[LLVMbugs] [Bug 15484] New: Block types are not covariant in their return type and contravariant in their argument types with respect to C++ classes

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=15484

            Bug ID: 15484
           Summary: Block types are not covariant in their return type and
                    contravariant in their argument types with respect to
                    C++ classes
           Product: clang
           Version: unspecified
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jppellet at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

This bug is related to this StackOverflow question:
http://stackoverflow.com/questions/15312774/assigning-block-pointers-differences-between-objective-c-vs-c-classes

Imagine I have this simple Objective-C class hierarchy:

@interface Fruit : NSObject
@end

@interface Apple : Fruit
@end
Then I can write stuff like this:

Fruit *(^getFruit)();
Apple *(^getApple)();
getFruit = getApple;

This is all good and well — in Objective-C. Now let's try that in C++ or
Objective-C++, supposing we have these similar C++ classes:

class FruitCpp {};

class AppleCpp : public FruitCpp {};

class OrangeCpp : public FruitCpp {};
Sadly, these block assignments don't compile any more:

FruitCpp *(^getFruitCpp)();
AppleCpp *(^getAppleCpp)();
getFruitCpp = getAppleCpp; // error!

Why should it work for Objective-C classes and not C++ classes?

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