diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -1524,8 +1524,6 @@
 def err_private_module_fragment_expected_semi : Error<
   "expected ';' after private module fragment declaration">;
 def err_missing_before_module_end : Error<"expected %0 at end of module">;
-def err_unsupported_module_partition : Error<
-  "sorry, module partitions are not yet supported">;
 
 def err_export_empty : Error<"export declaration cannot be empty">;
 }
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10933,6 +10933,11 @@
   "import of module '%0' appears within same top-level module '%1'">;
 def err_module_import_in_implementation : Error<
   "@import of module '%0' in implementation of '%1'; use #import">;
+def err_module_import_in_non_module : Error <
+   "import module partition isn't allowed in non module.">;
+def err_private_module_fragment_in_module_partition : Error <
+   "private module fragment is not allowed to appear in a module partition "
+   "since a module unit with a private-module-fragment shall be the only module unit of its module.">;
 
 // C++ Modules
 def err_module_decl_not_at_start : Error<
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
@@ -530,6 +530,18 @@
     return getTopLevelModule()->Name;
   }
 
+  /// Retrieve the name which contains a C++20 module partitions.
+  StringRef getTopLevelPrimaryModuleName() const {
+    if (Kind != ModuleInterfaceUnit)
+      return getTopLevelModuleName();
+    return getTopLevelModuleName().split('-').first;
+  }
+  StringRef getPrimaryModuleName() const {
+    if (Kind != ModuleInterfaceUnit)
+      return Name;
+    return StringRef(Name).split('-').first;
+  }
+
   /// The serialized AST file for this module, if one was created.
   OptionalFileEntryRefDegradesToFileEntryPtr getASTFile() const {
     return getTopLevelModule()->ASTFile;
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
@@ -2215,6 +2215,8 @@
   Module *getOwningModule(const Decl *Entity) {
     return Entity->getOwningModule();
   }
+  StringRef getCurrentModuleName() const;
+  bool IsCurrentModulePartition() const;
 
   /// Make a merged definition of an existing hidden definition \p ND
   /// visible at the specified location.
@@ -2915,13 +2917,16 @@
   enum class ModuleDeclKind {
     Interface,      ///< 'export module X;'
     Implementation, ///< 'module X;'
+    Partition,      ///< 'export module X:Y;'
+                    ///< 'module X:Y;'
   };
 
   /// The parser has processed a module-declaration that begins the definition
   /// of a module interface or implementation.
   DeclGroupPtrTy ActOnModuleDecl(SourceLocation StartLoc,
                                  SourceLocation ModuleLoc, ModuleDeclKind MDK,
-                                 ModuleIdPath Path, bool IsFirstDecl);
+                                 ModuleIdPath Path, ModuleIdPath PartitionPath,
+                                 bool IsFirstDecl);
 
   /// The parser has processed a global-module-fragment declaration that begins
   /// the definition of the global module fragment of the current module unit.
@@ -2944,11 +2949,13 @@
   /// \param Path The module access path.
   DeclResult ActOnModuleImport(SourceLocation StartLoc,
                                SourceLocation ExportLoc,
-                               SourceLocation ImportLoc, ModuleIdPath Path);
+                               SourceLocation ImportLoc, ModuleIdPath Path,
+                               bool IsPartition = false);
   DeclResult ActOnModuleImport(SourceLocation StartLoc,
                                SourceLocation ExportLoc,
                                SourceLocation ImportLoc, Module *M,
-                               ModuleIdPath Path = {});
+                               ModuleIdPath Path = {},
+                               bool IsPartition = false);
 
   /// The parser has processed a module import translated from a
   /// #include or similar preprocessing directive.
diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -2341,17 +2341,14 @@
   if (ParseModuleName(ModuleLoc, Path, /*IsImport*/false))
     return nullptr;
 
+  SmallVector<std::pair<IdentifierInfo *, SourceLocation>, 2> Partition;
   // Parse the optional module-partition.
   if (Tok.is(tok::colon)) {
-    SourceLocation ColonLoc = ConsumeToken();
-    SmallVector<std::pair<IdentifierInfo *, SourceLocation>, 2> Partition;
+    ConsumeToken();
     if (ParseModuleName(ModuleLoc, Partition, /*IsImport*/false))
       return nullptr;
 
