Change grammar, enforce capabilities

Author: julian-elastic

x-pack/plugin/esql/src/main/antlr/parser/Join.g4

@@ -18,16 +18,8 @@ joinTarget
     ;
 
 joinCondition
-    : ON qualifiedName (COMMA qualifiedName)*                                #fieldBasedLookupJoin
-    | ON joinPredicateExpression (AND joinPredicateExpression)*              #expressionBasedLookupJoin
+    : ON booleanExpression (COMMA booleanExpression)*
     ;
 
-joinPredicate
-    : valueExpression
-    ;
-
-joinPredicateExpression
-    : left=operatorExpression comparisonOperator right=operatorExpression
-    ;
 
 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/enrich/ExpressionQueryList.java

@@ -37,6 +37,7 @@
 import java.util.ArrayList;
 import java.util.List;
 
+import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.LOOKUP_JOIN_ON_BOOLEAN_EXPRESSION;
 import static org.elasticsearch.xpack.esql.planner.TranslatorHandler.TRANSLATOR_HANDLER;
 
 /**
@@ -120,6 +121,9 @@ public static ExpressionQueryList expressionBasedJoin(
         AliasFilter aliasFilter,
         Warnings warnings
     ) {
+        if (LOOKUP_JOIN_ON_BOOLEAN_EXPRESSION.isEnabled() == false) {
+            throw new UnsupportedOperationException("Lookup Join on Boolean Expression capability is not enabled");
+        }
         if (request.getJoinOnConditions() == null) {
             throw new IllegalStateException("expressionBasedJoin must have join conditions");
         }