Remove non-valued return

(cherry picked from commit 42e515f43523ffbfe0b28c2f3f5c342e4d4b1c1d)

worldedit-core/src/main/antlr/com/sk89q/worldedit/antlr/Expression.g4

@@ -114,7 +114,7 @@ breakStatement : BREAK ;
 
 continueStatement : CONTINUE ;
 
-returnStatement : RETURN value=expression? ;
+returnStatement : RETURN value=expression ;
 
 switchStatement : SWITCH '(' target=expression ')' '{' (labels+=switchLabel ':' bodies+=statements )+ '}' ;
 

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

@@ -34,6 +34,8 @@ import java.time.Instant;
 import java.util.List;
 import java.util.Objects;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * Compiles and evaluates expressions.
  *
@@ -119,7 +121,9 @@ public class Expression {
 
         Instant deadline = Instant.now().plusMillis(timeout);
         // evaluation exceptions are thrown out of this method
-        return compiledExpression.execute(new ExecutionData(slots, functions, deadline));
+        Double result = compiledExpression.execute(new ExecutionData(slots, functions, deadline));
+        checkNotNull(result, "Expression must result in a value");
+        return result;
     }
 
     public void optimize() {

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

@@ -227,19 +227,12 @@ class CompilingVisitor extends ExpressionBaseVisitor<MethodHandle> {
 
     @Override
     public MethodHandle visitReturnStatement(ExpressionParser.ReturnStatementContext ctx) {
-        // MH:returnValue = (ExecutionData)Double
-        MethodHandle returnValue;
-        if (ctx.value != null) {
-            returnValue = evaluate(ctx.value).handle;
-        } else {
-            returnValue = defaultResult();
-        }
         return MethodHandles.filterArguments(
             // take the (Double)Double return statement
             RETURN_STATEMENT_BASE,
             0,
             // map the Double back to ExecutionData via the returnValue
-            returnValue
+            evaluate(ctx.value).handle
         );
     }
 

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

@@ -103,6 +103,12 @@ class ExpressionTest extends BaseExpressionTest {
                 () -> compile("x()"));
             assertEquals(0, e.getPosition(), "Error position");
         }
+        {
+            // verify that you must return a value
+            ExpressionException e = assertThrows(ExpressionException.class,
+                () -> compile("return"));
+            assertEquals(6, e.getPosition(), "Error position");
+        }
         assertThrows(ExpressionException.class,
             () -> compile("("));
         assertThrows(ExpressionException.class,
@@ -156,6 +162,24 @@ class ExpressionTest extends BaseExpressionTest {
     public void testWhile() throws ExpressionException {
         checkTestCase("c=5; a=0; while (c > 0) { ++a; --c; } a", 5);
         checkTestCase("c=5; a=0; do { ++a; --c; } while (c > 0); a", 5);
+        checkTestCase("" +
+            "c=5;" +
+            "a=0;" +
+            "while (c > 0) {" +
+            "   ++a;" +
+            "   --c;" +
+            "   if (c == 1) break;" +
+            "}" +
+            "a", 4);
+        checkTestCase("" +
+            "c=5;" +
+            "a=0;" +
+            "while (c > 0) {" +
+            "   ++a;" +
+            "   if (a < 5) continue;" +
+            "   --c;" +
+            "}" +
+            "a", 9);
     }
 
     @Test