Avoid referencing a vtable when not required

[rnk]

Summary

This fixes compilation errors about incomplete types used with WebKit's
RefPtr template. Simply calling an out of line constructor should not
instantiate all inline and defaulted virtual methods.

[thakis]

(fixes PR20221)

[rnk]

Ping. I think we agreed this was safe.

[rsmith]

I think I've convinced myself that this is fine, but please test it against some large projects before committing.

[rnk]

Closed by commit rL213109 (authored by @rnk).

lib/Sema/SemaExpr.cpp

if (Constructor->isDefaulted() && !Constructor->isDeleted()) {
       } else if (Constructor->isCopyConstructor()) {
         DefineImplicitCopyConstructor(Loc, Constructor);
       } else if (Constructor->isMoveConstructor()) {
         DefineImplicitMoveConstructor(Loc, Constructor);
       }
     } else if (Constructor->getInheritedConstructor()) {
       DefineInheritingConstructor(Loc, Constructor);
     }
-
-    MarkVTableUsed(Loc, Constructor->getParent());
   } else if (CXXDestructorDecl *Destructor =
                  dyn_cast<CXXDestructorDecl>(Func)) {
     Destructor = cast<CXXDestructorDecl>(Destructor->getFirstDecl());
     if (Destructor->isDefaulted() && !Destructor->isDeleted())
       DefineImplicitDestructor(Loc, Destructor);
     if (Destructor->isVirtual())
       MarkVTableUsed(Loc, Destructor->getParent());
   } else if (CXXMethodDecl *MethodDecl = dyn_cast<CXXMethodDecl>(Func)) {

test/SemaTemplate/virtual-member-functions.cpp

 struct Derived : Base<T> {
   virtual void foo() { }
 };
 
 template struct Derived<int>; // expected-note {{in instantiation of member function 'Base<int>::~Base' requested here}}
 
 template<typename T>
 struct HasOutOfLineKey {
-  HasOutOfLineKey() { } 
+  HasOutOfLineKey() { } // expected-note{{in instantiation of member function 'HasOutOfLineKey<int>::f' requested here}}
   virtual T *f(float *fp);
 };
 
 template<typename T>
 T *HasOutOfLineKey<T>::f(float *fp) {
   return fp; // expected-error{{cannot initialize return object of type 'int *' with an lvalue of type 'float *'}}
 }
 
-HasOutOfLineKey<int> out_of_line; // expected-note{{in instantiation of member function 'HasOutOfLineKey<int>::f' requested here}}
+HasOutOfLineKey<int> out_of_line; // expected-note{{in instantiation of member function 'HasOutOfLineKey<int>::HasOutOfLineKey' requested here}}
 
 namespace std {
   class type_info;
 }
 
 namespace PR7114 {
   class A { virtual ~A(); }; // expected-note{{declared private here}}
 
template<typename T> struct X : virtual Y {
     virtual void foo() { T x; } // expected-error {{variable has incomplete type 'void'}}
   };
   template<typename T> struct X2 : virtual Y {
     virtual void foo() { T x; }
   };
   Y* f(X<void>* x) { return dynamic_cast<Y*>(x); } // expected-note {{in instantiation of member function 'DynamicCast::X<void>::foo' requested here}}
   Y* f2(X<void>* x) { return dynamic_cast<Y*>(x); }
 }
+
+namespace avoid_using_vtable {
+// We shouldn't emit the vtable for this code, in any ABI.  If we emit the
+// vtable, we emit an implicit virtual dtor, which calls ~RefPtr, which requires
+// a complete type for DeclaredOnly.
+//
+// Previously we would reference the vtable in the MS C++ ABI, even though we
+// don't need to emit either the ctor or the dtor.  In the Itanium C++ ABI, the
+// 'trace' method is the key function, so even though we use the vtable, we
+// don't emit it.
+
+template <typename T>
+struct RefPtr {
+  T *m_ptr;
+  ~RefPtr() { m_ptr->deref(); }
+};
+struct DeclaredOnly;
+struct Base {
+  virtual ~Base();
+};
+
+struct AvoidVTable : Base {
+  RefPtr<DeclaredOnly> m_insertionStyle;
+  virtual void trace();
+  AvoidVTable();
+};
+// Don't call the dtor, because that will emit an implicit dtor, and require a
+// complete type for DeclaredOnly.
+void foo() { new AvoidVTable; }
+}
+
+namespace vtable_uses_incomplete {
+// Opposite of the previous test that avoids a vtable, this one tests that we
+// use the vtable when the ctor is defined inline.
+template <typename T>
+struct RefPtr {
+  T *m_ptr;
+  ~RefPtr() { m_ptr->deref(); }  // expected-error {{member access into incomplete type 'vtable_uses_incomplete::DeclaredOnly'}}
+};
+struct DeclaredOnly; // expected-note {{forward declaration of 'vtable_uses_incomplete::DeclaredOnly'}}
+struct Base {
+  virtual ~Base();
+};
+
+struct UsesVTable : Base {
+  RefPtr<DeclaredOnly> m_insertionStyle;
+  virtual void trace();
+  UsesVTable() {} // expected-note {{in instantiation of member function 'vtable_uses_incomplete::RefPtr<vtable_uses_incomplete::DeclaredOnly>::~RefPtr' requested here}}
+};
+}