From 274a69871322d072158d03ea4e9cf9e5a150ff91 Mon Sep 17 00:00:00 2001
From: Bartek Kryza <bkryza@gmail.com>
Date: Sat, 25 Feb 2023 01:50:07 +0100
Subject: [PATCH] Initial support for concept dependency relationships in class
 diagrams

---
 .../plantuml/class_diagram_generator.cc       | 160 +++++++++++++
 .../plantuml/class_diagram_generator.h        |   8 +
 src/class_diagram/model/class.h               |   4 -
 src/class_diagram/model/concept.cc            |  72 ++++++
 src/class_diagram/model/concept.h             |  59 +++++
 src/class_diagram/model/diagram.cc            | 105 ++++++++-
 src/class_diagram/model/diagram.h             |  14 ++
 .../visitor/translation_unit_visitor.cc       | 221 +++++++++++++++++-
 .../visitor/translation_unit_visitor.h        |  17 +-
 src/common/clang_utils.cc                     |   8 +
 src/common/clang_utils.h                      |   3 +
 src/common/model/template_parameter.cc        |  23 +-
 src/common/model/template_parameter.h         |   7 +
 src/util/util.h                               |  33 +++
 tests/t00056/t00056.cc                        |  50 ++--
 15 files changed, 750 insertions(+), 34 deletions(-)
 create mode 100644 src/class_diagram/model/concept.cc
 create mode 100644 src/class_diagram/model/concept.h

diff --git a/src/class_diagram/generators/plantuml/class_diagram_generator.cc b/src/class_diagram/generators/plantuml/class_diagram_generator.cc
index 8b18db8b..c41385cc 100644
--- a/src/class_diagram/generators/plantuml/class_diagram_generator.cc
+++ b/src/class_diagram/generators/plantuml/class_diagram_generator.cc
@@ -102,6 +102,23 @@ void generator::generate_alias(const enum_ &e, std::ostream &ostr) const
     m_generated_aliases.emplace(e.alias());
 }
 
+void generator::generate_alias(const concept_ &c, std::ostream &ostr) const
+{
+    print_debug(c, ostr);
+
+    if (m_config.generate_packages())
+        ostr << "annotation"
+             << " \"" << c.name();
+    else
+        ostr << "annotation"
+             << " \"" << render_name(c.full_name());
+
+    ostr << "\" as " << c.alias() << '\n';
+
+    // Register the added alias
+    m_generated_aliases.emplace(c.alias());
+}
+
 void generator::generate(const class_ &c, std::ostream &ostr) const
 {
     namespace plantuml_common = clanguml::common::generators::plantuml;
@@ -279,6 +296,26 @@ void generator::generate(const class_ &c, std::ostream &ostr) const
         generate_member_notes(ostr, method, c.alias());
 }
 
+void generator::generate(const concept_ &c, std::ostream &ostr) const
+{
+    namespace plantuml_common = clanguml::common::generators::plantuml;
+
+    std::string class_type{"annotation"};
+
+    ostr << class_type << " " << c.alias() << " <<concept>>";
+
+    if (m_config.generate_links) {
+        common_generator<diagram_config, diagram_model>::generate_link(ostr, c);
+    }
+
+    if (!c.style().empty())
+        ostr << " " << c.style();
+
+    ostr << " {" << '\n';
+
+    ostr << "}" << '\n';
+}
+
 void generator::generate_member_notes(std::ostream &ostr,
     const class_element &member, const std::string &alias) const
 {
@@ -309,6 +346,11 @@ void generator::generate_relationships(std::ostream &ostr) const
                 generate_relationships(*enm, ostr);
             }
         }
+        else if (auto *cpt = dynamic_cast<concept_ *>(p.get()); cpt) {
+            if (m_model.should_include(*cpt)) {
+                generate_relationships(*cpt, ostr);
+            }
+        }
     }
 }
 
@@ -409,6 +451,82 @@ void generator::generate_relationships(
     ostr << all_relations_str.str();
 }
 
