feat: Implement IfExpression handling in JVM backend

Adds the capability to compile IfExpressions in the KayJam language to JVM bytecode.

Implemented `visitIfExpression` in:
- `TypeKayJamExpressionVisitor.java`: Performs type checking on the
  condition (must be boolean) and ensures type consistency between
  the true and false branches if an else clause exists.
- `JVMKayJamExpressionVisitor.java`: Generates JVM bytecode for
  if-else statements using appropriate labels and conditional/unconditional
  jump instructions (IFEQ, GOTO).

A new unit test `JVMIfExpressionTest.java` has been added. This test
compiles a simple KayJam class containing an if-expression within a
constant initializer and verifies that the corresponding .class file is
generated by the JVM backend compiler.

This provides foundational support for conditional logic in the language.

Author: google-labs-jules[bot]

backend/jvm/src/main/java/com/github/kayjamlang/backend/jvm/JVMKayJamExpressionVisitor.java

@@ -12,6 +12,7 @@
 import com.github.kayjamlang.core.expressions.loops.WhileExpression;
 import org.objectweb.asm.ClassWriter;
 import org.objectweb.asm.FieldVisitor;
+import org.objectweb.asm.Label;
 import org.objectweb.asm.MethodVisitor;
 
 import java.util.ArrayDeque;
@@ -219,6 +220,33 @@ public Object visitGetExpression(GetExpression expression) {
 
     @Override
     public Object visitIfExpression(IfExpression expression) {
+        MethodVisitor mv = currentMethodVisitor;
+        Label elseLabel = new Label();
+        Label endIfLabel = new Label();
+
+        // 1. Visit condition
+        visit(expression.condition); // Leaves boolean on stack
+        mv.visitJumpInsn(IFEQ, elseLabel); // Jumps if condition is false (0)
+        popStack(1); // IFEQ consumes the boolean from the stack
+
+        // 2. True branch
+        visit(expression.ifTrue);
+        if (expression.ifFalse != null) {
+            mv.visitJumpInsn(GOTO, endIfLabel); // Skip false branch
+        }
+        // If expression.ifFalse is null, execution naturally falls through to endIfLabel if condition was true.
+
+        // 3. False branch
+        mv.visitLabel(elseLabel);
+        if (expression.ifFalse != null) {
+            visit(expression.ifFalse);
+        }
+        // If ifFalse is null, and condition was false, we jump to elseLabel,
+        // execute nothing here, and then proceed to endIfLabel.
+
+        // 4. End-if
+        mv.visitLabel(endIfLabel);
+
         return null;
     }
 

backend/jvm/src/main/java/com/github/kayjamlang/backend/jvm/TypeKayJamExpressionVisitor.java

@@ -67,7 +67,22 @@ public JVMType visitGetExpression(GetExpression expression) {
 
     @Override
     public JVMType visitIfExpression(IfExpression expression) {
-        return null;
+        JVMType conditionType = visit(expression.condition);
+        if (!(conditionType instanceof BooleanJVMType)) {
+            throw new RuntimeException("If expression condition must be boolean, got " + conditionType);
+        }
+
+        JVMType trueBranchType = visit(expression.ifTrue);
+
+        if (expression.ifFalse != null) {
+            JVMType falseBranchType = visit(expression.ifFalse);
+            if (!trueBranchType.equals(falseBranchType)) {
+                throw new RuntimeException("If expression branches must have the same type, got " + trueBranchType + " and " + falseBranchType);
+            }
+            return trueBranchType;
+        } else {
+            return trueBranchType;
+        }
     }
 
     @Override

backend/jvm/src/test/java/com/github/kayjamlang/backend/jvm/JVMIfExpressionTest.java

@@ -0,0 +1,76 @@
+package com.github.kayjamlang.backend.jvm;
+
+import com.github.kayjamlang.core.KayJamFile; // May not be directly used, but good for context
+import org.junit.Test;
+import java.io.File;
+import java.io.FileWriter;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import static org.junit.Assert.assertTrue;
+
+public class JVMIfExpressionTest {
+
+    @Test
+    public void testIfElseExpressionCompilation() throws Exception {
+        Path inputDir = null;
+        Path outputDir = null;
+        try {
+            // a. Create temporary input and output directories
+            inputDir = Files.createTempDirectory("kayjam_input_");
+            outputDir = Files.createTempDirectory("kayjam_output_");
+
+            // b. Create a KayJam source file string
+            String fileName = "MyIfTest";
+            // Assuming KayJam syntax for a class with a constant initialized by an if-expression.
+            // The constant 'MyVal' will be part of the static initializer (<clinit>) of MyIfTestKJ.class
+            String kayJamSource = "class " + fileName + " { const MyVal = if (true) { 1 } else { 2 } }";
+            File sourceFile = new File(inputDir.toFile(), fileName + ".kj");
+
+            try (FileWriter writer = new FileWriter(sourceFile)) {
+                writer.write(kayJamSource);
+            }
+
+            // c. Create JVMBackendOptions
+            JVMBackendOptions options = new JVMBackendOptions();
+            options.setInputDir(inputDir.toFile());
+            options.setOutputDir(outputDir.toFile());
+            // options.setNamespace(""); // Assuming default empty namespace for now
+
+            // d. Compile
+            JVMBackendCompiler compiler = JVMBackendCompiler.INSTANCE;
+            compiler.compile(options);
+
+            // e. Check if the class file was created
+            // Based on JVMKayJamExpressionVisitor:
+            // namespace is normalized. If empty, it's likely just the class name.
+            // className is normalizeClassName(container.name) + "KJ" for files, or normalizeClassName(classContainer.name) for classes.
+            // For a class named MyIfTest, this should become MyIfTest.class (if it's a class container direct output)
+            // or MyIfTestKJ.class if it's treated like a file container.
+            // The current KayJamFile logic wraps classes within a file, so the file name matters.
+            // The file is MyIfTest.kj, so it might generate MyIfTestKJ.class in the root of outputDir.
+            // However, the KayJamSource defines a class "MyIfTest".
+            // The visitKayJamFile method in JVMKayJamExpressionVisitor processes classes defined within the file.
+            // It uses `packageName + "/" + normalizeClassName(classContainer.name)`
+            // Adjust class name based on actual naming convention by compiler
+            // As per subtask description, checking for fileName + "KJ.class"
+            File expectedClassFile = new File(outputDir.toFile(), fileName + "KJ.class"); 
+            assertTrue("Expected class file was not generated: " + expectedClassFile.getAbsolutePath(), expectedClassFile.exists());
+
+        } finally {
+            // f. Cleanup (optional for worker, but good practice)
+            if (inputDir != null) {
+                // Simple cleanup, could be more robust
+                Files.walk(inputDir)
+                        .sorted(java.util.Comparator.reverseOrder())
+                        .map(Path::toFile)
+                        .forEach(File::delete);
+            }
+            if (outputDir != null) {
+                Files.walk(outputDir)
+                        .sorted(java.util.Comparator.reverseOrder())
+                        .map(Path::toFile)
+                        .forEach(File::delete);
+            }
+        }
+    }
+}