Index: clang/lib/Lex/HeaderSearch.cpp
===================================================================
--- clang/lib/Lex/HeaderSearch.cpp
+++ clang/lib/Lex/HeaderSearch.cpp
@@ -194,10 +194,20 @@
   for (const std::string &Dir : HSOpts->PrebuiltModulePaths) {
     SmallString<256> Result(Dir);
     llvm::sys::fs::make_absolute(Result);
-    llvm::sys::path::append(Result, ModuleName + ".pcm");
+    if (ModuleName.contains(':'))
+      // The separator of C++20 modules partitions (':') is not good for file
+      // systems, here we choose '-' by default since it is not a valid
+      // character of C++ indentifiers. So we could avoid conflicts. BTW, '-' is
+      // the choice of GCC too.
+      llvm::sys::path::append(Result, ModuleName.split(':').first + "-" +
+                                          ModuleName.split(':').second +
+                                          ".pcm");
+    else
+      llvm::sys::path::append(Result, ModuleName + ".pcm");
     if (getFileMgr().getFile(Result.str()))
       return std::string(Result);
   }
+
   return {};
 }
 
Index: clang/test/Modules/search-partitions.cppm
===================================================================
--- /dev/null
+++ clang/test/Modules/search-partitions.cppm
@@ -0,0 +1,44 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/partition1.cpp \
+// RUN:  -o %t/A-Part1.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/partition2.cpp \
+// RUN:  -o %t/A-Part2.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/partition3.cpp \
+// RUN:  -o %t/A-Part3.pcm
+
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/moduleA.cpp \
+// RUN:  -fprebuilt-module-path=%t
+
+// expected-no-diagnostics
+
+//--- partition1.cpp
+export module A : Part1;
+
+int part1();
+
+//--- partition2.cpp
+
+export module A : Part2;
+
+int part2();
+
+//--- partition3.cpp
+
+export module A : Part3;
+
+int part3();
+
+//--- moduleA.cpp
+
+export module A;
+
+import : Part1;
+export import : Part2;
+import : Part3;
+
+int foo();