Index: clang/include/clang/Basic/Module.h
===================================================================
--- clang/include/clang/Basic/Module.h
+++ clang/include/clang/Basic/Module.h
@@ -534,13 +534,13 @@
     return Kind == ModuleInterfaceUnit || isModulePartition();
   }
 
+  static StringRef getPrimaryModuleInterfaceName(StringRef Name) {
+    return Name.split(':').first;
+  }
+
   /// Get the primary module interface name from a partition.
   StringRef getPrimaryModuleInterfaceName() const {
-    if (isModulePartition()) {
-      auto pos = Name.find(':');
-      return StringRef(Name.data(), pos);
-    }
-    return Name;
+    return getPrimaryModuleInterfaceName(getTopLevelModuleName());
   }
 
   /// Retrieve the full name of this module, including the path from
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -2274,6 +2274,12 @@
 
   VisibleModuleSet VisibleModules;
 
+  /// Cache for module units which is in the same module with the current
+  /// module.
+  llvm::DenseSet<const Module *> CurrentModuleUnitsCache;
+
+  bool isInCurrentModule(const Module *M);
+
 public:
   /// Get the module owning an entity.
   Module *getOwningModule(const Decl *Entity) {
Index: clang/lib/Sema/SemaLookup.cpp
===================================================================
--- clang/lib/Sema/SemaLookup.cpp
+++ clang/lib/Sema/SemaLookup.cpp
@@ -1556,17 +1556,24 @@
   return LookupModulesCache;
 }
 
-/// Determine whether the module M is part of the current module from the
-/// perspective of a module-private visibility check.
-static bool isInCurrentModule(const Module *M, const LangOptions &LangOpts) {
+/// Determine whether the module M is in the same module with the current
+/// module.
+bool Sema::isInCurrentModule(const Module *M) {
+  assert(M && "We shouldn't check nullness for module here");
+  // Return quickly if we cached the result.
+  if (CurrentModuleUnitsCache.count(M))
+    return true;
+
   // If M is the global module fragment of a module that we've not yet finished
   // parsing, then it must be part of the current module.
-  // If it's a partition, then it must be visible to an importer (since only
-  // another partition or the named module can import it).
-  return M->getTopLevelModuleName() == LangOpts.CurrentModule ||
-         (M->Kind == Module::GlobalModuleFragment && !M->Parent) ||
-         M->Kind == Module::ModulePartitionInterface ||
-         M->Kind == Module::ModulePartitionImplementation;
+  if ((M->isGlobalModule() && !M->Parent) ||
+      (M->getPrimaryModuleInterfaceName() ==
+       Module::getPrimaryModuleInterfaceName(getLangOpts().CurrentModule))) {
+    CurrentModuleUnitsCache.insert(M);
+    return true;
+  }
+
+  return false;
 }
 
 bool Sema::hasVisibleMergedDefinition(NamedDecl *Def) {
@@ -1578,7 +1585,7 @@
 
 bool Sema::hasMergedDefinitionInCurrentModule(NamedDecl *Def) {
   for (const Module *Merged : Context.getModulesWithMergedDefinition(Def))
-    if (isInCurrentModule(Merged, getLangOpts()))
+    if (isInCurrentModule(Merged))
       return true;
   return false;
 }
@@ -1760,7 +1767,7 @@
   // means it is part of the current module. For any other query, that means it
   // is in our visible module set.
   if (ModulePrivate) {
-    if (isInCurrentModule(M, getLangOpts()))
+    if (isInCurrentModule(M))
       return true;
     else if (M->Kind == Module::ModuleKind::ModulePartitionImplementation &&
              isModuleDirectlyImported(M))
Index: clang/test/CXX/module/module.import/p2.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.import/p2.cpp
@@ -0,0 +1,38 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -std=c++20 %t/impl.cppm -emit-module-interface -o %t/M-impl.pcm
+// RUN: %clang_cc1 -std=c++20 %t/M.cppm -emit-module-interface -fprebuilt-module-path=%t -o %t/M.pcm
+// RUN: %clang_cc1 -std=c++20 %t/Use.cpp -fprebuilt-module-path=%t -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 %t/UseInPartA.cppm -fprebuilt-module-path=%t -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 %t/UseInAnotherModule.cppm -fprebuilt-module-path=%t -verify -fsyntax-only
+
+//--- impl.cppm
+module M:impl;
+class A {};
+
+//--- M.cppm
+export module M;
+import :impl;
+export A f();
+
+//--- Use.cpp
+import M;
+void test() {
+  A a; // expected-error {{unknown type name 'A'}}
+}
+
+//--- UseInPartA.cppm
+// expected-no-diagnostics
+export module M:partA;
+import :impl;
+void test() {
+  A a;
+}
+
+//--- UseInAnotherModule.cppm
+export module B;
+import M;
+void test() {
+  A a; // expected-error {{unknown type name 'A'}}
+}
Index: clang/test/Modules/cxx20-10-1-ex2.cpp
===================================================================
--- clang/test/Modules/cxx20-10-1-ex2.cpp
+++ clang/test/Modules/cxx20-10-1-ex2.cpp
@@ -56,7 +56,14 @@
 int &c = n; // expected-error {{use of undeclared identifier}}
 
 //--- std10-1-ex2-tu7.cpp
+// expected-no-diagnostics
 module B:X3; // does not implicitly import B
-import :X2;  // X2 is an implementation so exports nothing.
-             // error: n not visible here.
-int &c = n;  // expected-error {{use of undeclared identifier }}
+import : X2; // X2 is an implementation unit import B.
+// According to [module.import]p7:
+//   Additionally, when a module-import-declaration in a module unit of some
+//   module M imports another module unit U of M, it also imports all
+//   translation units imported by non-exported module-import-declarations in
+//   the module unit purview of U.
+//
+// So B is imported in B:X3 due to B:X2 imported B. So n is visible here.
+int &c = n;