+void generator::generate_relationships(
+    const concept_ &c, std::ostream &ostr) const
+{
+    namespace plantuml_common = clanguml::common::generators::plantuml;
+
+    //
+    // Process relationships
+    //
+    std::set<std::string> rendered_relations;
+
+    std::stringstream all_relations_str;
+    std::set<std::string> unique_relations;
+
+    for (const auto &r : c.relationships()) {
+        if (!m_model.should_include(r.type()))
+            continue;
+
+        LOG_DBG("== Processing relationship {}",
+            plantuml_common::to_plantuml(r.type(), r.style()));
+
+        std::stringstream relstr;
+        clanguml::common::id_t destination{0};
+        try {
+            destination = r.destination();
+
+            std::string puml_relation;
+            if (!r.multiplicity_source().empty())
+                puml_relation += "\"" + r.multiplicity_source() + "\" ";
+
+            puml_relation += plantuml_common::to_plantuml(r.type(), r.style());
+
+            if (!r.multiplicity_destination().empty())
+                puml_relation += " \"" + r.multiplicity_destination() + "\"";
+
+            std::string target_alias;
+            try {
+                target_alias = m_model.to_alias(destination);
+            }
+            catch (...) {
+                LOG_DBG("Failed to find alias to {}", destination);
+                continue;
+            }
+
+            if (m_generated_aliases.find(target_alias) ==
+                m_generated_aliases.end())
+                continue;
+
+            relstr << c.alias() << " " << puml_relation << " " << target_alias;
+
+            if (!r.label().empty()) {
+                relstr << " : " << plantuml_common::to_plantuml(r.access())
+                       << r.label();
+                rendered_relations.emplace(r.label());
+            }
+
+            if (unique_relations.count(relstr.str()) == 0) {
+                unique_relations.emplace(relstr.str());
+
+                relstr << '\n';
+
+                LOG_DBG("=== Adding relation {}", relstr.str());
+
+                all_relations_str << relstr.str();
+            }
+        }
+        catch (error::uml_alias_missing &e) {
+            LOG_DBG("=== Skipping {} relation from {} to {} due "
+                    "to: {}",
+                plantuml_common::to_plantuml(r.type(), r.style()),
+                c.full_name(), destination, e.what());
+        }
+    }
+
+    ostr << all_relations_str.str();
+}
+
 void generator::generate(const enum_ &e, std::ostream &ostr) const
 {
     ostr << "enum " << e.alias();
@@ -534,6 +652,20 @@ void generator::generate(const package &p, std::ostream &ostr) const
                 }
             }
         }
+        else if (auto *cpt = dynamic_cast<concept_ *>(subpackage.get()); cpt) {
+            if (m_model.should_include(*subpackage)) {
+                auto together_group =
+                    m_config.get_together_group(cpt->full_name(false));
+                if (together_group) {
+                    together_group_stack_.group_together(
+                        together_group.value(), cpt);
+                }
+                else {
+                    generate_alias(*cpt, ostr);
+                    generate(*cpt, ostr);
+                }
+            }
+        }
     }
 
     if (m_config.generate_packages()) {
@@ -552,6 +684,10 @@ void generator::generate(const package &p, std::ostream &ostr) const
                     generate_alias(*enm, ostr);
                     generate(*enm, ostr);
                 }
+                if (auto *cpt = dynamic_cast<concept_ *>(e); cpt) {
+                    generate_alias(*cpt, ostr);
+                    generate(*cpt, ostr);
+                }
             }
 
             ostr << "}\n";
@@ -589,6 +725,12 @@ void generator::generate_relationships(
                     dynamic_cast<enum_ &>(*subpackage), ostr);
             }
         }
+        else if (dynamic_cast<concept_ *>(subpackage.get()) != nullptr) {
+            if (m_model.should_include(*subpackage)) {
+                generate_relationships(
+                    dynamic_cast<concept_ &>(*subpackage), ostr);
+            }
+        }
     }
 }
 
@@ -648,6 +790,20 @@ void generator::generate_top_level_elements(std::ostream &ostr) const
                 }
             }
         }
+        else if (auto *cpt = dynamic_cast<concept_ *>(p.get()); cpt) {
+            if (m_model.should_include(*cpt)) {
+                auto together_group =
+                    m_config.get_together_group(cpt->full_name(false));
+                if (together_group) {
+                    together_group_stack_.group_together(
+                        together_group.value(), cpt);
+                }
+                else {
+                    generate_alias(*cpt, ostr);
+                    generate(*cpt, ostr);
+                }
+            }
+        }
     }
 }
 
@@ -666,6 +822,10 @@ void generator::generate_groups(std::ostream &ostr) const
                 generate_alias(*enm, ostr);
                 generate(*enm, ostr);
             }
+            if (auto *cpt = dynamic_cast<concept_ *>(e); cpt) {
+                generate_alias(*cpt, ostr);
+                generate(*cpt, ostr);
+            }
         }
 
         ostr << "}\n";
diff --git a/src/class_diagram/generators/plantuml/class_diagram_generator.h b/src/class_diagram/generators/plantuml/class_diagram_generator.h
index 063a286b..4929c7c1 100644
--- a/src/class_diagram/generators/plantuml/class_diagram_generator.h
+++ b/src/class_diagram/generators/plantuml/class_diagram_generator.h
@@ -18,6 +18,7 @@
 #pragma once
 
 #include "class_diagram/model/class.h"
+#include "class_diagram/model/concept.h"
 #include "class_diagram/model/diagram.h"
 #include "class_diagram/model/enum.h"
 #include "class_diagram/visitor/translation_unit_visitor.h"
@@ -47,6 +48,7 @@ using common_generator =
 
 using clanguml::class_diagram::model::class_;
 using clanguml::class_diagram::model::class_element;
+using clanguml::class_diagram::model::concept_;
 using clanguml::class_diagram::model::enum_;
 using clanguml::common::model::access_t;
 using clanguml::common::model::package;
@@ -65,6 +67,8 @@ public:
 
     void generate_alias(const enum_ &e, std::ostream &ostr) const;
 
+    void generate_alias(const concept_ &c, std::ostream &ostr) const;
+
     void generate(const class_ &c, std::ostream &ostr) const;
 
     void generate_top_level_elements(std::ostream &ostr) const;
@@ -77,6 +81,10 @@ public:
 
     void generate_relationships(const enum_ &c, std::ostream &ostr) const;
 
+    void generate(const concept_ &c, std::ostream &ostr) const;
+
+    void generate_relationships(const concept_ &c, std::ostream &ostr) const;
+
     void generate(const package &p, std::ostream &ostr) const;
 
     void generate_relationships(const package &p, std::ostream &ostr) const;
diff --git a/src/class_diagram/model/class.h b/src/class_diagram/model/class.h
index eb346177..97b55e19 100644
--- a/src/class_diagram/model/class.h
+++ b/src/class_diagram/model/class.h
@@ -79,9 +79,6 @@ public:
 
     bool is_abstract() const;
 
