[dllimport] A non-imported class with an imported key can't have a key

[rnk]

Summary

The vtable takes its DLL storage class from the class, not the key
function. When they disagree, the vtable won't be exported by the DLL
that defines the key function. The easiest way to ensure that importers
of the class emit their own vtable is to say that the class has no key
function.

[majnemer]

LGTM

lib/AST/RecordLayoutBuilder.cpp

for (const CXXMethodDecl *MD : RD->methods()) {
 
     // In certain ABIs, ignore functions with out-of-line inline definitions.
     if (!allowInlineFunctions) {
       const FunctionDecl *Def;
       if (MD->hasBody(Def) && Def->isInlineSpecified())
         continue;
     }
 
+    // If the key function is dllimport but the class isn't, then the class has
+    // no key function. The DLL that exports the key function won't export the
+    // vtable in this case.
+    if (MD->hasAttr<DLLImportAttr>() && !RD->hasAttr<DLLImportAttr>())
+      return nullptr;
+
     // We found it.
     return MD;
   }
 
   return nullptr;
 }
 
 DiagnosticBuilder ItaniumRecordLayoutBuilder::Diag(SourceLocation Loc,

test/CodeGenCXX/dllimport-rtti.cpp

 } u;
 
 struct __declspec(dllimport) V {
   virtual void f();
 } v;
 // GNU-DAG: @_ZTV1V = available_externally dllimport
 // GNU-DAG: @_ZTS1V = linkonce_odr
 // GNU-DAG: @_ZTI1V = linkonce_odr
+
+struct W {
+  __declspec(dllimport) virtual void f();
+  virtual void g();
+} w;
+// GNU-DAG: @_ZTV1W = linkonce_odr
+// GNU-DAG: @_ZTS1W = linkonce_odr
+// GNU-DAG: @_ZTI1W = linkonce_odr