C++: Support concept id expressions

cpp/ql/lib/semmle/code/cpp/Concept.qll

@@ -153,11 +153,110 @@ class NestedRequirementExpr extends Expr, @nested_requirement {
 
 /**
  * A C++ concept id expression.
+ *
+ * For example, if:
+ * ```cpp
+ * template<typename T, T X> concept C = ...;
+ * ...
+ * requires { C<int, 1>; };
+ * ```
+ * then `C<int, 1>` is a concept id expression that refers to
+ * the concept `C`.
  */
 class ConceptIdExpr extends RequirementExpr, @concept_id {
-  override string toString() { result = "concept<...>" }
+  override string toString() {
+    exists(string name |
+      concept_templates(this.getConcept(), name, _) and
+      result = name + "<...>"
+    )
+    or
+    // The following is for backward compatibility with databases created with
+    // CodeQL 2.19.3, 2.19.4, and 2.20.0. Those databases include concept id
+    // expressions, but do not include concept template information.
+    not exists(this.getConcept()) and
+    result = "concept<...>"
+  }
 
   override string getAPrimaryQlClass() { result = "ConceptIdExpr" }
+
+  /**
+   * Holds if the concept id is used as a type constraint.
+   *
+   * In this case, the first template argument is implicit.
+   */
+  predicate isTypeConstraint() { is_type_constraint(underlyingElement(this)) }
+
+  /**
+   * Gets the concept this concept id refers to.
+   */
+  Concept getConcept() { concept_instantiation(underlyingElement(this), unresolveElement(result)) }
+
+  /**
+   * Gets a template argument passed to the concept.
+   */
+  final Locatable getATemplateArgument() { result = this.getTemplateArgument(_) }
+
+  /**
+   * Gets the kind of a non-type template argument passed to the concept.
+   */
+  final Locatable getATemplateArgumentKind() { result = this.getTemplateArgumentKind(_) }
+
+  /**
+   * Gets the `i`th template argument passed to the concept.
+   *
+   * For example, if:
+   * ```cpp
+   * template<typename T, T X> concept C = ...;
+   * ...
+   * requires { C<int, 1>; };
+   * ```
+   * then `getTemplateArgument(0)` yields `int`, and `getTemplateArgument(1)`
+   * yields `1`.
+   *
+   * If the concept id is a type constraint, then `getTemplateArgument(0)`
+   * will not yield a result.
+   */
+  final Locatable getTemplateArgument(int index) {
+    if exists(this.getTemplateArgumentValue(index))
+    then result = this.getTemplateArgumentValue(index)
+    else result = this.getTemplateArgumentType(index)
+  }
+
+  /**
+   * Gets the kind of the `i`th template argument value passed to the concept.
+   *
+   * For example, if:
+   * ```cpp
+   * template<typename T, T X> concept C = ...;
+   * ...
+   * requires { C<int, 1>; };
+   * ```
+   * then `getTemplateArgumentKind(1)` yields `int`, and there is no result for
+   * `getTemplateArgumentKind(0)`.
+   */
+  final Locatable getTemplateArgumentKind(int index) {
+    exists(this.getTemplateArgumentValue(index)) and
+    result = this.getTemplateArgumentType(index)
+  }
+
+  /**
+   *  Gets the number of template arguments passed to the concept.
+   */
+  final int getNumberOfTemplateArguments() {
+    result = count(int i | exists(this.getTemplateArgument(i)))
+  }
+
+  private Type getTemplateArgumentType(int index) {
+    exists(int i | if this.isTypeConstraint() then i = index - 1 else i = index |
+      concept_template_argument(underlyingElement(this), i, unresolveElement(result))
+    )
+  }
+
+  private Expr getTemplateArgumentValue(int index) {
+    exists(int i | if this.isTypeConstraint() then i = index - 1 else i = index |
+      concept_template_argument_value(underlyingElement(this), i, unresolveElement(result))
+    )
+  }
 }
 
 /**
@@ -187,4 +286,9 @@ class Concept extends Declaration, @concept_template {
    * the constraint expression is `std::is_same<T, int>::value`.
    */
   Expr getExpr() { result.getParent() = this }
+
+  /**
+   * Gets a concept id expression that refers to this concept
+   */
+  ConceptIdExpr getAReferringConceptIdExpr() { this = result.getConcept() }
 }

cpp/ql/lib/semmle/code/cpp/Declaration.qll

@@ -235,7 +235,7 @@ class Declaration extends Locatable, @declaration {
    *
    * `Foo<int, 1> bar;`
    *
-   * Will have `getTemplateArgument())` return `int`, and
+   * Will have `getTemplateArgument(0)` return `int`, and
    * `getTemplateArgument(1)` return `1`.
    */
   final Locatable getTemplateArgument(int index) {

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -883,6 +883,11 @@ concept_templates(
     string name: string ref,
     int location: @location_default ref
 );
+concept_instantiation(
+    unique int to: @concept_id ref,
+    int from: @concept_template ref
+);
+is_type_constraint(int concept_id: @concept_id ref);
 concept_template_argument(
     int concept_id: @concept ref,
     int index: int ref,