diff --git a/lib/ClangImporter/ClangDerivedConformances.cpp b/lib/ClangImporter/ClangDerivedConformances.cpp
index 89bd5d8ddd66d..59a7a60410aaa 100644
--- a/lib/ClangImporter/ClangDerivedConformances.cpp
+++ b/lib/ClangImporter/ClangDerivedConformances.cpp
@@ -586,10 +586,33 @@ void swift::conformToCxxSetIfNeeded(ClangImporter::Implementation &impl,
   if (!valueType || !sizeType)
     return;
 
+  auto insertId = ctx.getIdentifier("__insertUnsafe");
+  auto inserts = lookupDirectWithoutExtensions(decl, insertId);
+  FuncDecl *insert = nullptr;
+  for (auto candidate : inserts) {
+    if (auto candidateMethod = dyn_cast<FuncDecl>(candidate)) {
+      if (!candidateMethod->hasParameterList())
+        continue;
+      auto params = candidateMethod->getParameters();
+      if (params->size() != 1)
+        continue;
+      auto param = params->front();
+      if (param->getType()->getCanonicalType() !=
+          valueType->getUnderlyingType()->getCanonicalType())
+        continue;
+      insert = candidateMethod;
+      break;
+    }
+  }
+  if (!insert)
+    return;
+
   impl.addSynthesizedTypealias(decl, ctx.Id_Element,
                                valueType->getUnderlyingType());
   impl.addSynthesizedTypealias(decl, ctx.getIdentifier("Size"),
                                sizeType->getUnderlyingType());
+  impl.addSynthesizedTypealias(decl, ctx.getIdentifier("InsertionResult"),
+                               insert->getResultInterfaceType());
   impl.addSynthesizedProtocolAttrs(decl, {KnownProtocolKind::CxxSet});
 }
 
diff --git a/stdlib/public/Cxx/CxxSet.swift b/stdlib/public/Cxx/CxxSet.swift
index 02dec94fb1193..e3f143da45b60 100644
--- a/stdlib/public/Cxx/CxxSet.swift
+++ b/stdlib/public/Cxx/CxxSet.swift
@@ -13,11 +13,31 @@
 public protocol CxxSet<Element> {
   associatedtype Element
   associatedtype Size: BinaryInteger
+  associatedtype InsertionResult // std::pair<iterator, bool>
+
+  init()
+
+  @discardableResult
+  mutating func __insertUnsafe(_ element: Element) -> InsertionResult
 
   func count(_ element: Element) -> Size
 }
 
 extension CxxSet {
+  /// Creates a C++ set containing the elements of a Swift Sequence.
+  ///
+  /// This initializes the set by copying every element of the sequence.
+  ///
+  /// - Complexity: O(*n*), where *n* is the number of elements in the Swift
+  ///   sequence
+  @inlinable
+  public init<S: Sequence>(_ sequence: S) where S.Element == Element {
+    self.init()
+    for item in sequence {
+      self.__insertUnsafe(item)
+    }
+  }
+
   @inlinable
   public func contains(_ element: Element) -> Bool {
     return count(element) > 0
diff --git a/test/Interop/Cxx/stdlib/use-std-set.swift b/test/Interop/Cxx/stdlib/use-std-set.swift
index 56f7b7672f166..95f0110908f10 100644
--- a/test/Interop/Cxx/stdlib/use-std-set.swift
+++ b/test/Interop/Cxx/stdlib/use-std-set.swift
@@ -47,4 +47,18 @@ StdSetTestSuite.test("MultisetOfCInt.contains") {
     expectFalse(s.contains(3))
 }
 
+StdSetTestSuite.test("SetOfCInt.init()") {
+    let s = SetOfCInt([1, 3, 5])
+    expectTrue(s.contains(1))
+    expectFalse(s.contains(2))
+    expectTrue(s.contains(3))
+}
+
+StdSetTestSuite.test("UnorderedSetOfCInt.init()") {
+    let s = UnorderedSetOfCInt([1, 3, 5])
+    expectTrue(s.contains(1))
+    expectFalse(s.contains(2))
+    expectTrue(s.contains(3))
+}
+
 runAllTests()