diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -18,6 +18,7 @@
 #include "clang/AST/DeclarationName.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/LLVM.h"
+#include "clang/Basic/Module.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/Specifiers.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -231,8 +232,15 @@
     /// module is imported.
     VisibleWhenImported,
 
+    /// This declaration has an owninig module, and is visible to lookups
+    /// that occurs within that module. And it is reachable to other module
+    /// when the owninig module is imported.
+    ReachableWhenImported,
+
     /// This declaration has an owning module, but is only visible to
     /// lookups that occur within that module.
+    /// The discarded declarations in global module fragment belongs
+    /// to this group too.
     ModulePrivate
   };
 
@@ -241,8 +249,8 @@
   /// DeclContext. These pointers form the linked list that is
   /// traversed via DeclContext's decls_begin()/decls_end().
   ///
-  /// The extra two bits are used for the ModuleOwnershipKind.
-  llvm::PointerIntPair<Decl *, 2, ModuleOwnershipKind> NextInContextAndBits;
+  /// The extra three bits are used for the ModuleOwnershipKind.
+  llvm::PointerIntPair<Decl *, 3, ModuleOwnershipKind> NextInContextAndBits;
 
 private:
   friend class DeclContext;
@@ -613,6 +621,19 @@
     return getModuleOwnershipKind() == ModuleOwnershipKind::ModulePrivate;
   }
 
+  bool isInvisibleOutsideTheOwningModule() const {
+    return getModuleOwnershipKind() > ModuleOwnershipKind::VisibleWhenImported;
+  }
+
+  bool isInGlobalModuleFragment() const {
+    Module *M = getOwningModule();
+    return M ? M->isGlobalModule() : false;
+  }
+
+  /// FIXME: Implement discarding declarations actually in global module
+  /// fragment. See [module.global.frag]p3,4 for details.
+  bool isDiscardedInGlobalModuleFragment() const { return false; }
+
   /// Return true if this declaration has an attribute which acts as
   /// definition of the entity, such as 'alias' or 'ifunc'.
   bool hasDefiningAttr() const;
diff --git a/clang/include/clang/Basic/Module.h b/clang/include/clang/Basic/Module.h
--- a/clang/include/clang/Basic/Module.h
+++ b/clang/include/clang/Basic/Module.h
@@ -153,6 +153,10 @@
     return Kind == ModuleInterfaceUnit || Kind == PrivateModuleFragment;
   }
 
+  /// Does this Module scope describe a fragment of the global module within
+  /// some C++ module.
+  bool isGlobalModule() const { return Kind == GlobalModuleFragment; }
+
 private:
   /// The submodules of this module, indexed by name.
   std::vector<Module *> SubModules;
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -2258,6 +2258,13 @@
     return hasVisibleDefinition(const_cast<NamedDecl*>(D), &Hidden);
   }
 
