C++: Basic lib support for template param values

cpp/ql/src/semmle/code/cpp/Class.qll

@@ -614,6 +614,15 @@ class Class extends UserType {
     class_template_argument(underlyingElement(this), i, unresolveElement(result))
   }
 
+  /**
+   * Gets the `i`th template argument value used to instantiate this class from a
+   * class template. When called on a class template, this will return the
+   * `i`th template parameter value.
+   */
+  override Expr getTemplateArgumentValue(int i) {
+    class_template_argument_value(underlyingElement(this), i, unresolveElement(result))
+  }
+
   /**
    * Holds if this class/struct is polymorphic (has a virtual function, or
    * inherits one).

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

@@ -203,6 +203,13 @@ abstract class Declaration extends Locatable, @declaration {
    */
   Type getTemplateArgument(int index) { none() }
 
+  /**
+   * Gets the `i`th template argument value used to instantiate this declaration
+   * from a template. When called on a template, this will return the `i`th template
+   * parameter value if it exists.
+   */
+  Expr getTemplateArgumentValue(int index) { none() }
+
   /** Gets the number of template arguments for this declaration. */
   final int getNumberOfTemplateArguments() {
     result = count(int i | exists(getTemplateArgument(i)))

cpp/ql/src/semmle/code/cpp/Function.qll

@@ -352,6 +352,16 @@ class Function extends Declaration, ControlFlowNode, AccessHolder, @function {
     function_template_argument(underlyingElement(this), index, unresolveElement(result))
   }
 
+  /**
+   * Gets the value of the `i`th template argument used to instantiate this
+   * function from a function template if that argument was a 'non-type'
+   * argument.  When called on a function template, this with return the value
+   * of the `i`th template parameter.
+   */
+  override Expr getTemplateArgumentValue(int index) {
+    function_template_argument_value(underlyingElement(this), index, unresolveElement(result))
+  }
+
   /**
    * Holds if this function is defined in several files. This is illegal in
    * C (though possible in some C++ compilers), and likely indicates that

cpp/ql/src/semmle/code/cpp/Variable.qll

@@ -164,6 +164,15 @@ class Variable extends Declaration, @variable {
     variable_template_argument(underlyingElement(this), index, unresolveElement(result))
   }
 
+  /**
+   * Gets the `i`th template argument value used to instantiate this variable from a
+   * variable template. When called on a variable template, this will return the
+   * `i`th template parameter value.
+   */
+  override Expr getTemplateArgumentValue(int index) {
+    variable_template_argument_value(underlyingElement(this), index, unresolveElement(result))
+  }
+
   /**
    * Holds if this is a compiler-generated variable. For example, a
    * [range-based for loop](http://en.cppreference.com/w/cpp/language/range-for)