-    // FIXME: Support module partition declarations.
-    Diag(ColonLoc, diag::err_unsupported_module_partition)
-      << SourceRange(ColonLoc, Partition.back().second);
-    // Recover by parsing as a non-partition.
+    MDK = Sema::ModuleDeclKind::Partition;
   }
 
   // We don't support any module attributes yet; just parse them and diagnose.
@@ -2361,7 +2358,8 @@
 
   ExpectAndConsumeSemi(diag::err_module_expected_semi);
 
-  return Actions.ActOnModuleDecl(StartLoc, ModuleLoc, MDK, Path, IsFirstDecl);
+  return Actions.ActOnModuleDecl(StartLoc, ModuleLoc, MDK, Path, Partition,
+                                 IsFirstDecl);
 }
 
 /// Parse a module import declaration. This is essentially the same for
@@ -2393,6 +2391,7 @@
 
   SmallVector<std::pair<IdentifierInfo *, SourceLocation>, 2> Path;
   Module *HeaderUnit = nullptr;
+  bool IsPartition = false;
 
   if (Tok.is(tok::header_name)) {
     // This is a header import that the preprocessor decided we should skip
@@ -2404,14 +2403,11 @@
     HeaderUnit = reinterpret_cast<Module *>(Tok.getAnnotationValue());
     ConsumeAnnotationToken();
   } else if (getLangOpts().CPlusPlusModules && Tok.is(tok::colon)) {
-    SourceLocation ColonLoc = ConsumeToken();
+    ConsumeToken();
     if (ParseModuleName(ImportLoc, Path, /*IsImport*/true))
       return nullptr;
 
-    // FIXME: Support module partition import.
-    Diag(ColonLoc, diag::err_unsupported_module_partition)
-      << SourceRange(ColonLoc, Path.back().second);
-    return nullptr;
+    IsPartition = true;
   } else {
     if (ParseModuleName(ImportLoc, Path, /*IsImport*/true))
       return nullptr;
@@ -2433,7 +2429,8 @@
     Import =
         Actions.ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, HeaderUnit);
   else if (!Path.empty())
-    Import = Actions.ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, Path);
+    Import = Actions.ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, Path,
+                                       IsPartition);
   ExpectAndConsumeSemi(diag::err_module_expected_semi);
   if (Import.isInvalid())
     return nullptr;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1599,6 +1599,12 @@
   if (NewM == OldM)
     return false;
 
+  // If NewM and OldM are two partitions of the same module so the redeclaration
+  // should be compatible.
+  if (NewM && OldM &&
+      NewM->getPrimaryModuleName() == OldM->getPrimaryModuleName())
+    return false;
+
   bool NewIsModuleInterface = NewM && NewM->isModulePurview();
   bool OldIsModuleInterface = OldM && OldM->isModulePurview();
   if (NewIsModuleInterface || OldIsModuleInterface) {
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
@@ -1558,10 +1558,10 @@
 
 /// 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) {
+static bool isInCurrentModule(const Module *M, const Sema &SemaRef) {
   // 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.
-  return M->getTopLevelModuleName() == LangOpts.CurrentModule ||
+  return M->getTopLevelPrimaryModuleName() == SemaRef.getCurrentModuleName() ||
          (M->Kind == Module::GlobalModuleFragment && !M->Parent);
 }
 
@@ -1574,7 +1574,7 @@
 
 bool Sema::hasMergedDefinitionInCurrentModule(NamedDecl *Def) {
   for (const Module *Merged : Context.getModulesWithMergedDefinition(Def))
-    if (isInCurrentModule(Merged, getLangOpts()))
+    if (isInCurrentModule(Merged, *this))
       return true;
   return false;
 }
@@ -1756,7 +1756,7 @@
   // 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, *this))
       return true;
   } else {
     if (VisibleModules.isVisible(M))
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
@@ -89,7 +89,8 @@
 