-    bool is_concept() const { return is_concept_; }
-    void is_concept(bool concept_) { is_concept_ = concept_; }
-
     void find_relationships(
         std::vector<std::pair<std::string, common::model::relationship_t>>
             &nested_relationships);
@@ -95,7 +92,6 @@ private:
     bool is_template_instantiation_{false};
     bool is_alias_{false};
     bool is_union_{false};
-    bool is_concept_{false};
     std::vector<class_member> members_;
     std::vector<class_method> methods_;
     std::vector<class_parent> bases_;
diff --git a/src/class_diagram/model/concept.cc b/src/class_diagram/model/concept.cc
new file mode 100644
index 00000000..fa4a4a18
--- /dev/null
+++ b/src/class_diagram/model/concept.cc
@@ -0,0 +1,72 @@
+/**
+ * src/class_diagram/model/concept.cc
+ *
+ * Copyright (c) 2021-2023 Bartek Kryza <bkryza@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "concept.h"
+
+#include <sstream>
+
+namespace clanguml::class_diagram::model {
+
+concept_::concept_(const common::model::namespace_ &using_namespace)
+    : element{using_namespace}
+{
+}
+
+bool operator==(const concept_ &l, const concept_ &r)
+{
+    return l.id() == r.id();
+}
+
+std::string concept_::full_name_no_ns() const
+{
+    using namespace clanguml::util;
+
+    std::ostringstream ostr;
+
+    ostr << name();
+
+    render_template_params(ostr, using_namespace(), false);
+
+    return ostr.str();
+}
+
+std::string concept_::full_name(bool relative) const
+{
+    using namespace clanguml::util;
+    using clanguml::common::model::namespace_;
+
+    std::ostringstream ostr;
+
+    ostr << name_and_ns();
+
+    render_template_params(ostr, using_namespace(), relative);
+
+    std::string res;
+
+    if (relative)
+        res = using_namespace().relative(ostr.str());
+    else
+        res = ostr.str();
+
+    if (res.empty())
+        return "<<anonymous>>";
+
+    return res;
+}
+
+}
diff --git a/src/class_diagram/model/concept.h b/src/class_diagram/model/concept.h
new file mode 100644
index 00000000..1ae9349e
--- /dev/null
+++ b/src/class_diagram/model/concept.h
@@ -0,0 +1,59 @@
+/**
+ * src/class_diagram/model/concept.h
+ *
+ * Copyright (c) 2021-2023 Bartek Kryza <bkryza@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include "common/model/element.h"
+#include "common/model/stylable_element.h"
+#include "common/model/template_parameter.h"
+#include "common/model/template_trait.h"
+#include "common/types.h"
+
+#include <string>
+#include <vector>
+
+namespace clanguml::class_diagram::model {
+
+struct requires_expression {
+    common::model::template_parameter parameter;
+    std::vector<std::string> requirements;
+};
+
+class concept_ : public common::model::element,
+                 public common::model::stylable_element,
+                 public common::model::template_trait {
+public:
+    concept_(const common::model::namespace_ &using_namespace);
+
+    concept_(const concept_ &) = delete;
+    concept_(concept_ &&) noexcept = default;
+    concept_ &operator=(const concept_ &) = delete;
+    concept_ &operator=(concept_ &&) = delete;
+
+    std::string type_name() const override { return "concept"; }
+
+    friend bool operator==(const concept_ &l, const concept_ &r);
+
+    std::string full_name(bool relative = true) const override;
+
+    std::string full_name_no_ns() const override;
+
+    std::vector<std::string> requires_expression_;
+
+    std::string full_name_;
+};
+}
\ No newline at end of file
diff --git a/src/class_diagram/model/diagram.cc b/src/class_diagram/model/diagram.cc
index 2a365684..56b37016 100644
--- a/src/class_diagram/model/diagram.cc
+++ b/src/class_diagram/model/diagram.cc
@@ -32,6 +32,11 @@ const common::reference_vector<class_> &diagram::classes() const
 
 const common::reference_vector<enum_> &diagram::enums() const { return enums_; }
 
+const common::reference_vector<concept_> &diagram::concepts() const
+{
+    return concepts_;
+}
+
 common::model::diagram_t diagram::type() const
 {
     return common::model::diagram_t::kClass;
@@ -48,6 +53,11 @@ common::optional_ref<clanguml::common::model::diagram_element> diagram::get(
 
     res = get_enum(full_name);
 
+    if (res.has_value())
+        return res;
+
+    res = get_concept(full_name);
+
     return res;
 }
 
@@ -63,6 +73,11 @@ common::optional_ref<clanguml::common::model::diagram_element> diagram::get(
 
     res = get_enum(id);
 
+    if (res.has_value())
+        return res;
+
+    res = get_concept(id);
+
     return res;
 }
 
@@ -78,6 +93,12 @@ bool diagram::has_enum(const enum_ &e) const
         [&e](const auto &ee) { return ee.get().full_name() == e.full_name(); });
 }
 
