Fix RuleContext invariant violations in test code

src/main/java/org/antlr/v4/runtime/RuleContext.java

@@ -30,7 +30,7 @@
  * current context is the bottom of the stack. The ParserRuleContext subclass as a children list so that we can turn
  * this data structure into a tree.
  * <p>
- * The root node always has a null pointer and invokingState of -1.
+ * The root node of an actual parse tree always has a null parent pointer and invokingState of -1.
  * <p>
  * Upon entry to parsing, the first invoked rule function creates a context object (a subclass specialized for that rule
  * such as SContext) and makes it the root of a parse tree, recorded by field Parser._ctx.
@@ -67,11 +67,9 @@ public class RuleContext implements RuleNode {
   public @Nullable RuleContext parent;
 
   /**
-   * What state invoked the rule associated with this context? The "return address" is the followState of invokingState
-   * If parent is null, this should be -1 this context object represents the start rule.
+   * What state invoked the rule associated with this context? The "return address" is the followState of invokingState.
+   * For the root node of an actual parse tree, where parent is null, this should be -1.
    */
-  // todo на самом деле все не совсем так, есть варианты, когда
-  // todo parent == null, а стейт отличен от -1
   public int invokingState = -1;
 
   public RuleContext(@Nullable RuleContext parent, int invokingState) {

src/test/java/org/antlr/v4/runtime/tree/TreesTest.java

@@ -52,7 +52,7 @@ void setUp() {
     terminal1 = new TerminalNodeImpl(token1);
     terminal2 = new TerminalNodeImpl(token2);
 
-    root = new TestParserRuleContext(null, 0, 0);
+    root = new TestParserRuleContext(null, -1, 0);
     child1 = new TestParserRuleContext(root, -1, 1);
     child2 = new TestParserRuleContext(root, -1, 1);
     grandChild = new TestParserRuleContext(child1, -1, 2);
@@ -167,7 +167,7 @@ void testGetTokensFromParseTree() {
 
   @Test
   void testGetTokensFromParseTreeEmpty() {
-    ParserRuleContext emptyContext = new TestParserRuleContext(null, 0, 0);
+    ParserRuleContext emptyContext = new TestParserRuleContext(null, -1, 0);
     List<Token> tokens = Trees.getTokensFromParseTree(emptyContext);
     assertThat(tokens).isEmpty();
   }

src/test/java/org/antlr/v4/test/tool/ExpectedTokensTest.java

@@ -160,14 +160,15 @@ void testFollowIncludedInLeftRecursiveRule() throws Exception {
     ATN atn = g.getATN();
 
     // Simulate call stack after input '(x' from rule s
-    ParserRuleContext callStackFrom_s = new ParserRuleContext(null, 4);
+    ParserRuleContext root = ParserRuleContext.emptyContext();
+    ParserRuleContext callStackFrom_s = new ParserRuleContext(root, 4);
     ParserRuleContext callStackFrom_expr = new ParserRuleContext(callStackFrom_s, 9);
     int afterID = 14;
     IntervalSet tokens = atn.getExpectedTokens(afterID, callStackFrom_expr);
     assertThat(tokens.toString(g.getVocabulary())).isEqualTo("{R, PLUS}");
 
     // Simulate call stack after input '(x' from within rule expr
-    callStackFrom_expr = new ParserRuleContext(null, 9);
+    callStackFrom_expr = new ParserRuleContext(ParserRuleContext.emptyContext(), 9);
     tokens = atn.getExpectedTokens(afterID, callStackFrom_expr);
     assertThat(tokens.toString(g.getVocabulary())).isEqualTo("{R, PLUS}");
   }