[llvm-bugs] [Bug 38944] New: Windows debug symbols don't show vtcast information

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38944

            Bug ID: 38944
           Summary: Windows debug symbols don't show vtcast information
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: dmajor at mozilla.com
                CC: llvm-bugs at lists.llvm.org, rnk at google.com,
                    zturner at google.com

struct MyInterface
{
   virtual int F() = 0;
};

struct MyClass1 : public MyInterface
{
  int F() { return 1; }
} g_myClass1;

struct MyClass2 : public MyInterface
{
  int F() { return 2; }
} g_myClass2;

int main(int argc, char* argv[])
{
  MyInterface* volatile ptr;
  ptr = &g_myClass1;
  if (argc) ptr = &g_myClass2;
  __debugbreak();
  int result = ptr->F();
  return result;
}

clang-cl -c -GR- -Z7 x.cpp
lld-link -debug -nodefaultlib -entry:main x.obj
windbg -g x.exe

At the breakpoint, open the Locals window (Alt+3).

With cl.exe + link.exe, you can expand the `ptr` node and see a vtcast option:

- ptr                   0x00007ff7`13ffff10 struct MyInterface *
  + __vtcast_MyClass2   struct MyClass2
  + __vfptr             0x00007ff7`13fec050 x!MyClass2::`vftable' = Array [1]

With clang-cl + lld-link, `ptr` isn't even expandable:

  ptr                   0x00007ff7`c8b73008 struct MyInterface *

Reduced from https://bugzilla.mozilla.org/show_bug.cgi?id=1490743

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