Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -10716,9 +10716,6 @@
           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);
@@ -10747,6 +10744,9 @@
         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)) {
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -10486,15 +10486,12 @@
 
   // 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>())
@@ -10502,6 +10499,8 @@
   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)
Index: clang/test/SemaCXX/conversion-function.cpp
===================================================================
--- clang/test/SemaCXX/conversion-function.cpp
+++ clang/test/SemaCXX/conversion-function.cpp
@@ -62,6 +62,24 @@
   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 {