Can't implicitly cast from class with multiple inheritance to a base of a virtually base class if public inheritance follows after private or protected, but not before them

[llvmbot]

Bugzilla Link	28591
Version	3.1
OS	MacOS X
Reporter	LLVM Bugzilla Contributor
CC	@Quuxplusone,@DougGregor,@miyuki,@rprichard,@vabr-g

Extended Description

# version: clang-703.0.31, 
# sample.cpp

class foo {};
class foofoo: public foo {};
class private_bar: private virtual foofoo {};
class public_bar: public virtual foofoo {};
class foobar_public_first: public public_bar, public private_bar {};
class foobar_private_first: public private_bar, public public_bar {};

void test_foobar() {
    foo *foo1 = new foobar_public_first(); // OK
    foo *foo2 = new foobar_private_first(); // compile error - "Cannot cast 'foobar_private_first' to its private base class 'foo'"
}

So, availability of casting to base depends on how ancestor classes are ordered in the inheritance list.

[d0027324-493a-46be-8448-4091c7735e73]

This issue also seems to have the same root cause as llvm/llvm-bugzilla-archive#32180 .

[rprichard]

This issue looks related to (or is a duplicate of) issue 19795: llvm/llvm-project#20169

[Quuxplusone]

I have also hit this bug, both on Xcode and on top-of-tree Clang. My test case is exactly isomorphic to Maxim's test case.