C++: Expose explicit and explicit(bool) function specifiers

cpp/ql/lib/change-notes/2024-08-01-explicit-bool.md

@@ -0,0 +1,5 @@
+---
+category: feature
+---
+* An `isExplicit` predicate was added to the `Function` class that determines whether the function was declared as explicit.
+* A `getExplicitExpr` predicate was added to the `Function` class that yields the constant boolean expression (if any) that conditionally determines whether the function is explicit.

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

@@ -158,6 +158,26 @@ class Function extends Declaration, ControlFlowNode, AccessHolder, @function {
    */
   predicate isConsteval() { this.hasSpecifier("is_consteval") }
 
+  /**
+   * Holds if this function is declared to be `explicit`.
+   */
+  predicate isExplicit() { this.hasSpecifier("explicit") }
+
+  /**
+   * Gets the constant expression that determines whether the function is explicit.
+   *
+   * For example, for the following code the result is is the expression `sizeof(T) == 1`:
+   * ```
+   * template<typename T> struct C {
+   *   explicit(sizeof(T) == 1)
+   *   C(const T);
+   * };
+   * ```
+   */
+  Expr getExplicitExpr() {
+    explicit_specifier_exprs(underlyingElement(this), unresolveElement(result))
+  }
+
   /**
    * Holds if this function is declared with `__attribute__((naked))` or
    * `__declspec(naked)`.

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -921,6 +921,11 @@ varspecifiers(
     int spec_id: @specifier ref
 );
 
+explicit_specifier_exprs(
+    unique int func_id: @function ref,
+    int constant: @expr ref
+)
+
 attributes(
     unique int id: @attribute,
     int kind: int ref,

cpp/ql/lib/upgrades/e197626a6ebccd052d5c891975fccf8aebcc9803/upgrade.properties

@@ -0,0 +1,2 @@
+description: Support explicit(bool) specifiers
+compatibility: partial