+bool diagram::has_concept(const concept_ &c) const
+{
+    return std::any_of(concepts_.cbegin(), concepts_.cend(),
+        [&c](const auto &cc) { return cc.get() == c; });
+}
+
 common::optional_ref<class_> diagram::get_class(const std::string &name) const
 {
     for (const auto &c : classes_) {
@@ -126,6 +147,32 @@ common::optional_ref<enum_> diagram::get_enum(
     return {};
 }
 
+common::optional_ref<concept_> diagram::get_concept(
+    const std::string &name) const
+{
+    for (const auto &c : concepts_) {
+        const auto full_name = c.get().full_name(false);
+
+        if (full_name == name) {
+            return {c};
+        }
+    }
+
+    return {};
+}
+
+common::optional_ref<concept_> diagram::get_concept(
+    clanguml::common::model::diagram_element::id_t id) const
+{
+    for (const auto &c : concepts_) {
+        if (c.get().id() == id) {
+            return {c};
+        }
+    }
+
+    return {};
+}
+
 bool diagram::add_package(std::unique_ptr<common::model::package> &&p)
 {
     LOG_DBG("Adding namespace package: {}, {}", p->name(), p->full_name(true));
@@ -173,8 +220,8 @@ bool diagram::add_class(std::unique_ptr<class_> &&c)
         }
     }
     catch (const std::runtime_error &e) {
-        LOG_WARN("Cannot add template specialization {} with id {} due to: {}",
-            name, id, e.what());
+        LOG_WARN(
+            "Cannot add concept {} with id {} due to: {}", name, id, e.what());
         return false;
     }
 
@@ -207,6 +254,55 @@ bool diagram::add_enum(std::unique_ptr<enum_> &&e)
     return false;
 }
 
+bool diagram::add_concept(std::unique_ptr<concept_> &&c)
+{
+    const auto base_name = c->name();
+    const auto full_name = c->full_name(false);
+
+    LOG_DBG("Adding concept: {}::{}, {}", c->get_namespace().to_string(),
+        base_name, full_name);
+
+    if (util::contains(base_name, "::"))
+        throw std::runtime_error("Name cannot contain namespace: " + base_name);
+
+    if (util::contains(base_name, "*"))
+        throw std::runtime_error("Name cannot contain *: " + base_name);
+
+    const auto ns = c->get_relative_namespace();
+    auto name = base_name;
+    auto name_with_ns = c->name_and_ns();
+    auto name_and_ns = ns | name;
+    auto &cc = *c;
+    auto id = cc.id();
+
+    try {
+        if (!has_concept(cc)) {
+            if (add_element(ns, std::move(c)))
+                concepts_.push_back(std::ref(cc));
+
+            const auto &el = get_element<concept_>(name_and_ns).value();
+
+            if ((el.name() != name) || !(el.get_relative_namespace() == ns))
+                throw std::runtime_error(
+                    "Invalid element stored in the diagram tree");
+
+            LOG_DBG("Added concept {} ({} - [{}])", base_name, full_name, id);
+
+            return true;
+        }
+    }
+    catch (const std::runtime_error &e) {
+        LOG_WARN(
+            "Cannot add concept {} with id {} due to: {}", name, id, e.what());
+        return false;
+    }
+
+    LOG_DBG("Concept {} ({} - [{}]) already in the model", base_name, full_name,
+        id);
+
+    return false;
+}
+
 void diagram::get_parents(
     clanguml::common::reference_set<class_> &parents) const
 {
@@ -257,6 +353,11 @@ std::string diagram::to_alias(
             return e.get().alias();
     }
 
+    for (const auto &c : concepts_) {
+        if (c.get().id() == id)
+            return c.get().alias();
+    }
+
     throw error::uml_alias_missing(fmt::format("Missing alias for {}", id));
 }
 
diff --git a/src/class_diagram/model/diagram.h b/src/class_diagram/model/diagram.h
index 61ffee1a..cb0e39b5 100644
--- a/src/class_diagram/model/diagram.h
+++ b/src/class_diagram/model/diagram.h
@@ -22,6 +22,7 @@
 #include "common/model/nested_trait.h"
 #include "common/model/package.h"
 #include "common/types.h"
+#include "concept.h"
 #include "enum.h"
 
 #include <string>
@@ -54,10 +55,14 @@ public:
 
     const common::reference_vector<enum_> &enums() const;
 
+    const common::reference_vector<concept_> &concepts() const;
+
     bool has_class(const class_ &c) const;
 
     bool has_enum(const enum_ &e) const;
 
+    bool has_concept(const concept_ &e) const;
+
     common::optional_ref<class_> get_class(const std::string &name) const;
 
     common::optional_ref<class_> get_class(
@@ -68,10 +73,17 @@ public:
     common::optional_ref<enum_> get_enum(
         clanguml::common::model::diagram_element::id_t id) const;
 
+    common::optional_ref<concept_> get_concept(const std::string &name) const;
+
+    common::optional_ref<concept_> get_concept(
+        clanguml::common::model::diagram_element::id_t id) const;
+
     bool add_class(std::unique_ptr<class_> &&c);
 
     bool add_enum(std::unique_ptr<enum_> &&e);
 
