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/lib/Sema/SemaLookup.cpp
===================================================================
--- clang/lib/Sema/SemaLookup.cpp
+++ clang/lib/Sema/SemaLookup.cpp
@@ -1561,12 +1561,11 @@
 static bool isInCurrentModule(const Module *M, const LangOptions &LangOpts) {
   // 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)
+    return true;
+
+  return M->getPrimaryModuleInterfaceName() ==
+         Module::getPrimaryModuleInterfaceName(LangOpts.CurrentModule);
 }
 
 bool Sema::hasVisibleMergedDefinition(NamedDecl *Def) {
Index: clang/test/CXX/module/module.import/p2.cpp
===================================================================
--- /dev/null
+++ clang/test/CXX/module/module.import/p2.cpp
@@ -0,0 +1,21 @@
+// 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
+
+//--- 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'}}
+}
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;