ADT: Add SFINAE to the generic IntrusiveRefCntPtr constructors

[dexonsmith]

Summary

Add an enable_if to the generic IntrusiveRefCntPtr constructors so
that std::is_convertible gives an honest answer when the underlying
pointers cannot be converted.

[dblaikie]

Looks good - some optional/separate patch suggestions.

llvm/include/llvm/ADT/IntrusiveRefCntPtr.h

173–178

Can we collapse this ctor with the const ref version by having just one version that accepts by value? (not 100% sure we can - genuinely asking. I think we can because IntrusiveRefCntPtr has real copy ctor/move ctor above, so we won't end up with infinite recursion where the copy ctor needs to use the copy ctor.. )

llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp

100–138

Since these are static_asserts, they could just be in the namespace/global scope, rather than being inside a test function. The test function itself does no work when executed, since this is all compile-time checking.

I don't mind too much either way, though.

[dexonsmith]

Thanks for the review! Pushed in 17c584551d573f1693990773e29fbe6b4b6fa4f4.

llvm/include/llvm/ADT/IntrusiveRefCntPtr.h

173–178

Nice catch; I added that in.

llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp

100–138

Yeah, it's a bit cleaner that way. I ended up putting them at namespace scope instead. I stripped it down just to the interesting cases (rejecting) and added an actual unit test for the positive cases.

llvm/include/llvm/ADT/IntrusiveRefCntPtr.h

 
 public:
   using element_type = T;
 
   explicit IntrusiveRefCntPtr() = default;
   IntrusiveRefCntPtr(T *obj) : Obj(obj) { retain(); }
   IntrusiveRefCntPtr(const IntrusiveRefCntPtr &S) : Obj(S.Obj) { retain(); }
   IntrusiveRefCntPtr(IntrusiveRefCntPtr &&S) : Obj(S.Obj) { S.Obj = nullptr; }
 
-  template <class X>
-  IntrusiveRefCntPtr(IntrusiveRefCntPtr<X> &&S) : Obj(S.get()) {
+  template <class X,
+            std::enable_if_t<std::is_convertible<X *, T *>::value, bool> = true>
+  IntrusiveRefCntPtr(IntrusiveRefCntPtr<X> S) : Obj(S.get()) {
     S.Obj = nullptr;
   }
 
-  template <class X>
+  template <class X,
+            std::enable_if_t<std::is_convertible<X *, T *>::value, bool> = true>
   IntrusiveRefCntPtr(std::unique_ptr<X> S) : Obj(S.release()) {
     retain();
   }
 
-  template <class X>
-  IntrusiveRefCntPtr(const IntrusiveRefCntPtr<X> &S) : Obj(S.get()) {
-    retain();
-  }
-
   ~IntrusiveRefCntPtr() { release(); }
 
   IntrusiveRefCntPtr &operator=(IntrusiveRefCntPtr S) {
     swap(S);
     return *this;
   }
 
   T &operator*() const { return *Obj; }

llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp

TEST(IntrusiveRefCntPtr, UsesTraitsToRetainAndRelease) {
   {
     InterceptRefCounted *I = new InterceptRefCounted(&Released, &Retained);
     IntrusiveRefCntPtr<InterceptRefCounted> R = I;
   }
   EXPECT_TRUE(Released);
   EXPECT_TRUE(Retained);
 }
 
+// Test that the generic constructors use SFINAE to disable invalid
+// conversions.
+struct X : RefCountedBase<X> {};
+struct Y : X {};
+struct Z : RefCountedBase<Z> {};
+static_assert(!std::is_convertible<IntrusiveRefCntPtr<X> &&,
+                                   IntrusiveRefCntPtr<Y>>::value,
+              "X&& -> Y should be rejected with SFINAE");
+static_assert(!std::is_convertible<const IntrusiveRefCntPtr<X> &,
+                                   IntrusiveRefCntPtr<Y>>::value,
+              "const X& -> Y should be rejected with SFINAE");
+static_assert(
+    !std::is_convertible<std::unique_ptr<X>, IntrusiveRefCntPtr<Y>>::value,
+    "X -> Y should be rejected with SFINAE");
+static_assert(!std::is_convertible<IntrusiveRefCntPtr<X> &&,
+                                   IntrusiveRefCntPtr<Z>>::value,
+              "X&& -> Z should be rejected with SFINAE");
+static_assert(!std::is_convertible<const IntrusiveRefCntPtr<X> &,
+                                   IntrusiveRefCntPtr<Z>>::value,
+              "cosnt X& -> Z should be rejected with SFINAE");
+static_assert(
+    !std::is_convertible<std::unique_ptr<X>, IntrusiveRefCntPtr<Z>>::value,
+    "X -> Z should be rejected with SFINAE");
+
+TEST(IntrusiveRefCntPtr, InteropsWithConvertible) {
+  // Check converting constructors and operator=.
+  auto Y1 = makeIntrusiveRefCnt<Y>();
+  auto Y2 = makeIntrusiveRefCnt<Y>();
+  auto Y3 = makeIntrusiveRefCnt<Y>();
+  auto Y4 = makeIntrusiveRefCnt<Y>();
+  const void *P1 = Y1.get();
+  const void *P2 = Y2.get();
+  const void *P3 = Y3.get();
+  const void *P4 = Y4.get();
+  IntrusiveRefCntPtr<X> X1 = std::move(Y1);
+  IntrusiveRefCntPtr<X> X2 = Y2;
+  IntrusiveRefCntPtr<X> X3;
+  IntrusiveRefCntPtr<X> X4;
+  X3 = std::move(Y3);
+  X4 = Y4;
+  EXPECT_EQ(P1, X1.get());
+  EXPECT_EQ(P2, X2.get());
+  EXPECT_EQ(P3, X3.get());
+  EXPECT_EQ(P4, X4.get());
+}
+
 } // end namespace llvm