Swift: add ConstructorDecl.isFailable/0

swift/ql/lib/codeql/swift/dataflow/internal/DataFlowPrivate.qll

@@ -602,12 +602,12 @@ predicate storeStep(Node node1, ContentSet c, Node node2) {
     c instanceof OptionalSomeContentSet
   )
   or
-  // creation of an optional by returning from an optional initializer (`init?`)
-  exists(ConstructorDecl init, OptionalType initRetType |
+  // creation of an optional by returning from a failable initializer (`init?`)
+  exists(ConstructorDecl init |
     node1.asExpr().(CallExpr).getStaticTarget() = init and
     node2 = node1 and // HACK: again, we should ideally have a separate Node case here, and not reuse the CallExpr
     c instanceof OptionalSomeContentSet and
-    init.getInterfaceType().(FunctionType).getResult().(FunctionType).getResult() = initRetType
+    init.isFailable()
   )
   or
   FlowSummaryImpl::Private::Steps::summaryStoreStep(node1, c, node2)

swift/ql/lib/codeql/swift/elements/decl/ConstructorDecl.qll

@@ -1,9 +1,17 @@
 private import codeql.swift.generated.decl.ConstructorDecl
 private import codeql.swift.elements.decl.MethodDecl
+private import codeql.swift.elements.type.FunctionType
+private import codeql.swift.elements.type.OptionalType
 
 /**
  * An initializer of a class, struct, enum or protocol.
  */
 class ConstructorDecl extends Generated::ConstructorDecl, MethodDecl {
   override string toString() { result = this.getSelfParam().getType() + "." + super.toString() }
+
+  /** Holds if this initializer returns an optional type. Failable initializers are written as `init?`. */
+  predicate isFailable() {
+    this.getInterfaceType().(FunctionType).getResult().(FunctionType).getResult() instanceof
+      OptionalType
+  }
 }