[ADT] Fix DenseMapInfo<variant>::isEqual to delegate to DenseMapInfo, not ==

[sammccall]

steal tests from D151079

[sammccall]

sorry, sent this prematurely

[sammccall]

fixed

[IncludeGuardian]

llvm/include/llvm/ADT/DenseMapInfo.h

328–330

I think we may be able to use std::visit twice and rely on the type of the parameter in the second visit matching the original type of the value we cast to const void *.

const void *ErasedLHSValue = std::visit([](const auto& LHSValue){
    return static_cast<const void *>(&LHSValue);
}, l);

return std::visit([=](const auto& RHSValue){
    using T = std::remove_reference_t<decltype(RHSValue)>;
    return DenseMapInfo<T>::isEqual(*static_cast<T *>(ErasedLHSValue), RHSValue);
}, r);

[sammccall]

simplify isEqual by using two std::visit calls with generic lambdas

[sammccall]

llvm/include/llvm/ADT/DenseMapInfo.h

328–330

Right! I always forget that auto-params in lambdas are good for something other than saving typing :-)

[kadircet]

thanks!

[IncludeGuardian]

LGTM!

llvm/include/llvm/ADT/DenseMapInfo.h

return std::visit(
           return detail::combineHashValue(
               DenseMapInfo<size_t>::getHashValue(Val.index()),
               DenseMapInfo<T>::getHashValue(Alternative));
         },
         Val);
   }
 
   static bool isEqual(const Variant &LHS, const Variant &RHS) {
-    return LHS == RHS;
+    if (LHS.index() != RHS.index())
+      return false;
+    if (LHS.valueless_by_exception())
+      return true;
+    // We want to dispatch to DenseMapInfo<T>::isEqual(LHS.get(I), RHS.get(I))
+    // We know the types are the same, but std::visit(V, LHS, RHS) doesn't.
+    // We erase the type held in LHS to void*, and dispatch over RHS.
+    const void *ErasedLHS =
+        std::visit([](const auto &LHS) -> const void * { return &LHS; }, LHS);
+    return std::visit(
+        [&](const auto &RHS) -> bool {
+          using T = std::remove_cv_t<std::remove_reference_t<decltype(RHS)>>;
+          return DenseMapInfo<T>::isEqual(*static_cast<const T *>(ErasedLHS),
+                                          RHS);
+        },
+        RHS);
   }
 };
 } // end namespace llvm
 
 #endif // LLVM_ADT_DENSEMAPINFO_H

llvm/unittests/ADT/DenseMapTest.cpp

 namespace {
 struct A {
   A(int value) : value(value) {}
   int value;
 };
 struct B : public A {
   using A::A;
 };
+
+struct AlwaysEqType {
+  bool operator==(const AlwaysEqType &RHS) const { return true; }
+};
 } // namespace
 
 namespace llvm {
 template <typename T>
 struct DenseMapInfo<T, std::enable_if_t<std::is_base_of_v<A, T>>> {
   static inline T getEmptyKey() { return {static_cast<int>(~0)}; }
   static inline T getTombstoneKey() { return {static_cast<int>(~0U - 1)}; }
   static unsigned getHashValue(const T &Val) { return Val.value; }
   static bool isEqual(const T &LHS, const T &RHS) {
     return LHS.value == RHS.value;
   }
 };
+
+template <> struct DenseMapInfo<AlwaysEqType> {
+  using T = AlwaysEqType;
+  static inline T getEmptyKey() { return {}; }
+  static inline T getTombstoneKey() { return {}; }
+  static unsigned getHashValue(const T &Val) { return 0; }
+  static bool isEqual(const T &LHS, const T &RHS) {
+    return false;
+  }
+};
 } // namespace llvm
 
 namespace {
 TEST(DenseMapCustomTest, SFINAEMapInfo) {
   // Test that we can use a pointer to an incomplete type as a DenseMap key.
   // This is an important build time optimization, since many classes have
   // DenseMap members.
   DenseMap<B, int> Map;
   B Keys[3] = {{0}, {1}, {2}};
   Map.insert({Keys[0], 1});
   Map.insert({Keys[1], 2});
   Map.insert({Keys[2], 3});
   EXPECT_EQ(Map.count(Keys[0]), 1u);
   EXPECT_EQ(Map[Keys[0]], 1);
   EXPECT_EQ(Map[Keys[1]], 2);
   EXPECT_EQ(Map[Keys[2]], 3);
   Map.clear();
   EXPECT_EQ(Map.find(Keys[0]), Map.end());
   EXPECT_EQ(Map.find(Keys[1]), Map.end());
   EXPECT_EQ(Map.find(Keys[2]), Map.end());
 }
 
 TEST(DenseMapCustomTest, VariantSupport) {
-  using variant = std::variant<int, int>;
+  using variant = std::variant<int, int, AlwaysEqType>;
   DenseMap<variant, int> Map;
   variant Keys[] = {
       variant(std::in_place_index<0>, 1),
       variant(std::in_place_index<1>, 1),
+      variant(std::in_place_index<2>),
   };
   Map.try_emplace(Keys[0], 0);
   Map.try_emplace(Keys[1], 1);
   EXPECT_THAT(Map, testing::SizeIs(2));
   EXPECT_NE(DenseMapInfo<variant>::getHashValue(Keys[0]),
             DenseMapInfo<variant>::getHashValue(Keys[1]));
+  // Check that isEqual dispatches to isEqual of underlying type, and not to
+  // operator==.
+  EXPECT_FALSE(DenseMapInfo<variant>::isEqual(Keys[2], Keys[2]));
 }
 } // namespace