+  bool hasReachableDefinition(NamedDecl *D, NamedDecl **Suggested,
+                              bool OnlyNeedComplete = false);
+  bool hasReachableDefinition(NamedDecl *D) {
+    NamedDecl *Hidden;
+    return hasVisibleDefinition(D, &Hidden);
+  }
+
   /// Determine if the template parameter \p D has a visible default argument.
   bool
   hasVisibleDefaultArgument(const NamedDecl *D,
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -579,6 +579,7 @@
   // FIXME: Handle isModulePrivate.
   switch (D->getModuleOwnershipKind()) {
   case Decl::ModuleOwnershipKind::Unowned:
+  case Decl::ModuleOwnershipKind::ReachableWhenImported:
   case Decl::ModuleOwnershipKind::ModulePrivate:
     return false;
   case Decl::ModuleOwnershipKind::Visible:
diff --git a/clang/lib/Sema/SemaCXXScopeSpec.cpp b/clang/lib/Sema/SemaCXXScopeSpec.cpp
--- a/clang/lib/Sema/SemaCXXScopeSpec.cpp
+++ b/clang/lib/Sema/SemaCXXScopeSpec.cpp
@@ -121,7 +121,7 @@
             // entering the context, and that can't happen in a SFINAE context.
             assert(!isSFINAEContext() &&
                    "partial specialization scope specifier in SFINAE context?");
-            if (!hasVisibleDeclaration(PartialSpec))
+            if (!hasReachableDefinition(PartialSpec))
               diagnoseMissingImport(SS.getLastQualifierNameLoc(), PartialSpec,
                                     MissingImportKind::PartialSpecialization,
                                     /*Recover*/true);
@@ -243,8 +243,8 @@
   if (EnumD->isCompleteDefinition()) {
     // If we know about the definition but it is not visible, complain.
     NamedDecl *SuggestedDef = nullptr;
-    if (!hasVisibleDefinition(EnumD, &SuggestedDef,
-                              /*OnlyNeedComplete*/false)) {
+    if (!hasReachableDefinition(EnumD, &SuggestedDef,
+                                /*OnlyNeedComplete*/ false)) {
       // If the user is going to see an error here, recover by making the
       // definition visible.
       bool TreatAsComplete = !isSFINAEContext();
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -1684,7 +1684,8 @@
   assert(DeclModule && "hidden decl has no owning module");
 
   // If the owning module is visible, the decl is visible.
-  if (SemaRef.isModuleVisible(DeclModule, D->isModulePrivate()))
+  if (SemaRef.isModuleVisible(DeclModule,
+                              D->isInvisibleOutsideTheOwningModule()))
     return true;
 
   // Determine whether a decl context is a file context for the purpose of
@@ -1752,6 +1753,18 @@
 }
 
 bool Sema::isModuleVisible(const Module *M, bool ModulePrivate) {
+  // [module.global.frag]p2:
+  // A global-module-fragment specifies the contents of the global module
+  // fragment for a module unit. The global module fragment can be used to
+  // provide declarations that are attached to the global module and usable
+  // within the module unit.
+  //
+  // Global module fragment is special. Global Module fragment is only usable
+  // within the module unit it got defined [module.global.frag]p2. In this
+  // case, the global module fragment shouldn't own an AST File.
+  if (M->isGlobalModule() && M->getASTFile())
+    return false;
+
   // The module might be ordinarily visible. For a module-private query, that
   // means it is part of the current module. For any other query, that means it
   // is in our visible module set.
diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -80,7 +80,15 @@
 
   // All declarations created from now on are owned by the global module.
   auto *TU = Context.getTranslationUnitDecl();
-  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::Visible);
+  // [module.global.frag]p2
+  // A global-module-fragment specifies the contents of the global module
+  // fragment for a module unit. The global module fragment can be used to
+  // provide declarations that are attached to the global module and usable
+  // within the module unit.
+  //
+  // So that the global module fragment could only be used in current module
+  // unit.
+  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::ModulePrivate);
   TU->setLocalOwningModule(GlobalModule);
 
   // FIXME: Consider creating an explicit representation of this declaration.
@@ -232,10 +240,16 @@
   VisibleModules.setVisible(Mod, ModuleLoc);
 
   // From now on, we have an owning module for all declarations we see.
-  // However, those declarations are module-private unless explicitly
+  // In C++20 modules, those declaration would be reachable when imported
+  // unless explicitily exported.
+  // Otherwise, those declarations are module-private unless explicitly
   // exported.
   auto *TU = Context.getTranslationUnitDecl();
-  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::ModulePrivate);
+  if (getLangOpts().CPlusPlusModules)
+    TU->setModuleOwnershipKind(
+        Decl::ModuleOwnershipKind::ReachableWhenImported);
+  else
+    TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::ModulePrivate);
   TU->setLocalOwningModule(Mod);
 
