Perform ActOnConversionDeclarator after looking for any virtual functions it overrides

[rdwampler]

Summary

This allows for suppressing warnings about the conversion function never being called if it overrides a virtual function in a base class.

clang/lib/Sema/SemaDecl.cpp

if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(NewFD)) {
           = Context.DeclarationNames.getCXXDestructorName(
                                         Context.getCanonicalType(ClassType));
         if (NewFD->getDeclName() != Name) {
           Diag(NewFD->getLocation(), diag::err_destructor_name);
           NewFD->setInvalidDecl();
           return Redeclaration;
         }
       }
-    } else if (CXXConversionDecl *Conversion
-               = dyn_cast<CXXConversionDecl>(NewFD)) {
-      ActOnConversionDeclarator(Conversion);
     } else if (auto *Guide = dyn_cast<CXXDeductionGuideDecl>(NewFD)) {
       if (auto *TD = Guide->getDescribedFunctionTemplate())
         CheckDeductionGuideTemplate(TD);
 
       // A deduction guide is not on the list of entities that can be
       // explicitly specialized.
       if (Guide->getTemplateSpecializationKind() == TSK_ExplicitSpecialization)
         Diag(Guide->getBeginLoc(), diag::err_deduction_guide_specialized)
if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(NewFD)) {
         // A virtual method shall not have a requires-clause.
         Diag(NewFD->getTrailingRequiresClause()->getBeginLoc(),
              diag::err_constrained_virtual_method);
 
       if (Method->isStatic())
         checkThisInStaticMemberFunctionType(Method);
     }
 
+    if (CXXConversionDecl *Conversion = dyn_cast<CXXConversionDecl>(NewFD))
+      ActOnConversionDeclarator(Conversion);
+
     // Extra checking for C++ overloaded operators (C++ [over.oper]).
     if (NewFD->isOverloadedOperator() &&
         CheckOverloadedOperatorDeclaration(NewFD)) {
       NewFD->setInvalidDecl();
       return Redeclaration;
     }
 
     // Extra checking for C++0x literal operators (C++0x [over.literal]).

clang/lib/Sema/SemaDeclCXX.cpp

 /// class, if possible.
 Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
   assert(Conversion && "Expected to receive a conversion function declaration");
 
   CXXRecordDecl *ClassDecl = cast<CXXRecordDecl>(Conversion->getDeclContext());
 
   // Make sure we aren't redeclaring the conversion function.
   QualType ConvType = Context.getCanonicalType(Conversion->getConversionType());
-
   // C++ [class.conv.fct]p1:
   //   [...] A conversion function is never used to convert a
   //   (possibly cv-qualified) object to the (possibly cv-qualified)
   //   same object type (or a reference to it), to a (possibly
   //   cv-qualified) base class of that type (or a reference to it),
   //   or to (possibly cv-qualified) void.
-  // FIXME: Suppress this warning if the conversion function ends up being a
-  // virtual function that overrides a virtual function in a base class.
   QualType ClassType
     = Context.getCanonicalType(Context.getTypeDeclType(ClassDecl));
   if (const ReferenceType *ConvTypeRef = ConvType->getAs<ReferenceType>())
     ConvType = ConvTypeRef->getPointeeType();
   if (Conversion->getTemplateSpecializationKind() != TSK_Undeclared &&
       Conversion->getTemplateSpecializationKind() != TSK_ExplicitSpecialization)
     /* Suppress diagnostics for instantiations. */;
+  else if (Conversion->size_overridden_methods() != 0)
+    /* Suppress diagnostics for overriding virtual function in a base class. */;
   else if (ConvType->isRecordType()) {
     ConvType = Context.getCanonicalType(ConvType).getUnqualifiedType();
     if (ConvType == ClassType)
       Diag(Conversion->getLocation(), diag::warn_conv_to_self_not_used)
         << ClassType;
     else if (IsDerivedFrom(Conversion->getLocation(), ClassType, ConvType))
       Diag(Conversion->getLocation(), diag::warn_conv_to_base_not_used)
         <<  ClassType << ConvType;

clang/test/SemaCXX/conversion-function.cpp

 
 class B : public A {
 public:
   operator A&() const; // expected-warning{{conversion function converting 'B' to its base class 'A' will never be used}}
   operator const void() const; // expected-warning{{conversion function converting 'B' to 'const void' will never be used}}
   operator const B(); // expected-warning{{conversion function converting 'B' to itself will never be used}}
 };
 
+class BaseA {};
+class DerivedA;
+
+class BaseB {
+  virtual operator BaseA &() = 0;
+  virtual operator DerivedA &() = 0;
+};
+
+class DerivedA : public BaseA, BaseB {
+  virtual operator BaseA &();    // OK. Overrides BaseB::operatorBaseA&()
+  virtual operator DerivedA &(); // OK. Overrides BaseB::operatorDerivedA&()
+};
+
+class DerivedB : public BaseA {
+  virtual operator DerivedB &(); // expected-warning{{conversion function converting 'DerivedB' to itself will never be used}}
+  virtual operator BaseA &();    // expected-warning{{conversion function converting 'DerivedB' to its base class 'BaseA' will never be used}}
+};
+
 // This used to crash Clang.
 struct Flip;
 struct Flop {
   Flop();
   Flop(const Flip&); // expected-note{{candidate constructor}}
 };
 struct Flip {
   operator Flop() const; // expected-note{{candidate function}}