diff --git a/tool/src/org/antlr/v4/semantics/SemanticPipeline.java b/tool/src/org/antlr/v4/semantics/SemanticPipeline.java
index 127733b5d..a75fd4e74 100644
--- a/tool/src/org/antlr/v4/semantics/SemanticPipeline.java
+++ b/tool/src/org/antlr/v4/semantics/SemanticPipeline.java
@@ -31,6 +31,7 @@ package org.antlr.v4.semantics;
 
 import org.antlr.v4.analysis.LeftRecursiveRuleTransformer;
 import org.antlr.v4.parse.ANTLRParser;
+import org.antlr.v4.runtime.Token;
 import org.antlr.v4.tool.*;
 import org.antlr.v4.tool.ast.GrammarAST;
 
@@ -174,6 +175,9 @@ public class SemanticPipeline {
 				String lit = alias.getChild(1).getText();
 				g.defineTokenAlias(name, lit);
 			}
+			else {
+				g.defineTokenName(alias.getText());
+			}
 		}
 
 		// DEFINE TOKEN TYPES FOR X : 'x' ; RULES
@@ -187,7 +191,13 @@ public class SemanticPipeline {
 		   */
 
 		// DEFINE TOKEN TYPES FOR TOKEN REFS LIKE ID, INT
-		for (GrammarAST idAST : tokenIDs) { g.defineTokenName(idAST.getText()); }
+		for (GrammarAST idAST : tokenIDs) {
+			if (g.getTokenType(idAST.getText()) == Token.INVALID_TYPE) {
+				g.tool.errMgr.grammarError(ErrorType.IMPLICIT_TOKEN_DEFINITION, g.fileName, idAST.token, idAST.getText());
+			}
+
+			g.defineTokenName(idAST.getText());
+		}
 
 		// DEFINE TOKEN TYPES FOR STRING LITERAL REFS LIKE 'while', ';'
 		for (String s : strings) { g.defineStringLiteral(s); }
diff --git a/tool/src/org/antlr/v4/tool/ErrorType.java b/tool/src/org/antlr/v4/tool/ErrorType.java
index 3d406d9dd..07ca65eec 100644
--- a/tool/src/org/antlr/v4/tool/ErrorType.java
+++ b/tool/src/org/antlr/v4/tool/ErrorType.java
@@ -144,6 +144,8 @@ public enum ErrorType {
 	RULE_WITH_TOO_FEW_ALT_LABELS(122, "rule <arg>: must label all alternatives or none", ErrorSeverity.ERROR),
 	ALT_LABEL_REDEF(123, "rule alt label <arg> redefined in rule <arg2>, originally in <arg3>", ErrorSeverity.ERROR),
 	ALT_LABEL_CONFLICTS_WITH_RULE(124, "rule alt label <arg> conflicts with rule <arg2>", ErrorSeverity.ERROR),
+	IMPLICIT_TOKEN_DEFINITION(125, "implicit definition of token <arg> in parser", ErrorSeverity.WARNING),
+
 	/** Documentation comment is unterminated */
     //UNTERMINATED_DOC_COMMENT(, "", ErrorSeverity.ERROR),