   // FIXME: Create a ModuleDecl.
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -795,8 +795,9 @@
 
   if (PatternDef && !IsEntityBeingDefined) {
     NamedDecl *SuggestedDef = nullptr;
-    if (!hasVisibleDefinition(const_cast<NamedDecl*>(PatternDef), &SuggestedDef,
-                              /*OnlyNeedComplete*/false)) {
+    if (!hasReachableDefinition(const_cast<NamedDecl *>(PatternDef),
+                                &SuggestedDef,
+                                /*OnlyNeedComplete*/ false)) {
       // If we're allowed to diagnose this and recover, do so.
       bool Recover = Complain && !isSFINAEContext();
       if (Complain)
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -8571,6 +8571,56 @@
   return false;
 }
 
+// [module.reach]p3.1:
+// A declaration D is reachable from a point P if
+// - D appears prior to P in the same translation unit, or
+// - D is not discarded ([module.global.frag]), appears in a translation unit
+// that is reachable from P, and does not appear within a
+// private-module-fragment. A declaration is reachable if it is reachable from
+// any point in the instantiation context ([module.context]).
+//
+// [module.reach]p2:
+// All translation units that are necessarily reachable are reachable.
+// Additional translation units on which the point within the program has an
+// interface dependency may be considered reachable, but it is unspecified which
+// are and under what circumstances.
+bool Sema::hasReachableDefinition(NamedDecl *D, NamedDecl **Suggested,
+                                  bool OnlyNeedComplete) {
+  bool IsVisible = hasVisibleDefinition(D, Suggested, OnlyNeedComplete);
+
+  // If it didn't introduce C++ Modules, it is meaningless to talk about
+  // reachable definition.
+  if (!getLangOpts().CPlusPlusModules)
+    return IsVisible;
+
+  // Any visible declaration is reachable.
+  if (IsVisible)
+    return true;
+
+  // If D owns a module and the module is not visible. It implies that the
+  // module is not imported. So we should return false directly. This is
+  // possible in case of `#pragma clang module`. See
+  // clang/test/SemaCXX/compare-modules-cxx2a.cpp for example.
+  if (D->getOwningModule() && !VisibleModules.isVisible(D->getOwningModule()))
+    return false;
+
+  // If D isn't from AST file, it implies that D appears in the same TU.
+  // So it should be reachable.
+  if (!D->isFromASTFile())
+    return true;
+
+  if (D->isModulePrivate())
+    return false;
+
+  if (!D->isInGlobalModuleFragment())
+    return true;
+
+  if (D->isDiscardedInGlobalModuleFragment())
+    return false;
+
+  return true;
+}
+
 /// Locks in the inheritance model for the given class and all of its bases.
 static void assignInheritanceModel(Sema &S, CXXRecordDecl *RD) {
   RD = RD->getMostRecentNonInjectedDecl();
@@ -8644,16 +8694,15 @@
 
   // If we have a complete type, we're done.
   if (!Incomplete) {
-    // If we know about the definition but it is not visible, complain.
-    NamedDecl *SuggestedDef = nullptr;
+    NamedDecl *Suggested = nullptr;
     if (Def &&
-        !hasVisibleDefinition(Def, &SuggestedDef, /*OnlyNeedComplete*/true)) {
+        !hasReachableDefinition(Def, &Suggested, /*OnlyNeedComplete=*/true)) {
       // If the user is going to see an error here, recover by making the
       // definition visible.
       bool TreatAsComplete = Diagnoser && !isSFINAEContext();
-      if (Diagnoser && SuggestedDef)
-        diagnoseMissingImport(Loc, SuggestedDef, MissingImportKind::Definition,
-                              /*Recover*/TreatAsComplete);
+      if (Diagnoser && Suggested)
+        diagnoseMissingImport(Loc, Suggested, MissingImportKind::Definition,
+                              /*Recover*/ TreatAsComplete);
       return !TreatAsComplete;
     } else if (Def && !TemplateInstCallbacks.empty()) {
       CodeSynthesisContext TempInst;
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -603,15 +603,22 @@
   D->setTopLevelDeclInObjCContainer(Record.readInt());
   D->setAccess((AccessSpecifier)Record.readInt());
   D->FromASTFile = true;
-  bool ModulePrivate = Record.readInt();
+  auto ModuleOwnership = (Decl::ModuleOwnershipKind)Record.readInt();
+  bool ModulePrivate =
+      (ModuleOwnership == Decl::ModuleOwnershipKind::ModulePrivate);
 
