Open Radar

 
Summary:
Clang accepts code that synthesizes getters and setters for Objective-C properties with abstract C++ class reference types. No diagnostic is produced, but a backing ivar is silently generated with the abstract class type, which should not be possible: the generated code creates instances of the abstract class. When unrelated code attempts to call pure virtual methods via the returned reference, the C++ runtime produces an error that can be challenging to track down.

Clang should refuse to synthesize ivars and getters for Objective-C properties with abstract C++ types.

Example code:

  class TestInterface
  {
  public:
      TestInterface() {}
      virtual ~TestInterface() {}
      virtual int TestMethod() const = 0;
  };

  @interface TestClass : NSObject
  @property (nonatomic, readonly) const TestInterface& test; // This should not compile without an explicit getter.
  @end

  @implementation TestClass
  @end

  int main(int argc, const char * argv[])
  {
      @autoreleasepool {
          TestClass *testClass = [[TestClass alloc] init];
          const TestInterface& testInterface = testClass.test;
          NSLog(@"%d", testInterface.TestMethod()); // Runtime error: "libc++abi.dylib: pure virtual method called"
      }
      return 0;
  }

As far as I can see, the compiler generates code that is roughly equivalent to the following (invalid) implementation:

  @implementation TestClass
  {
      TestInterface _test;
  }

  - (const TestInterface&)test
  {
      return _test;
  }
 @end


Steps to Reproduce:
Open and run the attached project in Xcode 4.6.3 or Xcode 5 DP2.

Expected Results:
Compiler error complaining that @properties with abstract values cannot be synthesized.

Actual Results:
Runtime error "libc++abi.dylib: pure virtual method called" leading to abort().

Regression:
Unknown

Notes:
This issue is reproducible with the following compilers:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang --version
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.4.0
Thread model: posix
$ /Applications/Xcode5-DP2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang --version
Apple LLVM version 5.0 (clang-500.1.61) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin12.4.0
Thread model: posix