+    bool add_concept(std::unique_ptr<concept_> &&e);
+
     bool add_package(std::unique_ptr<common::model::package> &&p);
 
     std::string to_alias(
@@ -90,6 +102,8 @@ private:
     common::reference_vector<class_> classes_;
 
     common::reference_vector<enum_> enums_;
+
+    common::reference_vector<concept_> concepts_;
 };
 } // namespace clanguml::class_diagram::model
 
diff --git a/src/class_diagram/visitor/translation_unit_visitor.cc b/src/class_diagram/visitor/translation_unit_visitor.cc
index 62ddcafa..5bfc8bc0 100644
--- a/src/class_diagram/visitor/translation_unit_visitor.cc
+++ b/src/class_diagram/visitor/translation_unit_visitor.cc
@@ -19,6 +19,7 @@
 #include "translation_unit_visitor.h"
 #include "common/clang_utils.h"
 
+#include <clang/AST/ExprConcepts.h>
 #include <clang/Basic/FileManager.h>
 #include <clang/Lex/Preprocessor.h>
 #include <spdlog/spdlog.h>
@@ -284,7 +285,7 @@ bool translation_unit_visitor::VisitClassTemplateDecl(
     // Override the id with the template id, for now we don't care about the
     // underlying templated class id
 
-    process_template_parameters(*cls, *c_ptr);
+    process_template_parameters(*cls, *c_ptr, *c_ptr);
 
     const auto cls_full_name = c_ptr->full_name(false);
     const auto id = common::to_id(cls_full_name);
@@ -293,6 +294,15 @@ bool translation_unit_visitor::VisitClassTemplateDecl(
 
     set_ast_local_id(cls->getID(), id);
 
+    llvm::SmallVector<const clang::Expr *, 24> constraints{};
+    if (cls->hasAssociatedConstraints()) {
+        cls->getAssociatedConstraints(constraints);
+    }
+
+    for (const auto *expr : constraints) {
+        find_relationships_in_constraint_expression(*c_ptr, expr);
+    }
+
     if (!cls->getTemplatedDecl()->isCompleteDefinition()) {
         forward_declarations_.emplace(id, std::move(c_ptr));
         return true;
@@ -367,6 +377,155 @@ bool translation_unit_visitor::VisitRecordDecl(clang::RecordDecl *rec)
     return true;
 }
 
+bool translation_unit_visitor::TraverseConceptDecl(clang::ConceptDecl *cpt)
+{
+    // Skip system headers
+    if (source_manager().isInSystemHeader(cpt->getSourceRange().getBegin()))
+        return true;
+
+    if (!diagram().should_include(cpt->getQualifiedNameAsString()))
+        return true;
+
+    LOG_DBG("= Visiting concept (isType: {}) declaration {} at {}",
+        cpt->isTypeConcept(), cpt->getQualifiedNameAsString(),
+        cpt->getLocation().printToString(source_manager()));
+
+    auto concept_model = create_concept_declaration(cpt);
+
+    if (!concept_model)
+        return true;
+
+    const auto concept_id = concept_model->id();
+
+    set_ast_local_id(cpt->getID(), concept_id);
+
+    process_template_parameters(*cpt, *concept_model);
+
+    if (const auto *constraint =
+            clang::dyn_cast<clang::RequiresExpr>(cpt->getConstraintExpr());
+        constraint) {
+
+        auto constraint_source = common::to_string(constraint);
+
+        LOG_DBG("== Processing constraint: '{}'", constraint_source);
+
+        for (const auto *requirement : constraint->getRequirements()) {
+            LOG_DBG("== Processing requirement: '{}'", requirement->getKind());
+        }
+
+        // process 'requires (...)' declaration
+        for (const auto *decl : constraint->getBody()->decls()) {
+            // decl->dump();
+
+            if (const auto *parm_var_decl =
+                    clang::dyn_cast<clang::ParmVarDecl>(decl);
+                parm_var_decl) {
+                parm_var_decl->getQualifiedNameAsString();
+
+                LOG_DBG("=== Processing parameter variable declaration: {}, {}",
+                    parm_var_decl->getQualifiedNameAsString(),
+                    common::to_string(
+                        parm_var_decl->getType(), cpt->getASTContext()));
+            }
+            else {
+                LOG_DBG(
+                    "=== Processing some other declaration: {}", decl->getID());
+            }
+        }
+
+        // process concept body requirements '{ }' if any
+        for (const auto *req : constraint->getRequirements()) {
+            if (req->getKind() == clang::concepts::Requirement::RK_Simple) {
+                const auto *simple_req =
+                    clang::dyn_cast<clang::concepts::ExprRequirement>(req);
+                LOG_DBG("=== Processing expression requirement: {}",
+                    common::to_string(simple_req->getExpr()));
+            }
+            else if (req->getKind() == clang::concepts::Requirement::RK_Type) {
+                const auto *type_req =
+                    clang::dyn_cast<clang::concepts::TypeRequirement>(req);
+                LOG_DBG(
+                    "=== Processing type requirement: {}", type_req->getKind());
+            }
+            else if (req->getKind() ==
+                clang::concepts::Requirement::RK_Nested) {
+                const auto *nested_req =
+                    clang::dyn_cast<clang::concepts::NestedRequirement>(req);
+                LOG_DBG("=== Processing nested requirement: {}",
+                    common::to_string(nested_req->getConstraintExpr()));
+            }
+            else if (req->getKind() ==
+                clang::concepts::Requirement::RK_Compound) {
+                const auto *nested_req =
+                    clang::dyn_cast<clang::concepts::ExprRequirement>(req);
+                LOG_DBG("=== Processing compound requirement: {}",
+                    common::to_string(nested_req->getExpr()));
+            }
+        }
+    }
+    else {
+        // TODO
+    }
+
+    if (cpt->getConstraintExpr())
+        find_relationships_in_constraint_expression(
+            *concept_model, cpt->getConstraintExpr());
+
+    if (diagram_.should_include(*concept_model)) {
+        LOG_DBG("Adding concept {} with id {}", concept_model->full_name(false),
+            concept_model->id());
+
+        diagram_.add_concept(std::move(concept_model));
+    }
+    else {
+        LOG_DBG("Skipping concept {} with id {}", concept_model->full_name(),
+            concept_model->id());
+    }
+
+    return true;
+}
+
+void translation_unit_visitor::find_relationships_in_constraint_expression(
+    clanguml::common::model::element &c, const clang::Expr *expr)
+{
+    if (expr == nullptr)
+        return;
+
+    if (const auto *concept_specialization =
+            clang::dyn_cast<clang::ConceptSpecializationExpr>(expr);
+        concept_specialization) {
+        if (concept_specialization->getNamedConcept() &&
+            diagram().should_include(concept_specialization->getNamedConcept()
+                                         ->getQualifiedNameAsString())) {
+            auto target_id = get_ast_local_id(
+                concept_specialization->getNamedConcept()->getID())
+                                 .value();
+
+            for (const auto ta :
+                concept_specialization->getTemplateArguments()) {
+                if (ta.getKind() == clang::TemplateArgument::Template)
+                    ta.getAsTemplateOrTemplatePattern().dump();
+            }
+
+            c.add_relationship({relationship_t::kDependency, target_id});
+        }
+    }
+    else if (const auto *constraint =
+                 clang::dyn_cast<clang::RequiresExpr>(expr);
+             constraint) {
+        // TODO
+    }
+    else if (const auto *binop = clang::dyn_cast<clang::BinaryOperator>(expr);
+             binop) {
+        find_relationships_in_constraint_expression(c, binop->getLHS());
+        find_relationships_in_constraint_expression(c, binop->getRHS());
+    }
+    else if (const auto *unop = clang::dyn_cast<clang::UnaryOperator>(expr);
+             unop) {
+        find_relationships_in_constraint_expression(c, unop->getSubExpr());
+    }
+}
+
 bool translation_unit_visitor::VisitCXXRecordDecl(clang::CXXRecordDecl *cls)
 {
     // Skip system headers
@@ -440,6 +599,37 @@ bool translation_unit_visitor::VisitCXXRecordDecl(clang::CXXRecordDecl *cls)
     return true;
 }
 
