Fix pre- and post- ops

(cherry picked from commit 0f787a89b8f2b0f29e0e4a2327224a3b0d050171)

worldedit-core/src/main/java/com/sk89q/worldedit/internal/expression/invoke/CompilingVisitor.java

@@ -269,13 +269,14 @@ class CompilingVisitor extends ExpressionBaseVisitor<MethodHandle> {
         return ExpressionHandles.call(data -> {
             LocalSlot.Variable variable = ExpressionHandles.getVariable(data, target);
             double value = variable.getValue();
+            double result = value;
             if (opType == INCREMENT) {
                 value++;
             } else {
                 value--;
             }
             variable.setValue(value);
-            return value;
+            return result;
         });
     }
 
@@ -286,14 +287,13 @@ class CompilingVisitor extends ExpressionBaseVisitor<MethodHandle> {
         return ExpressionHandles.call(data -> {
             LocalSlot.Variable variable = ExpressionHandles.getVariable(data, target);
             double value = variable.getValue();
-            double result = value;
             if (opType == INCREMENT) {
                 value++;
             } else {
                 value--;
             }
             variable.setValue(value);
-            return result;
+            return value;
         });
     }
 

worldedit-core/src/test/java/com/sk89q/worldedit/internal/expression/ExpressionTest.java

@@ -84,6 +84,14 @@ class ExpressionTest extends BaseExpressionTest {
         checkTestCase("3+1", 4);
     }
 
+    @Test
+    void testPostPreOps() {
+        checkTestCase("a=0; b=a++; a+b", 1);
+        checkTestCase("a=0; b=++a; a+b", 2);
+        checkTestCase("a=0; b=a--; a+b", -1);
+        checkTestCase("a=0; b=--a; a+b", -2);
+    }
+
     @Test
     public void testErrors() {
         // test lexer errors