diff --git a/lib/checkstl.cpp b/lib/checkstl.cpp
index 844aa68a9..3b234196d 100644
--- a/lib/checkstl.cpp
+++ b/lib/checkstl.cpp
@@ -235,7 +235,7 @@ namespace {
             << "partition_point" << "pop_heap" << "prev_permutation" << "push_heap" << "random_shuffle" << "remove" << "remove_copy"
             << "remove_copy_if" << "remove_if" << "replace" << "replace_copy" << "replace_copy_if" << "replace_if" << "reverse" << "reverse_copy"
             << "search_n" << "shuffle" << "sort" << "sort_heap" << "stable_partition" << "stable_sort" << "swap_ranges" << "transform" << "unique"
-            << "unique_copy" << "upper_bound";
+            << "unique_copy" << "upper_bound" << "string" << "wstring" << "u16string" << "u32string";
     const std::set<std::string> algorithm22 = make_container< std::set<std::string> >() // func(begin1 << end1 << begin2 << end2
             << "find_end" << "find_first_of" << "includes" << "lexicographical_compare" << "merge" << "partial_sort_copy"
             << "search" << "set_difference" << "set_intersection" << "set_symmetric_difference" << "set_union";
@@ -289,6 +289,14 @@ void CheckStl::mismatchingContainers()
             tok = arg1->linkAt(-1);
         }
     }
+    for (unsigned int varid = 0; varid < symbolDatabase->getVariableListSize(); varid++) {
+        const Variable* var = symbolDatabase->getVariableFromVarId(varid);
+        if (var && var->isStlStringType() && Token::Match(var->nameToken(), "%var% (") && Token::Match(var->nameToken()->tokAt(2), pattern2.c_str())) {
+            if (var->nameToken()->strAt(2) != var->nameToken()->strAt(8)) {
+                mismatchingContainersError(var->nameToken());
+            }
+        }
+    }
 }
 
 
diff --git a/test/teststl.cpp b/test/teststl.cpp
index 46755a765..2a31e0c75 100644
--- a/test/teststl.cpp
+++ b/test/teststl.cpp
@@ -296,6 +296,20 @@ private:
               "    std::vector<int>::iterator it = std::find_first_of(ints1.begin(), ints1.end(), ints2.begin(), ints2.end());\n"
               "}");
         ASSERT_EQUALS("", errout.str());
+
+        // #6839
+        check("void f(const std::wstring& a, const std::wstring& b) {\n"
+              "    const std::string tp1 = std::string(a.begin(), b.end());\n"
+              "    const std::wstring tp2 = std::string(b.begin(), a.end());\n"
+              "    const std::u16string tp3(a.begin(), b.end());\n"
+              "    const std::u32string tp4(b.begin(), a.end());\n"
+              "    const std::string fp1 = std::string(a.begin(), a.end());\n"
+              "    const std::string tp2(a.begin(), a.end());\n"
+              "}");
+        ASSERT_EQUALS("[test.cpp:2]: (error) Iterators of different containers are used together.\n"
+                      "[test.cpp:3]: (error) Iterators of different containers are used together.\n"
+                      "[test.cpp:4]: (error) Iterators of different containers are used together.\n"
+                      "[test.cpp:5]: (error) Iterators of different containers are used together.\n", errout.str());
     }
 
     void iterator9() {