+std::unique_ptr<clanguml::class_diagram::model::concept_>
+translation_unit_visitor::create_concept_declaration(clang::ConceptDecl *cpt)
+{
+    assert(cpt != nullptr);
+
+    auto concept_ptr{
+        std::make_unique<model::concept_>(config_.using_namespace())};
+    auto &concept_model = *concept_ptr;
+
+    auto qualified_name = cpt->getQualifiedNameAsString();
+
+    if (!diagram().should_include(qualified_name))
+        return {};
+
+    auto ns = common::get_template_namespace(*cpt);
+
+    concept_model.set_name(cpt->getNameAsString());
+    concept_model.set_namespace(ns);
+    concept_model.set_id(common::to_id(concept_model.full_name(false)));
+
+    process_comment(*cpt, concept_model);
+    set_source_location(*cpt, concept_model);
+
+    if (concept_model.skip())
+        return {};
+
+    concept_model.set_style(concept_model.style_spec());
+
+    return concept_ptr;
+}
+
 std::unique_ptr<class_> translation_unit_visitor::create_record_declaration(
     clang::RecordDecl *rec)
 {
@@ -490,7 +680,7 @@ std::unique_ptr<class_> translation_unit_visitor::create_class_declaration(
     if (!diagram().should_include(qualified_name))
         return {};
 
-    auto ns = common::get_tag_namespace(*cls);
+    auto ns{common::get_tag_namespace(*cls)};
 
     process_record_parent(cls, c, ns);
 
@@ -531,7 +721,8 @@ void translation_unit_visitor::process_record_parent(
             // regular class
             id_opt = get_ast_local_id(local_id);
 
-            // If not, check if the parent template declaration is in the model
+            // If not, check if the parent template declaration is in the
+            // model
             if (!id_opt) {
                 if (parent_record_decl->getDescribedTemplate() != nullptr) {
                     local_id =
@@ -600,7 +791,8 @@ void translation_unit_visitor::process_class_declaration(
 
 bool translation_unit_visitor::process_template_parameters(
     const clang::TemplateDecl &template_declaration,
-    common::model::template_trait &c)
+    common::model::template_trait &c,
+    common::optional_ref<common::model::element> templated_element)
 {
     LOG_DBG("Processing class {} template parameters...",
         common::get_qualified_name(template_declaration));
@@ -621,6 +813,16 @@ bool translation_unit_visitor::process_template_parameters(
             ct.set_default_value("");
             ct.is_variadic(template_type_parameter->isParameterPack());
 
