[SetOperations] Support set containers with remove_if (#96613)

The current set_intersect implementation only works for std::set style
sets that have a value-erase method that does not invalidate iterators.
As such, it cannot be used for set containers like SetVector, which only
has iterator-invalidating erase.

Support such set containers by calling the remove_if method instead, if
it exists. The detection code is adopted from how contains() is detected
inside llvm::is_contained().

Author: nikic

llvm/include/llvm/ADT/SetOperations.h

@@ -15,8 +15,20 @@
 #ifndef LLVM_ADT_SETOPERATIONS_H
 #define LLVM_ADT_SETOPERATIONS_H
 
+#include "llvm/ADT/STLExtras.h"
+
 namespace llvm {
 
+namespace detail {
+template <typename Set, typename Fn>
+using check_has_member_remove_if_t =
+    decltype(std::declval<Set>().remove_if(std::declval<Fn>()));
+
+template <typename Set, typename Fn>
+static constexpr bool HasMemberRemoveIf =
+    is_detected<check_has_member_remove_if_t, Set, Fn>::value;
+} // namespace detail
+
 /// set_union(A, B) - Compute A := A u B, return whether A changed.
 ///
 template <class S1Ty, class S2Ty> bool set_union(S1Ty &S1, const S2Ty &S2) {
@@ -36,11 +48,16 @@ template <class S1Ty, class S2Ty> bool set_union(S1Ty &S1, const S2Ty &S2) {
 /// elements that are not contained in S2.
 ///
 template <class S1Ty, class S2Ty> void set_intersect(S1Ty &S1, const S2Ty &S2) {
-  for (typename S1Ty::iterator I = S1.begin(); I != S1.end();) {
-    const auto &E = *I;
-    ++I;
-    if (!S2.count(E))
-      S1.erase(E); // Erase element if not in S2
+  auto Pred = [&S2](const auto &E) { return !S2.count(E); };
+  if constexpr (detail::HasMemberRemoveIf<S1Ty, decltype(Pred)>) {
+    S1.remove_if(Pred);
+  } else {
+    for (typename S1Ty::iterator I = S1.begin(); I != S1.end();) {
+      const auto &E = *I;
+      ++I;
+      if (!S2.count(E))
+        S1.erase(E); // Erase element if not in S2
+    }
   }
 }
 

llvm/unittests/ADT/SetOperationsTest.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SetOperations.h"
+#include "llvm/ADT/SetVector.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
@@ -65,6 +66,16 @@ TEST(SetOperationsTest, SetIntersect) {
   // is empty as they are non-overlapping.
   EXPECT_THAT(Set1, IsEmpty());
   EXPECT_EQ(ExpectedSet2, Set2);
+
+  // Check that set_intersect works on SetVector via remove_if.
+  SmallSetVector<int, 4> SV;
+  SV.insert(3);
+  SV.insert(6);
+  SV.insert(4);
+  SV.insert(5);
+  set_intersect(SV, Set2);
+  // SV should contain only 6 and 5 now.
+  EXPECT_THAT(SV, testing::ElementsAre(6, 5));
 }
 
 TEST(SetOperationsTest, SetIntersection) {