C++: Align definition of anonymous union with the C/C++ standard

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

@@ -18,6 +18,12 @@ class Union extends Struct {
   override string explain() { result = "union " + this.getName() }
 
   override predicate isDeeplyConstBelow() { any() } // No subparts
+
+  /** Holds if this type is anonymous. */
+  final predicate isAnonymous() {
+    getName() = "union <unnamed>" and
+    exists(Variable v | v.getType() = this and v.getName() = "(null)")
+  }
 }
 
 /**
@@ -64,3 +70,16 @@ class NestedUnion extends Union {
   /** Holds if this member is public. */
   predicate isPublic() { this.hasSpecifier("public") }
 }
+
+/**
+ * A C/C++ anonymous union. For example:
+ * ```
+ * union {
+ *   int i;
+ *   float f;
+ * };
+ * ```
+ */
+class AnonymousUnion extends Union {
+  AnonymousUnion() { this.isAnonymous() }
+}

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

@@ -30,9 +30,6 @@ class UserType extends Type, Declaration, NameQualifyingElement, AccessHolder, @
 
   override predicate hasName(string name) { usertypes(underlyingElement(this), name, _) }
 
-  /** Holds if this type is anonymous. */
-  predicate isAnonymous() { getName().matches("(unnamed%") }
-
   override predicate hasSpecifier(string s) { Type.super.hasSpecifier(s) }
 
   override Specifier getASpecifier() { result = Type.super.getASpecifier() }

cpp/ql/test/library-tests/unions/Unions1.expected

@@ -4,3 +4,5 @@
 | unions.cpp:27:9:27:20 | MyLocalUnion | LocalUnion, Struct, Union |  | operator= |
 | unions.cpp:33:7:33:13 | MyClass |  |  | operator= |
 | unions.cpp:36:9:36:21 | MyNestedUnion | NestedUnion, Struct, Union |  | operator= |
+| unions.cpp:43:9:43:9 | union <unnamed> | AnonymousUnion, LocalUnion, Struct, Union |  | operator= |
+| unions.cpp:48:9:48:9 | union <unnamed> | LocalUnion, Struct, Union |  | operator= |

cpp/ql/test/library-tests/unions/Unions1.ql

@@ -12,6 +12,9 @@ string describe(Class c) {
   or
   c instanceof NestedUnion and
   result = "NestedUnion"
+  or
+  c instanceof AnonymousUnion and
+  result = "AnonymousUnion"
 }
 
 from Class c

cpp/ql/test/library-tests/unions/unions.cpp

@@ -38,3 +38,15 @@ class MyClass
     float f;
   };
 };
+
+void test_anonymous_union() {
+  union {
+    int u1;
+    char* u2;
+  };
+
+  union {
+    int u3;
+    char* u4;
+  } not_an_anonymous_union;
+}