[llvm-bugs] [Bug 27811] New: [ms] clang-cl emits scalar deleting ctor and vtable for templates with dllimported instantiation declaration

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27811

            Bug ID: 27811
           Summary: [ms] clang-cl emits scalar deleting ctor and vtable
                    for templates with dllimported instantiation
                    declaration
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nicolasweber at gmx.de
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Somewhat similar to bug 27810, but I'm less sure that it's wrong -- I vaguely
remember that we emit some members more often than cl on purpose.
Anyhow...consider:

template <class T>
struct codecvt {
  virtual ~codecvt() {
  }
};
template class __declspec(dllimport) codecvt<char>;

clang-cl emits a whole lot of stuff for this. cl emits nothing. Regular
non-windows clang also emits nothing if this is converted to an `extern
template class codecvt<char>`.


This too causes us to emit many functions (order of 60) into every TU including
MS C++ headers. Other than bug 27810, these functions disappear after -O2, but
it'd probably be much faster to not emit them in the first place, instead of
relying on the optimizer to clean them up.

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