[clang] [alpha.webkit.NoDeleteChecker] Allow no-delete default constructors (PR #201544)

[Ryosuke Niwa via cfe-commits]

================
@@ -701,3 +701,45 @@ Ref<RefCountable> [[clang::annotate_type("webkit.nodelete")]] returnTypedefPrval
 
 } // namespace returned_prvalue_typedef
 
+namespace create_with_default_constructor {
+
+  struct ObjectWithDefaultConstructorWithoutMemberVariables {
+    void ref() const;
+    void deref() const;
+
+    static auto [[clang::annotate_type("webkit.nodelete")]] create() {
+      return adoptRef(*new ObjectWithDefaultConstructorWithoutMemberVariables());
+    }
+  };
+
+  struct ObjectWithDefaultConstructorWithPODMemberVariables {
+    void ref() const;
+    void deref() const;
+
+    static auto [[clang::annotate_type("webkit.nodelete")]] create() {
+      return adoptRef(*new ObjectWithDefaultConstructorWithPODMemberVariables());
+    }
+
+  private:
+    int value { 0 };
+    RefCountable* ptr { nullptr };
+  };
+
+  struct ObjectWithOpaqueCtor {
+    ObjectWithOpaqueCtor();
+  };
+
+  struct ObjectWithDefaultConstructorWithOpaqueCtorMemberVariables {
+    void ref() const;
+    void deref() const;
+
+    static auto [[clang::annotate_type("webkit.nodelete")]] create() {
+      return adoptRef(*new ObjectWithDefaultConstructorWithOpaqueCtorMemberVariables());
+      // expected-warning at -1{{A function 'create' has [[clang::annotate_type("webkit.nodelete")]] but it contains code that could destruct an object}}
+    }
+
+  private:
+    ObjectWithOpaqueCtor obj;
+  };
+
+} // namespace create_with_default_constructor
----------------
rniwa wrote:

Because adding an empty constructor makes the warning go away. My initial thinking was same as you that we're failing to detect that a destructor won't be called here but that's not what's happening.

The following code will generate a warning:
```
  struct Foo {
    void ref() const;
    void deref() const;

    static auto [[clang::annotate_type("webkit.nodelete")]] create() {
      return adoptRef(*new Foo());
    }
  };
```

While the following code does not:
```
  struct Bar {
    Bar() { }
    void ref() const;
    void deref() const;

    static auto [[clang::annotate_type("webkit.nodelete")]] create() {
      return adoptRef(*new Bar());
    }
  };
```

As a matter of fact, I added logging in various places and it showed me that we're returning `false` due to the lack of the function body.

https://github.com/llvm/llvm-project/pull/201544