+            if (template_type_parameter->getTypeConstraint()) {
+                util::apply_if_not_null(
+                    template_type_parameter->getTypeConstraint()
+                        ->getNamedConcept(),
+                    [&ct](const clang::ConceptDecl *named_concept) mutable {
+                        ct.set_concept_constraint(
+                            named_concept->getQualifiedNameAsString());
+                    });
+            }
+
             c.add_template(std::move(ct));
         }
         else if (clang::dyn_cast_or_null<clang::NonTypeTemplateParmDecl>(
@@ -2405,6 +2607,17 @@ void translation_unit_visitor::resolve_local_to_global_ids()
             }
         }
     }
+    for (const auto &cpt : diagram().concepts()) {
+        for (auto &rel : cpt.get().relationships()) {
+            const auto maybe_local_id = rel.destination();
+            if (get_ast_local_id(maybe_local_id)) {
+                LOG_DBG("= Resolved instantiation destination from local "
+                        "id {} to global id {}",
+                    maybe_local_id, *get_ast_local_id(maybe_local_id));
+                rel.set_destination(*get_ast_local_id(maybe_local_id));
+            }
+        }
+    }
 }
 
 void translation_unit_visitor::finalize()
diff --git a/src/class_diagram/visitor/translation_unit_visitor.h b/src/class_diagram/visitor/translation_unit_visitor.h
index 63758ecb..2c8d65d7 100644
--- a/src/class_diagram/visitor/translation_unit_visitor.h
+++ b/src/class_diagram/visitor/translation_unit_visitor.h
@@ -18,6 +18,7 @@
 #pragma once
 
 #include "class_diagram/model/class.h"
+#include "class_diagram/model/concept.h"
 #include "class_diagram/model/diagram.h"
 #include "common/model/enums.h"
 #include "common/model/template_trait.h"
@@ -83,12 +84,7 @@ public:
 
     virtual bool VisitTypeAliasTemplateDecl(clang::TypeAliasTemplateDecl *cls);
 
-//    bool TraverseTypeConstraint(const clang::TypeConstraint *C);
-//    bool TraverseConceptRequirement(clang::concepts::Requirement *R);
-//    bool TraverseConceptTypeRequirement(clang::concepts::TypeRequirement *R);
-//    bool TraverseConceptExprRequirement(clang::concepts::ExprRequirement *R);
-//    bool TraverseConceptNestedRequirement(
-//        clang::concepts::NestedRequirement *R);
+    virtual bool TraverseConceptDecl(clang::ConceptDecl *cpt);
 
     /**
      * @brief Get diagram model reference
@@ -121,6 +117,9 @@ private:
     std::unique_ptr<clanguml::class_diagram::model::class_>
     create_record_declaration(clang::RecordDecl *rec);
 
+    std::unique_ptr<clanguml::class_diagram::model::concept_>
+    create_concept_declaration(clang::ConceptDecl *cpt);
+
     void process_class_declaration(const clang::CXXRecordDecl &cls,
         clanguml::class_diagram::model::class_ &c);
 
@@ -141,7 +140,8 @@ private:
 
     bool process_template_parameters(
         const clang::TemplateDecl &template_declaration,
-        clanguml::common::model::template_trait &t);
+        clanguml::common::model::template_trait &t,
+        common::optional_ref<common::model::element> templated_element = {});
 
     void process_template_specialization_argument(
         const clang::ClassTemplateSpecializationDecl *cls,
@@ -249,6 +249,9 @@ private:
         const std::set<std::string> &template_parameter_names,
         const clang::TemplateSpecializationType &template_instantiation_type);
 
+    void find_relationships_in_constraint_expression(
+        clanguml::common::model::element &c, const clang::Expr *expr);
+
     void process_unexposed_template_specialization_parameters(
         const std::string &tspec,
         clanguml::common::model::template_parameter &tp,
diff --git a/src/common/clang_utils.cc b/src/common/clang_utils.cc
index ce94c1ff..88d033e3 100644
--- a/src/common/clang_utils.cc
+++ b/src/common/clang_utils.cc
@@ -90,6 +90,14 @@ model::namespace_ get_tag_namespace(const clang::TagDecl &declaration)
     return ns;
 }
 
+model::namespace_ get_template_namespace(const clang::TemplateDecl &declaration)
+{
+    model::namespace_ ns{declaration.getQualifiedNameAsString()};
+    ns.pop_back();
+
+    return ns;
+}
+
 std::string get_tag_name(const clang::TagDecl &declaration)
 {
     auto base_name = declaration.getNameAsString();
diff --git a/src/common/clang_utils.h b/src/common/clang_utils.h
index 6e40ee6d..6b64ef0a 100644
--- a/src/common/clang_utils.h
+++ b/src/common/clang_utils.h
@@ -72,6 +72,9 @@ template <typename T> std::string get_qualified_name(const T &declaration)
 
 model::namespace_ get_tag_namespace(const clang::TagDecl &declaration);
 
+model::namespace_ get_template_namespace(
+    const clang::TemplateDecl &declaration);
+
 std::optional<clanguml::common::model::namespace_> get_enclosing_namespace(
     const clang::DeclContext *decl);
 
diff --git a/src/common/model/template_parameter.cc b/src/common/model/template_parameter.cc
index ed276b6c..87ec2361 100644
--- a/src/common/model/template_parameter.cc
+++ b/src/common/model/template_parameter.cc
@@ -138,6 +138,8 @@ std::string template_parameter::to_string(
 {
     using clanguml::common::model::namespace_;
 
+    assert(!(!type().empty() && concept_constraint().has_value()));
+
     std::string res;
     if (!type().empty()) {
         if (!relative)
@@ -146,8 +148,17 @@ std::string template_parameter::to_string(
             res += namespace_{type()}.relative_to(using_namespace).to_string();
     }
 
