Add some debugging code (inspired by #6021)

lib/templatesimplifier.cpp

@@ -1274,6 +1274,7 @@ bool TemplateSimplifier::simplifyTemplateInstantiations(
             }
         }
         Token * const tok2 = *iter2;
+        assert(tokenlist.validateToken(tok2)); // that assertion fails on examples from #6021
         if (tok2->str() != name)
             continue;
 

lib/tokenlist.cpp

@@ -1107,3 +1107,14 @@ std::string TokenList::fileLine(const Token *tok) const
 {
     return ErrorLogger::ErrorMessage::FileLocation(tok, this).stringify();
 }
+
+bool TokenList::validateToken(const Token* tok) const
+{
+    if (!tok)
+        return true;
+    for (Token *t = _front; t; t = t->next()) {
+        if (tok==t)
+            return true;
+    }
+    return false;
+}

lib/tokenlist.h

@@ -137,6 +137,14 @@ public:
      */
     void validateAst();
 
+    /**
+     * Verify that the given token is an element of the tokenlist.
+     * That method is implemented for debugging purposes.
+     * @param[in] tok token to be checked
+     * \return true if token was found in tokenlist, false else. In case of nullptr true is returned.
+     */
+    bool validateToken(const Token* tok) const;
+
 private:
 
     /** Disable copy constructor, no implementation */