diff --git a/lib/AST/ModuleNameLookup.cpp b/lib/AST/ModuleNameLookup.cpp
index 288625bb5ab20..cd5915b468fd0 100644
--- a/lib/AST/ModuleNameLookup.cpp
+++ b/lib/AST/ModuleNameLookup.cpp
@@ -17,6 +17,8 @@
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/NameLookupRequests.h"
 #include "swift/Basic/Assertions.h"
+#include "clang/AST/Attr.h"
+#include "clang/AST/ExprCXX.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace swift;
@@ -278,6 +280,43 @@ void ModuleNameLookup<LookupStrategy>::lookupInModule(
   if (decls.size() - initialCount <= 1)
     return;
 
+  // Some functions like `memchr` are defined both in libc and in libc++.
+  // Importing both would result in ambiguities, but there are some attributes
+  // that mark the preferred overloads. See _LIBCPP_PREFERRED_OVERLOAD.
+  llvm::SmallPtrSet<ValueDecl *, 4> declsToRemove;
+  bool hasPreferredOverload = false;
+  for (auto decl : decls)
+    if (const auto *clangDecl = decl->getClangDecl()) {
+      if (clangDecl->hasAttr<clang::EnableIfAttr>()) {
+        // FIXME: at some point we might want to call into Clang to implement
+        // the full enable_if semantics including the constant evaluation of the
+        // conditions. For now, just look for the first enable_if(true, "...")
+        // and assume all the rest of the enable_ifs evaluate to true.
+        bool thisDeclHasPreferredOverload = false;
+        for (auto clangAttr :
+             clangDecl->specific_attrs<clang::EnableIfAttr>()) {
+          if (auto litExpr =
+                  dyn_cast<clang::CXXBoolLiteralExpr>(clangAttr->getCond())) {
+            if (litExpr->getValue()) {
+              thisDeclHasPreferredOverload = hasPreferredOverload = true;
+              break;
+            }
+          }
+        }
+        if (!thisDeclHasPreferredOverload)
+          declsToRemove.insert(decl);
+      } else
+        declsToRemove.insert(decl);
+    }
+
+  if (hasPreferredOverload) {
+    decls.erase(std::remove_if(decls.begin() + initialCount, decls.end(),
+                               [&](ValueDecl *d) -> bool {
+                                 return declsToRemove.contains(d);
+                               }),
+                decls.end());
+  }
+
   // Remove duplicated declarations, which can happen when the same module is
   // imported with multiple access paths.
   llvm::SmallPtrSet<ValueDecl *, 4> knownDecls;
diff --git a/test/Interop/Cxx/modules/Inputs/ambiguous_a.h b/test/Interop/Cxx/modules/Inputs/ambiguous_a.h
new file mode 100644
index 0000000000000..623fe07e1f579
--- /dev/null
+++ b/test/Interop/Cxx/modules/Inputs/ambiguous_a.h
@@ -0,0 +1,3 @@
+#pragma once
+
+void f(int a);
diff --git a/test/Interop/Cxx/modules/Inputs/ambiguous_b.h b/test/Interop/Cxx/modules/Inputs/ambiguous_b.h
new file mode 100644
index 0000000000000..c93b9861d8d3a
--- /dev/null
+++ b/test/Interop/Cxx/modules/Inputs/ambiguous_b.h
@@ -0,0 +1,3 @@
+#pragma once
+
+void f(int a) __attribute__((enable_if(a > 0, ""))) __attribute__((enable_if(true, "")));
diff --git a/test/Interop/Cxx/modules/Inputs/module.modulemap b/test/Interop/Cxx/modules/Inputs/module.modulemap
index 02f06f1f8f09f..1c760fd08e66b 100644
--- a/test/Interop/Cxx/modules/Inputs/module.modulemap
+++ b/test/Interop/Cxx/modules/Inputs/module.modulemap
@@ -17,3 +17,13 @@ module TopLevelModule {
   }
   export *
 }
+
+module AmbiguousA {
+  header "ambiguous_a.h"
+  requires cplusplus
+}
+
+module AmbiguousB {
+  header "ambiguous_a.h"
+  requires cplusplus
+}
diff --git a/test/Interop/Cxx/modules/preferred-overload.swift b/test/Interop/Cxx/modules/preferred-overload.swift
new file mode 100644
index 0000000000000..f594191325433
--- /dev/null
+++ b/test/Interop/Cxx/modules/preferred-overload.swift
@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=default
+
+import AmbiguousA
+import AmbiguousB
+
+func g(_ x: CInt) {
+    f(x)
+}
diff --git a/test/Interop/Cxx/objc-correctness/memchr.swift b/test/Interop/Cxx/objc-correctness/memchr.swift
new file mode 100644
index 0000000000000..d0618aa574847
--- /dev/null
+++ b/test/Interop/Cxx/objc-correctness/memchr.swift
@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -cxx-interoperability-mode=default -typecheck -verify -I %S/Inputs %s
+
+// REQUIRES: objc_interop
+// REQUIRES: VENDOR=apple
+
+import Darwin
+
+func test_memchr() {
+    var src = "hello"
+    var _ = src.withUTF8 { srcBuf in
+        Darwin.memchr(srcBuf.baseAddress!, 137, src.count)
+    }
+}