   // Determine whether this declaration is part of a (sub)module. If so, it
   // may not yet be visible.
   if (unsigned SubmoduleID = readSubmoduleID()) {
+    if (ModuleOwnership == Decl::ModuleOwnershipKind::Visible)
+      ModuleOwnership = Decl::ModuleOwnershipKind::VisibleWhenImported;
+
+    if ((int)ModuleOwnership > 4)
+      llvm::report_fatal_error(
+          "The size of sizeModuleOwnership is larger than 4.\n");
+
+    D->setModuleOwnershipKind(ModuleOwnership);
     // Store the owning submodule ID in the declaration.
-    D->setModuleOwnershipKind(
-        ModulePrivate ? Decl::ModuleOwnershipKind::ModulePrivate
-                      : Decl::ModuleOwnershipKind::VisibleWhenImported);
     D->setOwningModuleID(SubmoduleID);
 
     if (ModulePrivate) {
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -310,7 +310,7 @@
   Record.push_back(D->isReferenced());
   Record.push_back(D->isTopLevelDeclInObjCContainer());
   Record.push_back(D->getAccess());
-  Record.push_back(D->isModulePrivate());
+  Record.push_back((uint64_t)D->getModuleOwnershipKind());
   Record.push_back(Writer.getSubmoduleID(D->getOwningModule()));
 
   // If this declaration injected a name into a context different from its
@@ -1921,7 +1921,7 @@
   Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 2));  // AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3));  // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -1954,7 +1954,7 @@
   Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 2));  // AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3));  // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -1992,7 +1992,7 @@
   Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(AS_none));                 // C++ AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -2042,7 +2042,7 @@
   Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(AS_none));                 // C++ AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -2104,7 +2104,7 @@
   Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(AS_none));                 // C++ AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -2152,7 +2152,7 @@
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 2)); // C++ AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -2181,7 +2181,7 @@
   Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(0));                   // TopLevelDeclInObjCContainer
   Abv->Add(BitCodeAbbrevOp(AS_none));                 // C++ AccessSpecifier
-  Abv->Add(BitCodeAbbrevOp(0));                       // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6)); // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(0));                       // NameKind = Identifier
@@ -2233,7 +2233,7 @@
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // Referenced
   Abv->Add(BitCodeAbbrevOp(0));                         // InObjCContainer
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 2)); // Access
-  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // ModulePrivate
+  Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 3)); // ModulePrivate
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // SubmoduleID
   // NamedDecl
   Abv->Add(BitCodeAbbrevOp(DeclarationName::Identifier)); // NameKind
diff --git a/clang/test/CXX/basic/basic.scope/basic.scope.namespace/p2.cpp b/clang/test/CXX/basic/basic.scope/basic.scope.namespace/p2.cpp
--- a/clang/test/CXX/basic/basic.scope/basic.scope.namespace/p2.cpp
+++ b/clang/test/CXX/basic/basic.scope/basic.scope.namespace/p2.cpp
@@ -29,18 +29,18 @@
 #endif
 
 void test_early() {
-  in_header = 1; // expected-error {{missing '#include "foo.h"'; 'in_header' must be declared before it is used}}
+  in_header = 1; // expected-error {{use of undeclared identifier 'in_header'}}
   // expected-note@*{{not visible}}
 
