Require $$ for custom widgets, and that overridden JS widgets must exist

See https://github.com/Jermolene/TiddlyWiki5/pull/6666#issuecomment-1133637763

core/modules/parsers/wikiparser/rules/html.js

@@ -93,9 +93,6 @@ exports.parseTag = function(source,pos,options) {
 		return null;
 	}
 	node.tag = token.match[1];
-	if(node.tag.slice(1).indexOf("$") !== -1) {
-		return null;
-	}
 	if(node.tag.charAt(0) === "$") {
 		node.type = node.tag.substr(1);
 	}

core/modules/widgets/widget.js

@@ -435,10 +435,14 @@ options include:
 Widget.prototype.makeChildWidget = function(parseTreeNode,options) {
 	var self = this;
 	options = options || {};
-	// Check whether this node type is defined by a custom macro definition
+	// Check whether this node type is defined by a custom widget definition
 	var variableDefinitionName = "$" + parseTreeNode.type,
-		variableInfo = this.variables[variableDefinitionName];
+		variableInfo = this.variables[variableDefinitionName],
-	if(!parseTreeNode.isNotRemappable && variableInfo && variableInfo.value && variableInfo.isWidgetDefinition) {
+		isOverrideable = function() {
+			// Widget is overrideable if it has a double dollar user defined name, or if it is an existing JS widget
+			return parseTreeNode.type.charAt(0) === "$" || !!self.widgetClasses[parseTreeNode.type];
+		};
+	if(!parseTreeNode.isNotRemappable && isOverrideable() && variableInfo && variableInfo.value && variableInfo.isWidgetDefinition) {
 		var newParseTreeNode = {
 			type: "transclude",
 			children: [

editions/test/tiddlers/tests/data/transclude/CustomWidget-ActionWidget.tid

@@ -12,15 +12,15 @@ title: Output
 title: Actions
 
 \whitespace trim
-<!-- Define the <$action-mywidget> widget by defining a transcludable variable with that name -->
+<!-- Define the <$$action-mywidget> widget by defining a transcludable variable with that name -->
-\widget $action-mywidget(one:'Jaguar')
+\widget $$action-mywidget(one:'Jaguar')
 \whitespace trim
 <$action-setfield $tiddler="Result" $field="text" $value=<<one>>/>
 \end
 
-<$action-mywidget one="Dingo">
+<$$action-mywidget one="Dingo">
 	Crocodile
-</$action-mywidget>
+</$$action-mywidget>
 +
 title: ExpectedResult
 

editions/test/tiddlers/tests/data/transclude/CustomWidget-Fail.tid

@@ -0,0 +1,26 @@
+title: Transclude/CustomWidget/Fail
+description: Custom widget failed definition
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+\whitespace trim
+<!-- Attempt to define the <$non-existent-widget> widget by defining a transcludable variable with that name -->
+\widget $non-existent-widget(one:'Jaguar')
+\whitespace trim
+<$text text=<<one>>/>
+<$slot $name="ts-body">
+	Whale
+</$slot>
+\end
+<$non-existent-widget one="Dingo">
+	Crocodile
+</$non-existent-widget>
+<$non-existent-widget one="BumbleBee">
+	Squirrel
+</$non-existent-widget>
++
+title: ExpectedResult
+
+<p>Undefined widget 'non-existent-widget'Undefined widget 'non-existent-widget'</p>

editions/test/tiddlers/tests/data/transclude/CustomWidget-Simple.tid

@@ -12,20 +12,20 @@ title: Output
 title: TiddlerOne
 
 \whitespace trim
-<!-- Define the <$mywidget> widget by defining a transcludable variable with that name -->
+<!-- Define the <$$mywidget> widget by defining a transcludable variable with that name -->
-\widget $mywidget(one:'Jaguar')
+\widget $$mywidget(one:'Jaguar')
 \whitespace trim
 <$text text=<<one>>/>
 <$slot $name="ts-body">
 	Whale
 </$slot>
 \end
-<$mywidget one="Dingo">
+<$$mywidget one="Dingo">
 	Crocodile
-</$mywidget>
+</$$mywidget>
-<$mywidget one="BumbleBee">
+<$$mywidget one="BumbleBee">
 	Squirrel
-</$mywidget>
+</$$mywidget>
 +
 title: ExpectedResult
 

editions/test/tiddlers/tests/data/transclude/CustomWidget-VariableAttribute.tid

@@ -12,17 +12,17 @@ title: Output
 title: TiddlerOne
 
 \whitespace trim
-<!-- Redefine the <$mywidget> widget by defining a transcludable variable with that name -->
+<!-- Redefine the <$$mywidget> widget by defining a transcludable variable with that name -->
-\widget $mywidget($variable:'Jaguar')
+\widget $$mywidget($variable:'Jaguar')
 \whitespace trim
 <$text text=<<$variable>>/>
 <$slot $name="ts-body">
 	Whale
 </$slot>
 \end
-<$mywidget $variable="Dingo">
+<$$mywidget $variable="Dingo">
 	Crocodile
-</$mywidget>
+</$$mywidget>
 +
 title: ExpectedResult