Index: clang/lib/AST/ItaniumMangle.cpp
===================================================================
--- clang/lib/AST/ItaniumMangle.cpp
+++ clang/lib/AST/ItaniumMangle.cpp
@@ -1537,6 +1537,25 @@
       }
     }
 
+    // Handle modules specially to avoid breaking ABI.
+    // Otherwise it is possible to mangle two different lambda to the same name,
+    // See issue 52857 for example.
+    if (!TD->isExternallyVisible() || TD->getOwningModuleForLinkage()) {
+      // Get a unique id for the anonymous struct. If it is not a real output
+      // ID doesn't matter so use fake one.
+      unsigned AnonStructId = NullOut ? 0 : Context.getAnonymousStructId(TD);
+
+      // Mangle it as a source name in the form
+      // [n] $_<id>
+      // where n is the length of the string.
+      SmallString<8> Str;
+      Str += "$_";
+      Str += llvm::utostr(AnonStructId);
+
+      Out << Str.size();
+      Out << Str;
+    }
+
     if (TD->isExternallyVisible()) {
       unsigned UnnamedMangle = getASTContext().getManglingNumber(TD);
       Out << "Ut";
@@ -1544,22 +1563,8 @@
         Out << UnnamedMangle - 2;
       Out << '_';
       writeAbiTags(TD, AdditionalAbiTags);
-      break;
     }
 
-    // Get a unique id for the anonymous struct. If it is not a real output
-    // ID doesn't matter so use fake one.
-    unsigned AnonStructId = NullOut ? 0 : Context.getAnonymousStructId(TD);
-
-    // Mangle it as a source name in the form
-    // [n] $_<id>
-    // where n is the length of the string.
-    SmallString<8> Str;
-    Str += "$_";
-    Str += llvm::utostr(AnonStructId);
-
-    Out << Str.size();
-    Out << Str;
     break;
   }
 
Index: clang/test/CodeGenCXX/cxx20-lambda-modules.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/cxx20-lambda-modules.cpp
@@ -0,0 +1,18 @@
+// Tests that the lambda in modules would be mangled properly.
+
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: echo "export module test;" >> %t/test.cppm
+// RUN: echo "export namespace test {" >> %t/test.cppm
+// RUN: echo "auto foo = [] {return 43;};" >> %t/test.cppm
+// RUN: echo "auto bar = [] {return 'a';};" >> %t/test.cppm
+// RUN: echo "}" >> %t/test.cppm
+// RUN: %clang_cc1  -std=c++20 %t/test.cppm -emit-module-interface -o %t/test.pcm
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fprebuilt-module-path=%t -std=c++20 %s -emit-llvm -o - | FileCheck %s
+import test;
+void use() {
+  test::foo();
+  test::bar();
+}
+// CHECK: call noundef i32 @"_ZW4testENK4test3$_0Ut_clEv"
+// CHECK: call noundef signext i8 @"_ZW4testENK4test3$_1Ut_clEv"