 Sema::DeclGroupPtrTy
 Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
-                      ModuleDeclKind MDK, ModuleIdPath Path, bool IsFirstDecl) {
+                      ModuleDeclKind MDK, ModuleIdPath Path,
+                      ModuleIdPath PartitionPath, bool IsFirstDecl) {
   assert((getLangOpts().ModulesTS || getLangOpts().CPlusPlusModules) &&
          "should only have module decl in Modules TS or C++20");
 
@@ -165,6 +166,16 @@
     ModuleName += Piece.first->getName();
   }
 
+  if (MDK == ModuleDeclKind::Partition) {
+    std::string PartitionName;
+    for (auto &Piece : PartitionPath) {
+      if (!PartitionName.empty())
+        PartitionName += ".";
+      PartitionName += Piece.first->getName();
+    }
+    ModuleName += "-" + PartitionName;
+  }
+
   // If a module name was explicitly specified on the command line, it must be
   // correct.
   if (!getLangOpts().CurrentModule.empty() &&
@@ -180,6 +191,7 @@
   Module *Mod;
 
   switch (MDK) {
+  case ModuleDeclKind::Partition:
   case ModuleDeclKind::Interface: {
     // We can't have parsed or imported a definition of this module or parsed a
     // module map defining it already.
@@ -197,6 +209,7 @@
     // Create a Module for the module that we're defining.
     Mod = Map.createModuleForInterfaceUnit(ModuleLoc, ModuleName,
                                            GlobalModuleFragment);
+
     assert(Mod && "module creation should not fail");
     break;
   }
@@ -272,6 +285,11 @@
     return nullptr;
   }
 
+  if (IsCurrentModulePartition()) {
+    Diag(PrivateLoc, diag::err_private_module_fragment_in_module_partition);
+    return nullptr;
+  }
+
   // FIXME: Check this isn't a module interface partition.
   // FIXME: Check that this translation unit does not import any partitions;
   // such imports would violate [basic.link]/2's "shall be the only module unit"
@@ -306,17 +324,25 @@
 
 DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
                                    SourceLocation ExportLoc,
-                                   SourceLocation ImportLoc,
-                                   ModuleIdPath Path) {
+                                   SourceLocation ImportLoc, ModuleIdPath Path,
+                                   bool IsPartition) {
   // Flatten the module path for a Modules TS module name.
   std::pair<IdentifierInfo *, SourceLocation> ModuleNameLoc;
-  if (getLangOpts().ModulesTS) {
+  if (getLangOpts().ModulesTS || IsPartition) {
     std::string ModuleName;
     for (auto &Piece : Path) {
       if (!ModuleName.empty())
         ModuleName += ".";
       ModuleName += Piece.first->getName();
     }
+
+    if (IsPartition) {
+      assert(getLangOpts().CPlusPlusModules &&
+             "Partitions is only allowed in C++20 modules.");
+      if (getLangOpts().CurrentModule.empty())
+        Diag(ImportLoc, diag::err_module_import_in_non_module);
+      ModuleName = std::string(getCurrentModuleName()) + "-" + ModuleName;
+    }
     ModuleNameLoc = {PP.getIdentifierInfo(ModuleName), Path[0].second};
     Path = ModuleIdPath(ModuleNameLoc);
   }
@@ -327,7 +353,8 @@
   if (!Mod)
     return true;
 
-  return ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, Mod, Path);
+  return ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, Mod, Path,
+                           IsPartition);
 }
 
 /// Determine whether \p D is lexically within an export-declaration.
@@ -340,8 +367,8 @@
 
 DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
                                    SourceLocation ExportLoc,
-                                   SourceLocation ImportLoc,
-                                   Module *Mod, ModuleIdPath Path) {
+                                   SourceLocation ImportLoc, Module *Mod,
+                                   ModuleIdPath Path, bool IsPartition) {
   VisibleModules.setVisible(Mod, ImportLoc);
 
   checkModuleImportContext(*this, Mod, ImportLoc, CurContext);
@@ -351,14 +378,13 @@
   // silently ignoring the import.
   // Import-from-implementation is valid in the Modules TS. FIXME: Should we
   // warn on a redundant import of the current module?
-  // FIXME: Import of a module from an implementation partition of the same
-  // module is permitted.
-  if (Mod->getTopLevelModuleName() == getLangOpts().CurrentModule &&
-      (getLangOpts().isCompilingModule() || !getLangOpts().ModulesTS)) {
+  if (Mod->getTopLevelPrimaryModuleName() == getCurrentModuleName() &&
+      (getLangOpts().isCompilingModule() || !getLangOpts().ModulesTS) &&
+      !IsPartition && !IsCurrentModulePartition()) {
     Diag(ImportLoc, getLangOpts().isCompilingModule()
                         ? diag::err_module_self_import
                         : diag::err_module_import_in_implementation)
-        << Mod->getFullModuleName() << getLangOpts().CurrentModule;
+        << Mod->getFullModuleName() << getCurrentModuleName();
   }
 
