C++: Support __is_assignable builtin

While here fix the documentation of `__is_trivially_assignable` and
`__is_nothrow_assignable`.

cpp/ql/lib/semmle/code/cpp/exprs/BuiltInOperations.qll

@@ -612,13 +612,10 @@ class BuiltInOperationIsTriviallyDestructible extends BuiltInOperation, @istrivi
  * The `__is_trivially_assignable` built-in operation (used by some
  * implementations of the `<type_traits>` header).
  *
- * Returns `true` if the assignment operator `C::operator =(const C& c)` is
- * trivial.
+ * Returns `true` if the assignment operator `C::operator =(const D& d)` is
+ * trivial (i.e., it will not call any operation that is non-trivial).
  * ```
- * template<typename T>
- *   struct is_trivially_assignable
- *   : public integral_constant<bool, __is_trivially_assignable(T) >
- *   { };
+ * bool v = __is_trivially_assignable(MyType1, MyType2);
  * ```
  */
 class BuiltInOperationIsTriviallyAssignable extends BuiltInOperation, @istriviallyassignableexpr {
@@ -631,10 +628,10 @@ class BuiltInOperationIsTriviallyAssignable extends BuiltInOperation, @istrivial
  * The `__is_nothrow_assignable` built-in operation (used by some
  * implementations of the `<type_traits>` header).
  *
- * Returns true if there exists a `C::operator =(const C& c) nothrow`
+ * Returns true if there exists a `C::operator =(const D& d) nothrow`
  * assignment operator (i.e, with an empty exception specification).
  * ```
- * bool v = __is_nothrow_assignable(MyType);
+ * bool v = __is_nothrow_assignable(MyType1, MyType2);
  * ```
  */
 class BuiltInOperationIsNothrowAssignable extends BuiltInOperation, @isnothrowassignableexpr {
@@ -643,6 +640,22 @@ class BuiltInOperationIsNothrowAssignable extends BuiltInOperation, @isnothrowas
   override string getAPrimaryQlClass() { result = "BuiltInOperationIsNothrowAssignable" }
 }
 
+/**
+ * The `__is_assignable` built-in operation (used by some implementations
+ * of the `<type_traits>` header).
+ *
+ * Returns true if there exists a `C::operator =(const D& d)` assignment
+ * operator.
+ * ```
+ * bool v = __is_assignable(MyType1, MyType2);
+ * ```
+ */
+class BuiltInOperationIsAssignable extends BuiltInOperation, @isassignable {
+  override string toString() { result = "__is_assignable" }
+
+  override string getAPrimaryQlClass() { result = "BuiltInOperationIsAssignable" }
+}
+
 /**
  * The `__is_standard_layout` built-in operation (used by some implementations
  * of the `<type_traits>` header).

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -1650,6 +1650,7 @@ case @expr.kind of
 | 327 = @co_await
 | 328 = @co_yield
 | 329 = @temp_init
+| 330 = @isassignable
 ;
 
 @var_args_expr = @vastartexpr
@@ -1711,6 +1712,7 @@ case @expr.kind of
             | @isfinalexpr
             | @builtinchooseexpr
             | @builtincomplex
+            | @isassignable
             ;
 
 new_allocated_type(

cpp/ql/test/library-tests/builtins/type_traits/expr.expected

@@ -296,3 +296,12 @@
 | ms.cpp:255:24:255:43 | a_struct |  | <none> |
 | ms.cpp:256:24:256:49 | __is_final | a_final_struct | 1 |
 | ms.cpp:256:24:256:49 | a_final_struct |  | <none> |
+| ms.cpp:258:29:258:62 | __is_assignable | a_struct,a_struct | 1 |
+| ms.cpp:258:29:258:62 | a_struct |  | <none> |
+| ms.cpp:258:29:258:62 | a_struct |  | <none> |
+| ms.cpp:259:29:259:59 | __is_assignable | a_struct,empty | 0 |
+| ms.cpp:259:29:259:59 | a_struct |  | <none> |
+| ms.cpp:259:29:259:59 | empty |  | <none> |
+| ms.cpp:260:29:260:57 | __is_assignable | a_struct,int | 0 |
+| ms.cpp:260:29:260:57 | a_struct |  | <none> |
+| ms.cpp:260:29:260:57 | int |  | <none> |

cpp/ql/test/library-tests/builtins/type_traits/ms.cpp

@@ -254,5 +254,8 @@ void f(void) {
 
     bool b_is_final1 = __is_final(a_struct);
     bool b_is_final2 = __is_final(a_final_struct);
-}
 
+    bool b_is_assignable1 = __is_assignable(a_struct,a_struct);
+    bool b_is_assignable2 = __is_assignable(a_struct,empty);
+    bool b_is_assignable3 = __is_assignable(a_struct,int);
+}