-  global_module_fragment = 1; // expected-error {{missing '#include'; 'global_module_fragment' must be declared before it is used}}
-  // expected-note@p2.cpp:16 {{not visible}}
+  global_module_fragment = 1; // expected-error {{use of undeclared identifier 'global_module_fragment'}}
 
   exported = 1; // expected-error {{must be imported from module 'A'}}
-  // expected-note@p2.cpp:18 {{not visible}}
 
-  not_exported = 1; // expected-error {{undeclared identifier}}
+  not_exported = 1; // expected-error {{declaration of 'not_exported' must be imported from module 'A' before it is required}}
 
-  internal = 1; // expected-error {{undeclared identifier}}
+  // FIXME: We need better diagnostic message for static variable.
+  internal = 1; // expected-error {{declaration of 'internal' must be imported from module 'A' before it is required}}
+                // expected-note@p2.cpp:20 {{declaration here is not visible}}
 
   not_exported_private = 1; // expected-error {{undeclared identifier}}
 
@@ -54,24 +54,23 @@
 #endif
 
 void test_late() {
-  in_header = 1; // expected-error {{missing '#include "foo.h"'; 'in_header' must be declared before it is used}}
+  in_header = 1; // expected-error {{use of undeclared identifier 'in_header'}}
   // expected-note@*{{not visible}}
 
-  global_module_fragment = 1; // expected-error {{missing '#include'; 'global_module_fragment' must be declared before it is used}}
-  // expected-note@p2.cpp:16 {{not visible}}
+  global_module_fragment = 1; // expected-error {{use of undeclared identifier 'global_module_fragment'}}
 
   exported = 1;
 
   not_exported = 1;
 #ifndef IMPLEMENTATION
-  // expected-error@-2 {{undeclared identifier 'not_exported'; did you mean 'exported'}}
-  // expected-note@p2.cpp:18 {{declared here}}
+  // expected-error@-2 {{declaration of 'not_exported' must be imported from module 'A' before it is required}}
+  // expected-note@p2.cpp:19 {{declaration here is not visible}}
 #endif
 
   internal = 1;
 #ifndef IMPLEMENTATION
-  // FIXME: should not be visible here
-  // expected-error@-3 {{undeclared identifier}}
+  // expected-error@-2 {{declaration of 'internal' must be imported from module 'A' before it is required}}
+  // expected-note@p2.cpp:20 {{declaration here is not visible}}
 #endif
 
   not_exported_private = 1;
diff --git a/clang/test/CXX/module/module.interface/p2.cpp b/clang/test/CXX/module/module.interface/p2.cpp
--- a/clang/test/CXX/module/module.interface/p2.cpp
+++ b/clang/test/CXX/module/module.interface/p2.cpp
@@ -69,22 +69,29 @@
 
 void use() {
   // namespace A is implicitly exported by the export of A::g.
-  A::f(); // expected-error {{no member named 'f' in namespace 'A'}}
+  A::f(); // expected-error {{declaration of 'f' must be imported from module 'p2' before it is required}}
+          // expected-note@* {{declaration here is not visible}}
   A::g();
-  A::h(); // expected-error {{no member named 'h' in namespace 'A'}}
-  using namespace A::inner; // expected-error {{expected namespace name}}
+  A::h();                   // expected-error {{declaration of 'h' must be imported from module 'p2' before it is required}}
+                            // expected-note@* {{declaration here is not visible}}
+  using namespace A::inner; // expected-error {{declaration of 'inner' must be imported from module 'p2' before it is required}}
+                            // expected-note@* {{declaration here is not visible}}
 
   // namespace B and B::inner are explicitly exported
   using namespace B;
   using namespace B::inner;
-  B::f(); // expected-error {{no member named 'f' in namespace 'B'}}
-  f(); // expected-error {{undeclared identifier 'f'}}
+  B::f(); // expected-error {{declaration of 'f' must be imported from module 'p2' before it is required}}
+          // expected-note@* {{declaration here is not visible}}
+  f();    // expected-error {{declaration of 'f' must be imported from module 'p2' before it is required}}
+          // expected-note@* {{declaration here is not visible}}
 
