diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -1906,11 +1906,39 @@
   return std::move(adl_begin(Range), adl_end(Range), Out);
 }
 
-/// Wrapper function around std::find to detect if an element exists
-/// in a container.
+namespace detail {
+template <typename Range, typename Element>
+using check_has_member_contains_t =
+    decltype(std::declval<Range &>().contains(std::declval<const Element &>()));
+
+template <typename Range, typename Element>
+static constexpr bool HasMemberContains =
+    is_detected<check_has_member_contains_t, Range, Element>::value;
+
+template <typename Range, typename Element>
+using check_has_member_find_t =
+    decltype(std::declval<Range &>().find(std::declval<const Element &>()) !=
+             std::declval<Range &>().end());
+
+template <typename Range, typename Element>
+static constexpr bool HasMemberFind =
+    is_detected<check_has_member_find_t, Range, Element>::value;
+
+} // namespace detail
+
+/// Returns true if \p Element is found in \p Range. Delegates the check to
+/// either `.contains(Element)`, `.find(Element)`, or `llvm::find`, in this
+/// order of preference. This is intended as the canonical way to check if an
+/// element exists in a range in generic code or range type that does not
+/// expose an `.contains(Element)` member.
 template <typename R, typename E>
 bool is_contained(R &&Range, const E &Element) {
-  return std::find(adl_begin(Range), adl_end(Range), Element) != adl_end(Range);
+  if constexpr (detail::HasMemberContains<R, E>)
+    return Range.contains(Element);
+  else if constexpr (detail::HasMemberFind<R, E>)
+    return Range.find(Element) != Range.end();
+  else
+    return llvm::find(Range, Element) != adl_end(Range);
 }
 
 /// Returns true iff \p Element exists in \p Set. This overload takes \p Set as
diff --git a/llvm/include/llvm/Analysis/LoopInfoImpl.h b/llvm/include/llvm/Analysis/LoopInfoImpl.h
--- a/llvm/include/llvm/Analysis/LoopInfoImpl.h
+++ b/llvm/include/llvm/Analysis/LoopInfoImpl.h
@@ -371,7 +371,7 @@
 
   // Check the parent loop pointer.
   if (ParentLoop) {
-    assert(is_contained(*ParentLoop, this) &&
+    assert(is_contained(ParentLoop->getSubLoops(), this) &&
            "Loop is not a subloop of its parent!");
   }
 #endif
diff --git a/llvm/unittests/ADT/STLExtrasTest.cpp b/llvm/unittests/ADT/STLExtrasTest.cpp
--- a/llvm/unittests/ADT/STLExtrasTest.cpp
+++ b/llvm/unittests/ADT/STLExtrasTest.cpp
@@ -1029,6 +1029,52 @@
   static_assert(!is_contained({1, 2, 3, 4}, 5), "It's not there :(");
 }
 
+TEST(STLExtrasTest, IsContainedMemberContains) {
+  // Check that `llvm::is_contained` uses the member `.contains()` when
+  // available.
+  struct Foo {
+    bool contains(int) const {
+      ++NumContainsCalls;
+      return ContainsResult;
+    }
+    bool ContainsResult = false;
+    mutable unsigned NumContainsCalls = 0;
+  } Container;
+
+  EXPECT_EQ(Container.NumContainsCalls, 0u);
+  EXPECT_FALSE(is_contained(Container, 1));
+  EXPECT_EQ(Container.NumContainsCalls, 1u);
+
+  Container.ContainsResult = true;
+  EXPECT_TRUE(is_contained(Container, 1));
+  EXPECT_EQ(Container.NumContainsCalls, 2u);
+}
+
+TEST(STLExtrasTest, IsContainedMemberFind) {
+  // Check that `llvm::is_contained` uses the member `.find(x)` when available.
+  struct Foo {
+    auto begin() const { return Data.begin(); }
+    auto end() const { return Data.end(); }
+    auto find(int X) const {
+      ++NumFindCalls;
+      return std::find(begin(), end(), X);
+    }
+
+    std::vector<int> Data;
+    mutable unsigned NumFindCalls = 0;
+  } Container;
+
+  Container.Data = {1, 2, 3};
+
+  EXPECT_EQ(Container.NumFindCalls, 0u);
+  EXPECT_TRUE(is_contained(Container, 1));
+  EXPECT_TRUE(is_contained(Container, 3));
+  EXPECT_EQ(Container.NumFindCalls, 2u);
+
+  EXPECT_FALSE(is_contained(Container, 4));
+  EXPECT_EQ(Container.NumFindCalls, 3u);
+}
+
 TEST(STLExtrasTest, addEnumValues) {
   enum A { Zero = 0, One = 1 };
   enum B { IntMax = INT_MAX, ULongLongMax = ULLONG_MAX };