Don't emit strong vtable definitions for imported classes with key functions (PR21355)

[hans]

Summary

Clang would previously assert on the following code when targeting MinGW:

struct __declspec(dllimport) S {
  virtual ~S();
};
S::~S() {}

Because ~S is a key function and the class is dllimport, we would try to emit a strong definition of the vtable, with dllimport - which is a conflict. We should not emit strong vtable definitions for imported classes.

[majnemer]

LGTM

[hans]

Closed by commit rL220532 (authored by @hans).

lib/CodeGen/CGVTables.cpp

 /// Note that we only call this at the end of the translation unit.
 llvm::GlobalVariable::LinkageTypes 
 CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
   if (!RD->isExternallyVisible())
     return llvm::GlobalVariable::InternalLinkage;
 
   // We're at the end of the translation unit, so the current key
   // function is fully correct.
-  if (const CXXMethodDecl *keyFunction = Context.getCurrentKeyFunction(RD)) {
+  const CXXMethodDecl *keyFunction = Context.getCurrentKeyFunction(RD);
+  if (keyFunction && !RD->hasAttr<DLLImportAttr>()) {
     // If this class has a key function, use that to determine the
     // linkage of the vtable.
     const FunctionDecl *def = nullptr;
     if (keyFunction->hasBody(def))
       keyFunction = cast<CXXMethodDecl>(def);
     
     switch (keyFunction->getTemplateSpecializationKind()) {
       case TSK_Undeclared:

test/CodeGenCXX/dllimport.cpp

// GNU2-NOT: @llvm.global_ctors
   template <int I> struct D { static int x, y; };
   template <int I> int D<I>::x = I + 1;
   template <int I> int D<I>::y = I + f();
   template struct __declspec(dllimport) D<42>;
   // MSC-DAG: @"\01?x@?$D@$0CK@@PR19933@@2HA" = available_externally dllimport global i32 43
   // MSC-DAG: @"\01?y@?$D@$0CK@@PR19933@@2HA" = available_externally dllimport global i32 0
 }
 
+namespace PR21355 {
+  struct __declspec(dllimport) S {
+    virtual ~S();
+  };
+  S::~S() {}
+
+  // S::~S is a key function, so we would ordinarily emit a strong definition for
+  // the vtable. However, S imported, so the vtable should be too.
+
+  // GNU-DAG: @_ZTVN7PR213551SE = available_externally dllimport unnamed_addr constant [4 x i8*]
+}
+
 // MS ignores DLL attributes on partial specializations.
 template <typename T> struct PartiallySpecializedClassTemplate {};
 template <typename T> struct __declspec(dllimport) PartiallySpecializedClassTemplate<T*> { void f() {} };
 USEMEMFUNC(PartiallySpecializedClassTemplate<void*>, f);
 // M32-DAG: define linkonce_odr x86_thiscallcc void @"\01?f@?$PartiallySpecializedClassTemplate@PAX@@QAEXXZ"
 // G32-DAG: {{declare|define available_externally}} dllimport x86_thiscallcc void @_ZN33PartiallySpecializedClassTemplateIPvE1fEv
 
 // Attributes on explicit specializations are honored.