[LLVMbugs] [Bug 22840] New: MS ABI: dllexport default closure constructor

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22840

            Bug ID: 22840
           Summary: MS ABI: dllexport default closure constructor
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: david.majnemer at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

consider:
struct __declspec(dllimport) S { S(int = 3) {} };

void f() {
  S s[5];
}

compiling with MSVC 2013 or 2015 yields a reference to __imp_??_FS@@QEAAXXZ
which is the default constructor closure.  The default constructor closure
evaluates all the default arguments and then calls the default constructor. 
Wrapping it this way makes it possible to call the "vector constructor
iterator" which MSVC employs as a code-size optimization.

If you change the dllimport to dllexport and compile with clang, no default
constructor is emitted and thus our program will not link.

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