   // namespace C is not exported
-  using namespace C; // expected-error {{expected namespace name}}
+  using namespace C; // expected-error {{declaration of 'C' must be imported from module 'p2' before it is required}}
+                     // expected-note@* {{declaration here is not visible}}
 
   // namespace D is exported, but D::f is not
-  D::f(); // expected-error {{no member named 'f' in namespace 'D'}}
+  D::f(); // expected-error {{declaration of 'f' must be imported from module 'p2' before it is required}}
+          // expected-note@* {{declaration here is not visible}}
 }
 
 int use_header() { return foo + bar::baz(); }
diff --git a/clang/test/CXX/module/module.reach/Inputs/p5-A.cppm b/clang/test/CXX/module/module.reach/Inputs/p5-A.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/Inputs/p5-A.cppm
@@ -0,0 +1,3 @@
+export module A;
+struct X {};
+export using Y = X;
diff --git a/clang/test/CXX/module/module.reach/p5.cpp b/clang/test/CXX/module/module.reach/p5.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/p5.cpp
@@ -0,0 +1,10 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/p5-A.cppm --precompile -o %t/A.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c -Xclang -verify
+
+export module B;
+import A;
+Y y;                // OK, definition of X is reachable
+X x;                // expected-error {{declaration of 'X' must be imported from module 'A' before it is required}}
+                    // expected-note@Inputs/p5-A.cppm:2 {{declaration here is not visible}}
diff --git a/clang/test/Modules/Inputs/Reachability-Private/Private.cppm b/clang/test/Modules/Inputs/Reachability-Private/Private.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/Reachability-Private/Private.cppm
@@ -0,0 +1,18 @@
+export module Private;
+inline void fn_m();             // OK, module-linkage inline function
+static void fn_s();
+export struct X;
+
+export void g(X *x) {
+  fn_s();                       // OK, call to static function in same translation unit
+  fn_m();                       // OK, call to module-linkage inline function
+}
+export X *factory();            // OK
+
+module :private;
+struct X {};                    // definition not reachable from importers of A
+X *factory() {
+  return new X ();
+}
+void fn_m() {}
+void fn_s() {}
diff --git a/clang/test/Modules/Inputs/Reachability-func-default-arg/func_default_arg.cppm b/clang/test/Modules/Inputs/Reachability-func-default-arg/func_default_arg.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/Reachability-func-default-arg/func_default_arg.cppm
@@ -0,0 +1,5 @@
+export module func_default_arg;
+struct t { };
+export t foo(t t1= t()) {
+    return t1;
+}
diff --git a/clang/test/Modules/Inputs/Reachability-func-ret/func_ret.cppm b/clang/test/Modules/Inputs/Reachability-func-ret/func_ret.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/Reachability-func-ret/func_ret.cppm
@@ -0,0 +1,5 @@
+export module func_ret;
+struct t { };
+export t foo() {
+    return t{};
+}
diff --git a/clang/test/Modules/Inputs/Reachability-template-default-arg/template_default_arg.cppm b/clang/test/Modules/Inputs/Reachability-template-default-arg/template_default_arg.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/Reachability-template-default-arg/template_default_arg.cppm
@@ -0,0 +1,8 @@
+export module template_default_arg;
+struct t { };
+
+export
+template <typename T = t>
+struct A {
+    T a;
+};
diff --git a/clang/test/Modules/Inputs/Reachability-using-templates/mod-templates.cppm b/clang/test/Modules/Inputs/Reachability-using-templates/mod-templates.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/Reachability-using-templates/mod-templates.cppm
@@ -0,0 +1,3 @@
+export module mod.templates;
+template<class> struct t { };
+export template<class T> using u = t<T>;
diff --git a/clang/test/Modules/Inputs/Reachability-using/mod.cppm b/clang/test/Modules/Inputs/Reachability-using/mod.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/Reachability-using/mod.cppm
@@ -0,0 +1,3 @@
+export module mod;
+struct t { };
+export using u = t;
