2.to_string_box().value, "-1");

    // Case 3: Unicode substring by character indices: "a😊b"[1..2] == "😊"
    let sig3 = FunctionSignature {
        name: "main".into(),
        params: vec![],
        return_type: MirType::String,
        effects: EffectMask::PURE,
    };
    let mut f3 = MirFunction::new(sig3, BasicBlockId::new(0));
    let bb3 = f3.entry_block;
    let s3 = f3.next_value_id();
    f3.get_block_mut(bb3)
        .unwrap()
        .add_instruction(MirInstruction::Const {
            dst: s3,
            value: ConstValue::String("a😊b".into()),
        });
    let sb3 = f3.next_value_id();
    f3.get_block_mut(bb3)
        .unwrap()
        .add_instruction(MirInstruction::NewBox {
            dst: sb3,
            box_type: "StringBox".into(),
            args: vec![s3],
        });
    // Create const values first to avoid borrow checker issues
    let v1 = f3.next_value_id();
    f3.get_block_mut(bb3)
        .unwrap()
        .add_instruction(MirInstruction::Const {
            dst: v1,
            value: ConstValue::Integer(1),
        });
    let v2 = f3.next_value_id();
    f3.get_block_mut(bb3)
        .unwrap()
        .add_instruction(MirInstruction::Const {
            dst: v2,
            value: ConstValue::Integer(2),
        });
    // Now create the BoxCall with the pre-created values
    let sub = f3.next_value_id();
    f3.get_block_mut(bb3)
        .unwrap()
        .add_instruction(MirInstruction::BoxCall {
            dst: Some(sub),
            box_val: sb3,
            method: "substring".into(),
            args: vec![v1, v2],
            method_id: Some(301),
            effects: EffectMask::PURE,
        });
    f3.get_block_mut(bb3)
        .unwrap()
        .add_instruction(MirInstruction::Return { value: Some(sub) });
    let mut m3 = MirModule::new("str_unicode_substring".into());
    m3.add_function(f3);
    let mut vm3 = VM::new();
    let out3 = vm3.execute_module(&m3).expect("vm exec");
    assert_eq!(out3.to_string_box().value, "😊");
}