[C++-Interop] Missing a matching ObjC ARC retain for C++ Copy Constructor to match C++ Destructor

[plotfi]

Description

Case is where there is a C++ type that contains an ARC type such as an ObjC Block where the type is passed across the Swift/C++ language boundary by value. The default copy constructor generation needed to facilitate this doesn't seem to account for the ARC retain in the cctor but in the dtor there is an ARC release. The he ObjC release in the callee functions as a double free. I have attached sanitized IR comparing the pass by const ref and pass by value case to this issue as well.

There is a repro project at https://github.com/plotfi/cxx-interop-test/tree/_Block_copy that covers this repro as well.

Reproducer

C++ Header (CXX.h)

typedef int (^UITextBlock)();

struct UITextAttribute {
  UITextAttribute() : textBlock((^{ return 42; })){};
  UITextAttribute(int text) : textBlock(^{ return text; }){};
  UITextBlock textBlock;

  // NOTE: If we manually add the right copy constructor then we avoid the loss 
  //       of the needed ARC retain at the ctor call site.
  #if 0
  UITextAttribute(const UITextAttribute &textAttribute) {
    textBlock = textAttribute.textBlock;
  }
  #endif
};

struct UIAccessibilityContext {
  // NOTE: if ints accessibilityIdentifier or extra are removed, then args
  //       handling code in IRGen seems to assert.
  int accessibilityIdentifier;
  UITextAttribute accessibilityLabel;
  int extra;

  // NOTE: If following line is pass by reference, we avoid the crash since we
  //       dont get an additional dtor ARC release for the reference pass.
  static UIAccessibilityContext build(
    #if 1
    UITextAttribute accessibilityLabel
    #else
    const UITextAttribute &accessibilityLabel
    #endif
  ) {
    return {.accessibilityLabel = accessibilityLabel};
  }
};

module.modulemap

module CXX {
  header "CXX.h"
  requires cplusplus
}

Swift (main.swift)

import CXX

var textAttr1 = UITextAttribute(24)
var a = textAttr1.textBlock()
print("Before. \(a)")

var context = UIAccessibilityContext.build(textAttr1)
var b = context.accessibilityLabel.textBlock()
print("Done. \(b)")

Repro

Copy above files CXX.h, module.modulemap and main.swift respectively into a test directory, cd into that directory and run:

  /path/to/swiftc -o test.exe  \
    -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.3.sdk \
    -Xfrontend -enable-objc-interop -Xfrontend -enable-experimental-cxx-interop \
    -I ./ main.swift
  
  ./text.exe

Expected

% ./test.exe
Before. 24
Done. 24

IR.zip

Actual

% ./test.exe
Before. 24
zsh: segmentation fault  ./test.exe

Mitigation

For now I see a mitigation as either implementing a manual copy constructor or passing by const ref. Without this the ObjC release in the callee functions as a double free.