[llvm-bugs] [Bug 26741] New: __builtin_object_size is not consistently conservative with C++ inheritance

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https://llvm.org/bugs/show_bug.cgi?id=26741

            Bug ID: 26741
           Summary: __builtin_object_size is not consistently conservative
                    with C++ inheritance
           Product: clang
           Version: trunk
          Hardware: Macintosh
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bob.wilson at apple.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15946
  --> https://llvm.org/bugs/attachment.cgi?id=15946&action=edit
patch for the object-size.cpp test

Clang r250488 added code to make __builtin_object_size more conservative when
dealing with an array of size 0 or 1 as the last member in a struct to support
the "writing off the end" idiom. The CodeGen/object-size.cpp test was added
along with that patch. I noticed that if I change the test2() function in that
test to use a buffer size of 1, then it does not report the object size in the
same conservative way. It didn't look like that was intentional. The
isDesignatorAtObjectEnd() function in lib/AST/ExprConstant.cpp is not handling
the case with C++ inheritance and so it gives up. The catch is that "giving up"
in this case does not lead to the conservative behavior. The attached patch to
the test shows what I would expect to happen.

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