[llvm] 17c5845 - ADT: Add SFINAE to the generic IntrusiveRefCntPtr constructors

[Duncan P. N. Exon Smith via llvm-commits]

Author: Duncan P. N. Exon Smith
Date: 2021-01-28T15:07:27-08:00
New Revision: 17c584551d573f1693990773e29fbe6b4b6fa4f4

URL: https://github.com/llvm/llvm-project/commit/17c584551d573f1693990773e29fbe6b4b6fa4f4
DIFF: https://github.com/llvm/llvm-project/commit/17c584551d573f1693990773e29fbe6b4b6fa4f4.diff

LOG: ADT: Add SFINAE to the generic IntrusiveRefCntPtr constructors

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. Added `static_assert`s to the test suite
to verify.

Also combine generic constructors from `IntrusiveRefCntPtr<X>&&` and
`const IntrusiveRefCntPtr<X>&`. At first glance this appears to be an
infinite loop, but the real copy/move constructors are spelled out
separately above. Added a unit test to verify.

Differential Revision: https://reviews.llvm.org/D95498

Added: 
    

Modified: 
    llvm/include/llvm/ADT/IntrusiveRefCntPtr.h
    llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp

Removed: 
    


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diff  --git a/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h b/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h
index ca4c40db48b9..f3dead8636d4 100644
--- a/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h
+++ b/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h
@@ -171,21 +171,18 @@ template <typename T> class IntrusiveRefCntPtr {
   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) {

diff  --git a/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp b/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp
index 5dbc1b53d3fe..6ba02ac5c3bc 100644
--- a/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp
+++ b/llvm/unittests/ADT/IntrusiveRefCntPtrTest.cpp
@@ -96,4 +96,50 @@ TEST(IntrusiveRefCntPtr, UsesTraitsToRetainAndRelease) {
   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