   SmallVector<SourceLocation, 2> IdentifierLocs;
@@ -708,3 +734,13 @@
 
   return D;
 }
+
+StringRef Sema::getCurrentModuleName() const {
+  if (!getLangOpts().CPlusPlusModules)
+    return getLangOpts().CurrentModule;
+  return StringRef(getLangOpts().CurrentModule).split('-').first;
+}
+
+bool Sema::IsCurrentModulePartition() const {
+  return StringRef(getLangOpts().CurrentModule).contains('-');
+}
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
@@ -8654,6 +8654,8 @@
       if (Diagnoser && SuggestedDef)
         diagnoseMissingImport(Loc, SuggestedDef, MissingImportKind::Definition,
                               /*Recover*/TreatAsComplete);
+      // FIXME: It is possible to emit duplicated diagnostic message. See
+      // CXX/module/module.reach/p4/X.cpp for example.
       return !TreatAsComplete;
     } else if (Def && !TemplateInstCallbacks.empty()) {
       CodeSynthesisContext TempInst;
diff --git a/clang/test/CXX/module/module.global.frag/Inputs/X-A.cppm b/clang/test/CXX/module/module.global.frag/Inputs/X-A.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.global.frag/Inputs/X-A.cppm
@@ -0,0 +1,3 @@
+module;
+#include "header.h"
+export module X:A;
diff --git a/clang/test/CXX/module/module.global.frag/Inputs/header.h b/clang/test/CXX/module/module.global.frag/Inputs/header.h
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.global.frag/Inputs/header.h
@@ -0,0 +1 @@
+constexpr int n = 1;
diff --git a/clang/test/CXX/module/module.global.frag/p2.cpp b/clang/test/CXX/module/module.global.frag/p2.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.global.frag/p2.cpp
@@ -0,0 +1,13 @@
+// This test tests that the declaration in the global module fragment 
+// couldn't be used in other module unit even they are in the same module.
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/Inputs/X-A.cppm -emit-module-interface -o %t/X-A.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+
+export module X:B;
+import :A;
+export int foo() {
+    return n; // expected-error {{'n' must be declared before it is used}}
+              // expected-note@Inputs/header.h:1 {{declaration here is not visible}}
+}
diff --git a/clang/test/CXX/module/module.import/Inputs/M-Part.cppm b/clang/test/CXX/module/module.import/Inputs/M-Part.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.import/Inputs/M-Part.cppm
@@ -0,0 +1 @@
+module M:Part;
diff --git a/clang/test/CXX/module/module.import/p8.cpp b/clang/test/CXX/module/module.import/p8.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.import/p8.cpp
@@ -0,0 +1,9 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/Inputs/M-Part.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/M-Part.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+// expected-no-diagnostics
+// FIXME: We couldn't export module M:Part since it is an implementation module.
+
+export module M;
+export import :Part;    // error: exported partition :Part is an implementation unit
diff --git a/clang/test/CXX/module/module.private.frag/p1.cpp b/clang/test/CXX/module/module.private.frag/p1.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.private.frag/p1.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+export module X:A;
+export struct S;
+module :private; // expected-error {{private module fragment is not allowed to appear in a module partition}}
+struct S{};
diff --git a/clang/test/CXX/module/module.reach/Inputs/p4/M-A.cppm b/clang/test/CXX/module/module.reach/Inputs/p4/M-A.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/Inputs/p4/M-A.cppm
@@ -0,0 +1,2 @@
+export module M:A;
+export struct B;
diff --git a/clang/test/CXX/module/module.reach/Inputs/p4/M-B.cppm b/clang/test/CXX/module/module.reach/Inputs/p4/M-B.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/Inputs/p4/M-B.cppm
@@ -0,0 +1,4 @@
+module M:B;
+struct B {
+  operator int();
+};
diff --git a/clang/test/CXX/module/module.reach/Inputs/p4/M.cppm b/clang/test/CXX/module/module.reach/Inputs/p4/M.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/Inputs/p4/M.cppm
@@ -0,0 +1,5 @@
+export module M;
+export import :A;
+import :B;
+B b2;
+export void f(B b = B());
diff --git a/clang/test/CXX/module/module.reach/p4/M-C.cppm b/clang/test/CXX/module/module.reach/p4/M-C.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/p4/M-C.cppm
@@ -0,0 +1,10 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/M-A.cppm -emit-module-interface -o %t/M-A.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+
+module M:C;
+import :A;                      // Since the compiler couldn't see the defintion of 'B' too, it couldn't
+                                // give better diagnostic message.
+B b1;                           // expected-error {{variable has incomplete type 'B'}}
+                                // expected-note@../Inputs/p4/M-A.cppm:2 {{forward declaration of 'B'}}
diff --git a/clang/test/CXX/module/module.reach/p4/X.cpp b/clang/test/CXX/module/module.reach/p4/X.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.reach/p4/X.cpp
@@ -0,0 +1,15 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/M-A.cppm -emit-module-interface -o %t/M-A.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/M-B.cppm -emit-module-interface -o %t/M-B.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/M.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/M.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+
+export module X;
+import M;
+                                // FIXME: Eliminate the duplicated diagnostics.
+B b3;                           // expected-error {{definition of 'B' must be imported}}
+                                // expected-error@-1 {{definition of 'B' must be imported}}
+                                // expected-note@../Inputs/p4/M-B.cppm:2 {{definition here is not reachable}}
+                                // expected-note@../Inputs/p4/M-B.cppm:2 {{definition here is not reachable}}
