--- a/mfbt/RefPtr.h
+++ b/mfbt/RefPtr.h
@@ -7,16 +7,18 @@
 #ifndef mozilla_RefPtr_h
 #define mozilla_RefPtr_h
 
 #include "mozilla/AlreadyAddRefed.h"
 #include "mozilla/Assertions.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/DbgMacro.h"
 
+#include <type_traits>
+
 /*****************************************************************************/
 
 // template <class T> class RefPtrGetterAddRefs;
 
 class nsQueryReferent;
 class nsCOMPtr_helper;
 class nsISupports;
 
@@ -105,39 +107,43 @@ class MOZ_IS_REFPTR RefPtr {
   MOZ_IMPLICIT RefPtr(T* aRawPtr) : mRawPtr(aRawPtr) {
     if (mRawPtr) {
       ConstRemovingRefPtrTraits<T>::AddRef(mRawPtr);
     }
   }
 
   MOZ_IMPLICIT RefPtr(decltype(nullptr)) : mRawPtr(nullptr) {}
 
-  template <typename I>
+  template <typename I,
+            typename = std::enable_if_t<std::is_convertible_v<I*, T*>>>
   MOZ_IMPLICIT RefPtr(already_AddRefed<I>& aSmartPtr)
       : mRawPtr(aSmartPtr.take())
   // construct from |already_AddRefed|
   {}
 
-  template <typename I>
+  template <typename I,
+            typename = std::enable_if_t<std::is_convertible_v<I*, T*>>>
   MOZ_IMPLICIT RefPtr(already_AddRefed<I>&& aSmartPtr)
       : mRawPtr(aSmartPtr.take())
   // construct from |otherRefPtr.forget()|
   {}
 
-  template <typename I>
+  template <typename I,
+            typename = std::enable_if_t<std::is_convertible_v<I*, T*>>>
   MOZ_IMPLICIT RefPtr(const RefPtr<I>& aSmartPtr)
       : mRawPtr(aSmartPtr.get())
   // copy-construct from a smart pointer with a related pointer type
   {
     if (mRawPtr) {
       ConstRemovingRefPtrTraits<T>::AddRef(mRawPtr);
     }
   }
 
-  template <typename I>
+  template <typename I,
+            typename = std::enable_if_t<std::is_convertible_v<I*, T*>>>
   MOZ_IMPLICIT RefPtr(RefPtr<I>&& aSmartPtr)
       : mRawPtr(aSmartPtr.forget().take())
   // construct from |Move(RefPtr<SomeSubclassOfT>)|.
   {}
 
   MOZ_IMPLICIT RefPtr(const nsQueryReferent& aHelper);
   MOZ_IMPLICIT RefPtr(const nsCOMPtr_helper& aHelper);
 #if defined(XP_WIN)

--- a/mfbt/tests/TestRefPtr.cpp
+++ b/mfbt/tests/TestRefPtr.cpp
@@ -109,10 +109,18 @@ int main() {
   MOZ_RELEASE_ASSERT(11 == Foo::sNumDestroyed);
 
   {
     RefPtr<Foo> f = GetNullFoo();
     MOZ_RELEASE_ASSERT(11 == Foo::sNumDestroyed);
   }
   MOZ_RELEASE_ASSERT(11 == Foo::sNumDestroyed);
 
+  {
+    bool condition = true;
+    const auto f =
+        condition ? mozilla::MakeRefPtr<Bar>() : mozilla::MakeRefPtr<Foo>();
+
+    MOZ_RELEASE_ASSERT(f);
+  }
+
   return 0;
 }