diff --git a/clang/test/Modules/Reachability-Private.cpp b/clang/test/Modules/Reachability-Private.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Reachability-Private.cpp
@@ -0,0 +1,19 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/Reachability-Private/Private.cppm --precompile -o %t/Private.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c -Xclang -verify
+
+import Private;
+void foo() {
+    X x; // expected-error {{definition of 'X' must be imported from module 'Private.<private>' before it is required}}
+         // expected-error@-1 {{definition of 'X' must be imported from module 'Private.<private>' before it is required}}
+         // expected-note@Inputs/Reachability-Private/Private.cppm:13 {{definition here is not reachable}}
+         // expected-note@Inputs/Reachability-Private/Private.cppm:13 {{definition here is not reachable}}
+    auto _ = factory();
+    auto *__ = factory();
+    X* ___ = factory();
+
+    g(__);
+    g(___);
+    g(factory());
+}
diff --git a/clang/test/Modules/Reachability-func-default-arg.cpp b/clang/test/Modules/Reachability-func-default-arg.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Reachability-func-default-arg.cpp
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/Reachability-func-default-arg/func_default_arg.cppm --precompile -o %t/func_default_arg.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c
+// expected-no-diagnostics
+import func_default_arg;
+void bar() {
+  auto ret = foo();
+}
diff --git a/clang/test/Modules/Reachability-func-ret.cpp b/clang/test/Modules/Reachability-func-ret.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Reachability-func-ret.cpp
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/Reachability-func-ret/func_ret.cppm --precompile -o %t/func_ret.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c
+// expected-no-diagnostics
+import func_ret;
+void bar() {
+  auto ret = foo();
+}
diff --git a/clang/test/Modules/Reachability-template-default-arg.cpp b/clang/test/Modules/Reachability-template-default-arg.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Reachability-template-default-arg.cpp
@@ -0,0 +1,11 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/Reachability-template-default-arg/template_default_arg.cppm --precompile -o %t/template_default_arg.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c -Xclang -verify
+
+import template_default_arg;
+void bar() {
+  A<> a0;
+  A<t> a1; // expected-error {{declaration of 't' must be imported from module 'template_default_arg' before it is required}}
+           // expected-note@Inputs/Reachability-template-default-arg/template_default_arg.cppm:2 {{declaration here is not visible}}
+}
diff --git a/clang/test/Modules/Reachability-using-templates.cpp b/clang/test/Modules/Reachability-using-templates.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Reachability-using-templates.cpp
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/Reachability-using-templates/mod-templates.cppm --precompile -o %t/mod.templates.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c
+// expected-no-diagnostics
+import mod.templates;
+void foo() {
+  u<int> v{};
+}
diff --git a/clang/test/Modules/Reachability-using.cpp b/clang/test/Modules/Reachability-using.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Reachability-using.cpp
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 %S/Inputs/Reachability-using/mod.cppm --precompile -o %t/mod.pcm
+// RUN: %clang -std=c++20 -fprebuilt-module-path=%t %s -c
+// expected-no-diagnostics
+import mod;
+void foo() {
+  u v{};
+}
diff --git a/clang/test/SemaCXX/compare-modules-cxx2a.cpp b/clang/test/SemaCXX/compare-modules-cxx2a.cpp
--- a/clang/test/SemaCXX/compare-modules-cxx2a.cpp
+++ b/clang/test/SemaCXX/compare-modules-cxx2a.cpp
@@ -1,3 +1,4 @@
+// This tests the unused declaration in global module fragment should be discarded.
 // RUN: %clang_cc1 -triple x86_64-apple-darwin -fcxx-exceptions -verify -std=c++2a -fmodules -I%S/Inputs %s -fno-modules-error-recovery
 
 #pragma clang module build compare