Index: clang/include/clang/Basic/Module.h
===================================================================
--- clang/include/clang/Basic/Module.h
+++ clang/include/clang/Basic/Module.h
@@ -168,6 +168,8 @@
   /// some C++ module.
   bool isGlobalModule() const { return Kind == GlobalModuleFragment; }
 
+  bool isPrivateModule() const { return Kind == PrivateModuleFragment; }
+
 private:
   /// The submodules of this module, indexed by name.
   std::vector<Module *> SubModules;
@@ -536,10 +538,23 @@
 
   /// Get the primary module interface name from a partition.
   StringRef getPrimaryModuleInterfaceName() const {
+    // Technically, global module fragment belongs to global module. And global
+    // module has no name: [module.unit]p6:
+    //   The global module has no name, no module interface unit, and is not
+    //   introduced by any module-declaration.
+    //
+    // <global> is the default name showed in module map.
+    if (isGlobalModule())
+      return "<global>";
+
     if (isModulePartition()) {
       auto pos = Name.find(':');
       return StringRef(Name.data(), pos);
     }
+
+    if (isPrivateModule())
+      return getTopLevelModuleName();
+
     return Name;
   }
 
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -2256,7 +2256,7 @@
   /// Namespace definitions that we will export when they finish.
   llvm::SmallPtrSet<const NamespaceDecl*, 8> DeferredExportedNamespaces;
 
-  /// Get the module whose scope we are currently within.
+  /// Get the module unit whose scope we are currently within.
   Module *getCurrentModule() const {
     return ModuleScopes.empty() ? nullptr : ModuleScopes.back().Module;
   }
@@ -2274,6 +2274,11 @@
 
   VisibleModuleSet VisibleModules;
 
+  /// Cache for module units which is usable for current module.
+  llvm::DenseSet<const Module *> UsableModuleUnitsCache;
+
+  bool isUsableModule(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 +1556,39 @@
   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 if we could use all the declarations in the module.
+bool Sema::isUsableModule(const Module *M) {
+  assert(M && "We shouldn't check nullness for module here");
+  // Return quickly if we cached the result.
+  if (UsableModuleUnitsCache.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;
+  // parsing, then M should be the global module fragment in current TU. So it
+  // should be usable.
+  // [module.global.frag]p1:
+  //   The global module fragment can be used to provide declarations that are
+  //   attached to the global module and usable within the module unit.
+  if ((M->isGlobalModule() && !M->Parent) ||
+      // If M is the private module fragment, it is usable only if it is within
+      // the current module unit. And it must be the current parsing module unit
+      // if it is within the current module unit according to the grammar of the
+      // private module fragment.
+      // NOTE: This is covered by the following condition. The intention of the
+      // check is to avoid string comparison as much as possible.
+      (M->isPrivateModule() && M == getCurrentModule()) ||
+      // The module unit which is in the same module with the current module
+      // unit is usable.
+      //
+      // FIXME: Here we judge if they are in the same module by comparing the
+      // string. Is there any better solution?
+      (M->getPrimaryModuleInterfaceName() ==
+       llvm::StringRef(getLangOpts().CurrentModule).split(':').first)) {
+    UsableModuleUnitsCache.insert(M);
+    return true;
+  }
+
+  return false;
 }
 
 bool Sema::hasVisibleMergedDefinition(NamedDecl *Def) {
@@ -1578,7 +1600,7 @@
 
 bool Sema::hasMergedDefinitionInCurrentModule(NamedDecl *Def) {
   for (const Module *Merged : Context.getModulesWithMergedDefinition(Def))
-    if (isInCurrentModule(Merged, getLangOpts()))
+    if (isUsableModule(Merged))
       return true;
   return false;
 }
@@ -1760,7 +1782,7 @@
   // 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 (isUsableModule(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 +1,73 @@
+// 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
+// RUN: %clang_cc1 -std=c++20 %t/Private.cppm -emit-module-interface -fprebuilt-module-path=%t -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 %t/TryUseFromPrivate.cpp -fprebuilt-module-path=%t -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 %t/Global.cppm -emit-module-interface -fprebuilt-module-path=%t -o %t/C.pcm
+// RUN: %clang_cc1 -std=c++20 %t/UseGlobal.cpp -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'}}
+}
+
+//--- Private.cppm
+export module A;
+module : private;
+
+class A {};
+void test() {
+  A a;
+}
+
+//--- TryUseFromPrivate.cpp
+
+import A;
+void test() {
+  A a; // expected-error {{unknown type name 'A'}}
+}
+
+//--- Global.cppm
+module;
+class A{};
+export module C;
+void test() {
+  A a;
+}
+
+//--- UseGlobal.cpp
+import C;
+void test() {
+  A a; // expected-error {{'A' must be declared before it is used}}
+       // expected-note@Global.cppm:2 {{declaration here is not visible}}
+}
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;