fix: fix incorrect model for empty for loop (#3108)

src/main/java/spoon/support/compiler/jdt/ParentExiter.java

@@ -543,7 +543,7 @@ public void visitCtFor(CtFor forLoop) {
 		} else if (isContainedInForUpdate() && child instanceof CtStatement) {
 			forLoop.addForUpdate((CtStatement) child);
 			return;
-		} else if (forLoop.getExpression() == null && child instanceof CtExpression) {
+		} else if (isContainedInForCondition() && child instanceof CtExpression) {
 			forLoop.setExpression((CtExpression<Boolean>) child);
 			return;
 		}
@@ -582,6 +582,14 @@ private boolean isContainedInForUpdate() {
 		return false;
 	}
 
+	private boolean isContainedInForCondition() {
+		if (!(jdtTreeBuilder.getContextBuilder().stack.peek().node instanceof ForStatement)) {
+			return false;
+		}
+		final ForStatement parent = (ForStatement) jdtTreeBuilder.getContextBuilder().stack.peek().node;
+		return parent.condition != null && parent.condition.equals(childJDT);
+	}
+
 	@Override
 	public void visitCtForEach(CtForEach foreach) {
 		if (foreach.getVariable() == null && child instanceof CtVariable) {

src/test/java/spoon/test/loop/LoopTest.java

@@ -17,6 +17,8 @@
 package spoon.test.loop;
 
 import org.junit.Test;
+import spoon.Launcher;
+import spoon.reflect.CtModel;
 import spoon.reflect.code.CtBlock;
 import spoon.reflect.code.CtFor;
 import spoon.reflect.code.CtForEach;
@@ -32,6 +34,7 @@
 import java.io.File;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 import static spoon.testing.utils.ModelUtils.build;
 
@@ -70,4 +73,16 @@ public void testForeachShouldHaveAlwaysABlockInItsBody() throws Exception {
 
 		assertEquals(expectedDebug, ctLoop.toString());
 	}
+
+	@Test
+	public void testEmptyForLoopExpression() {
+		Launcher launcher = new Launcher();
+		launcher.addInputResource("./src/test/java/spoon/test/loop/testclasses/EmptyLoops.java");
+		CtModel model = launcher.buildModel();
+		CtFor ctFor = model.getElements(new TypeFilter<>(CtFor.class)).get(0);
+		assertTrue(ctFor.getForInit().isEmpty());
+		assertNull(ctFor.getExpression());
+		assertTrue(ctFor.getForUpdate().isEmpty());
+		assertEquals("x = 5", ((CtBlock)ctFor.getBody()).getStatement(0).toString().trim());
+	}
 }

src/test/java/spoon/test/loop/testclasses/EmptyLoops.java

@@ -0,0 +1,8 @@
+package spoon.test.loop.testclasses;
+
+public class EmptyLoops {
+    void m1() {
+        int x = 0;
+        for (;;) x = 5;
+    }
+}