[llvm-bugs] [Bug 30578] New: 64-bit clang-cl does not discard the __unaligned attribute on pointers when casting

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30578

            Bug ID: 30578
           Summary: 64-bit clang-cl does not discard the __unaligned
                    attribute on pointers when casting
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: froydnj at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

This testcase, cut down from code in Firefox, compiles with 64-bit MSVC but
does not compile with 64-bit clang-cl:

typedef struct _ITEMIDLIST
{
  void* mkid;
} ITEMIDLIST;

typedef ITEMIDLIST __unaligned *LPITEMIDLIST;

LPITEMIDLIST ILCreateFromPathW(const char*);

ITEMIDLIST*
f()
{
  return static_cast<ITEMIDLIST*>(ILCreateFromPathW("nonsense"));
}

The results:

$ cl -c unaligned.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23918 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

unaligned.cpp

$ clang-cl unaligned.cpp -c -fms-compatibility-version=19.00.23918 -m64
unaligned.cpp(13,10):  error: static_cast from 'LPITEMIDLIST' (aka '__unaligned
      _ITEMIDLIST *') to 'ITEMIDLIST *' (aka '_ITEMIDLIST *'), which are not
      related by inheritance, is not allowed
  return static_cast<ITEMIDLIST*>(ILCreateFromPathW("nonsense"));
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

The actual definitions of all of these types come from <ShTypes.h> and
<ShlObj.h>.

Microsoft's documentation for __unaligned says that it is only accepted on
x86-64 and Itanium, but that it only has an effect on Itanium:

https://msdn.microsoft.com/en-us/library/ms177389.aspx

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