[dllexport] odr-use constexpr default args for constructor closures

[hans]

Summary

InstantiateDefaultCtorDefaultArgs() is supposed to mark default constructor args as odr-used, since those args will be used when emitting the constructor closure.

However, constexpr vars were not getting odr-used since DoMarkVarDeclReferenced() defers them in MaybeODRUseExprs, and the code was calling CleanupVarDeclMarking() which discarded those uses instead of processing them.

(This came up in Chromium, crbug.com/1312086)

[thakis]

LGTM

[xbolva00]

Please wait for some proper clang reviewer

@rsmith @aaron.ballman @rjmccall

[thakis]

What makes me an improper clang reviewer?

[xbolva00]

No, I mean just second look.

I believe @erichkeane is working in this area as well.

[hans]

I think @thakis's review is sufficient. I am of course happy to follow up on any extra review with follow-up patches.

[rnk]

Looks good to me after the fact.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

void Sema::InstantiateDefaultCtorDefaultArgs(CXXConstructorDecl *Ctor) {
   if (NumParams == 0)
     return;
   DLLExportAttr *Attr = Ctor->getAttr<DLLExportAttr>();
   if (!Attr)
     return;
   for (unsigned I = 0; I != NumParams; ++I) {
     (void)CheckCXXDefaultArgExpr(Attr->getLocation(), Ctor,
                                    Ctor->getParamDecl(I));
-    DiscardCleanupsInEvaluationContext();
+    CleanupVarDeclMarking();
   }
 }
 
 /// Get the previous declaration of a declaration for the purposes of template
 /// instantiation. If this finds a previous declaration, then the previous
 /// declaration of the instantiation of D should be an instantiation of the
 /// result of this function.
 template<typename DeclT>

clang/test/CodeGenCXX/dllexport-ctor-closure.cpp

struct __declspec(dllexport) CtorClosureOutOfLine {
   CtorClosureOutOfLine(const HasImplicitDtor2 &v = {});
 };
 CtorClosureOutOfLine::CtorClosureOutOfLine(const HasImplicitDtor2 &v) {}
 
 // CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_FCtorClosureInline@@QAEXXZ"
 // CHECK-LABEL: define linkonce_odr dso_local x86_thiscallcc void @"??1HasImplicitDtor1@@QAE@XZ"
 // CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_FCtorClosureOutOfLine@@QAEXXZ"
 // CHECK-LABEL: define linkonce_odr dso_local x86_thiscallcc void @"??1HasImplicitDtor2@@QAE@XZ"
+
+struct SomeStruct {};
+constexpr SomeStruct kConstexprStruct;
+struct __declspec(dllexport) ConstexprDefaultArg {
+  ConstexprDefaultArg(SomeStruct = kConstexprStruct) {}
+};
+// CHECK-LABEL: define weak_odr dso_local dllexport x86_thiscallcc void @"??_FConstexprDefaultArg@@QAEXXZ"