+    if (concept_constraint()) {
+        if (!relative)
+            res += namespace_{concept_constraint().value()}.to_string();
+        else
+            res += namespace_{concept_constraint().value()}
+                       .relative_to(using_namespace)
+                       .to_string();
+    }
+
     if (!name().empty()) {
-        if (!type().empty())
+        if (!type().empty() || concept_constraint())
             res += " ";
         if (!relative)
             res += namespace_{name()}.to_string();
@@ -216,4 +227,14 @@ bool template_parameter::find_nested_relationships(
     return added_aggregation_relationship;
 }
 
+void template_parameter::set_concept_constraint(std::string constraint)
+{
+    concept_constraint_ = std::move(constraint);
+}
+
+const std::optional<std::string> &template_parameter::concept_constraint() const
+{
+    return concept_constraint_;
+}
+
 } // namespace clanguml::common::model
diff --git a/src/common/model/template_parameter.h b/src/common/model/template_parameter.h
index 3ca0f173..b9ce70eb 100644
--- a/src/common/model/template_parameter.h
+++ b/src/common/model/template_parameter.h
@@ -100,6 +100,9 @@ public:
         const std::function<bool(const std::string &full_name)> &should_include)
         const;
 
+    void set_concept_constraint(std::string constraint);
+    const std::optional<std::string> &concept_constraint() const;
+
 private:
     /// Represents the type of non-type template parameters
     /// e.g. 'int'
@@ -125,6 +128,10 @@ private:
     /// Whether the argument specializes argument pack from parent template
     bool is_pack_{false};
 
+    /// Stores optional fully qualified name of constraint for this template
+    /// parameter
+    std::optional<std::string> concept_constraint_;
+
     // Nested template parameters
     std::vector<template_parameter> template_params_;
 
diff --git a/src/util/util.h b/src/util/util.h
index 1cddf3a4..8b1c7433 100644
--- a/src/util/util.h
+++ b/src/util/util.h
@@ -242,6 +242,39 @@ void for_each_if(const T &collection, C &&cond, F &&func)
         });
 }
 
+template <typename T, typename F, typename FElse>
+void apply_if_not_null(const T *pointer, F &&func, FElse &&func_else)
+{
+    if (pointer != nullptr) {
+        std::forward<F>(func)(pointer);
+    }
+    else if (func_else) {
+        std::forward<FElse>(func_else)();
+    }
+}
+
+template <typename T, typename F>
+void apply_if_not_null(const T *pointer, F &&func)
+{
+    apply_if_not_null(pointer, std::forward<F>(func), []() {});
+}
+
+template <typename F, typename FElse>
+void apply_if(const bool condition, F &&func, FElse &&func_else)
+{
+    if (condition) {
+        std::forward<F>(func)();
+    }
+    else if (func_else) {
+        std::forward<FElse>(func_else)();
+    }
+}
+
+template <typename F> void apply_if(const bool condition, F &&func)
+{
+    apply_if(condition, std::forward<F>(func), []() {});
+}
+
 std::size_t hash_seed(std::size_t seed);
 
 /**
diff --git a/tests/t00056/t00056.cc b/tests/t00056/t00056.cc
index 4d40a2e4..088b114b 100644
--- a/tests/t00056/t00056.cc
+++ b/tests/t00056/t00056.cc
@@ -2,53 +2,71 @@
 
 namespace clanguml {
 namespace t00056 {
+
+template <typename T, typename L>
+concept greater_than = sizeof(T) > sizeof(L);
+
+template <typename T, typename P>
+concept greater_than_requires = requires(T l, P r)
+{
+    sizeof(l) > sizeof(r);
+};
+
 // Constraint expression
 template <typename T>
-concept MaxFourBytes = sizeof(T) <= 4;
+concept max_four_bytes = sizeof(T) <= 4;
 
 // Simple requirement
 template <typename T>
-concept Iterable = requires(T container) {
-                       container.begin();
-                       container.end();
-                   };
+concept iterable = requires(T container)
+{
+    container.begin();
+    container.end();
+};
 
 // Type requirement
 template <typename T>
-concept HasValueType = requires { typename T::value_type; };
+concept has_value_type = requires
+{
+    typename T::value_type;
+};
 
 template <typename T>
-concept ConvertibleToString = requires(T s) { std::string{s}; };
+concept convertible_to_string = requires(T s)
+{
+    std::string{s};
+};
 
 // Compound requirement
 // ...
 
 // Combined concept
 template <typename T>
-concept IterableWithValueType = Iterable<T> && HasValueType<T>;
+concept iterable_with_value_type = iterable<T> && has_value_type<T>;
 
 template <typename T>
-concept IterableWithSmallValueType =
-    IterableWithValueType<T> && MaxFourBytes<T>;
+concept iterable_or_small_value_type =
+    iterable_with_value_type<T> || max_four_bytes<T>;
 
 // Simple type constraint
-template <MaxFourBytes T> struct A {
+template <max_four_bytes T> struct A {
     T a;
 };
 
 // Requires constant expression
 template <typename T>
-    requires MaxFourBytes<T>
+requires iterable_or_small_value_type<T>
 struct B {
     T b;
 };
 
 // Anonymous concept requirement
 template <typename T>
-    requires requires(T t) {
-                 --t;
-                 t--;
-             }
+requires requires(T t)
+{
+    --t;
+    t--;
+}
 struct C {
     T c;
 };