+void g() { f(); }               // FIXME: The default argument of f() shouldn't be reachable from here. 
diff --git a/clang/test/CXX/module/module.unit/Inputs/p3/X.cppm b/clang/test/CXX/module/module.unit/Inputs/p3/X.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p3/X.cppm
@@ -0,0 +1,6 @@
+export module X;
+export import :parta;
+import :partb;
+export int use() {
+    return a() + b();
+}
diff --git a/clang/test/CXX/module/module.unit/Inputs/p3/parta.cppm b/clang/test/CXX/module/module.unit/Inputs/p3/parta.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p3/parta.cppm
@@ -0,0 +1,5 @@
+export module X:parta;
+
+export int a() {
+    return 1;
+}
diff --git a/clang/test/CXX/module/module.unit/Inputs/p3/partb.cppm b/clang/test/CXX/module/module.unit/Inputs/p3/partb.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p3/partb.cppm
@@ -0,0 +1,5 @@
+module X:partb;
+import :parta;
+int b() {
+    return 2 + a();
+}
diff --git a/clang/test/CXX/module/module.unit/Inputs/p4/A-Foo.cppm b/clang/test/CXX/module/module.unit/Inputs/p4/A-Foo.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p4/A-Foo.cppm
@@ -0,0 +1,3 @@
+export module A:Foo;
+import :Internals;
+export int foo() { return 2 * (bar() + 1); }
diff --git a/clang/test/CXX/module/module.unit/Inputs/p4/A-Impl.cpp b/clang/test/CXX/module/module.unit/Inputs/p4/A-Impl.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p4/A-Impl.cpp
@@ -0,0 +1,5 @@
+// expected-no-diagnostics
+module A;
+import :Internals;
+int bar() { return baz() - 10; }
+int baz() { return 30; }
diff --git a/clang/test/CXX/module/module.unit/Inputs/p4/A-Internals.cppm b/clang/test/CXX/module/module.unit/Inputs/p4/A-Internals.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p4/A-Internals.cppm
@@ -0,0 +1,2 @@
+module A:Internals;
+int bar();
diff --git a/clang/test/CXX/module/module.unit/Inputs/p4/A.cppm b/clang/test/CXX/module/module.unit/Inputs/p4/A.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p4/A.cppm
@@ -0,0 +1,3 @@
+export module A;
+export import :Foo;
+export int baz();
diff --git a/clang/test/CXX/module/module.unit/Inputs/p8/B-X1.cppm b/clang/test/CXX/module/module.unit/Inputs/p8/B-X1.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p8/B-X1.cppm
@@ -0,0 +1,2 @@
+module B:X1;                    // does not implicitly import B
+int &a = n;                     // expected-error {{use of undeclared identifier 'n'}}
diff --git a/clang/test/CXX/module/module.unit/Inputs/p8/B-X2.cppm b/clang/test/CXX/module/module.unit/Inputs/p8/B-X2.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p8/B-X2.cppm
@@ -0,0 +1,4 @@
+// expected-no-diagnostics
+module B:X2;                    // does not implicitly import B
+import B;
+int &b = n;                     // OK
diff --git a/clang/test/CXX/module/module.unit/Inputs/p8/B-Y.cppm b/clang/test/CXX/module/module.unit/Inputs/p8/B-Y.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p8/B-Y.cppm
@@ -0,0 +1,5 @@
+module B:Y;                     // does not implicitly import B
+int y();
+#ifdef TESTING
+int _ = n;                      // expected-error {{use of undeclared identifier}}
+#endif
diff --git a/clang/test/CXX/module/module.unit/Inputs/p8/B.cppm b/clang/test/CXX/module/module.unit/Inputs/p8/B.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/Inputs/p8/B.cppm
@@ -0,0 +1,3 @@
+export module B;
+import :Y;                      // OK, does not create interface dependency cycle
+int n = y();
diff --git a/clang/test/CXX/module/module.unit/p3.cpp b/clang/test/CXX/module/module.unit/p3.cpp
deleted file mode 100644
--- a/clang/test/CXX/module/module.unit/p3.cpp
+++ /dev/null
@@ -1,4 +0,0 @@
-// RUN: %clang_cc1 -std=c++2a -verify %s
-
-export module foo:bar; // expected-error {{sorry, module partitions are not yet supported}}
-import :baz; // expected-error {{sorry, module partitions are not yet supported}}
diff --git a/clang/test/CXX/module/module.unit/p3/FromNonModule.cpp b/clang/test/CXX/module/module.unit/p3/FromNonModule.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/p3/FromNonModule.cpp
@@ -0,0 +1,11 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p3/parta.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/X-parta.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+
+import :parta; // expected-error {{import module partition isn't allowed in non module.}}
+               // expected-error@-1 {{not found}}
+
+int use() {
+    return a();
+}
diff --git a/clang/test/CXX/module/module.unit/p3/FromWrongModule.cpp b/clang/test/CXX/module/module.unit/p3/FromWrongModule.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/p3/FromWrongModule.cpp
@@ -0,0 +1,11 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p3/parta.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/Y-parta.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+
+export module Y;
+import :parta;  // expected-error {{error in loading module 'Y-parta' from prebuilt module path}}
+
+int use() {
+    return a();
+}
diff --git a/clang/test/CXX/module/module.unit/p3/simple.cpp b/clang/test/CXX/module/module.unit/p3/simple.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/p3/simple.cpp
@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+// expected-no-diagnostics
+
+export module foo:bar;
diff --git a/clang/test/CXX/module/module.unit/p3/user.cpp b/clang/test/CXX/module/module.unit/p3/user.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/p3/user.cpp
@@ -0,0 +1,13 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p3/parta.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/X-parta.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p3/partb.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/X-partb.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p3/X.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/X.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+// expected-no-diagnostics
+
+import X;
+
+int user() {
+    return use();
+}
diff --git a/clang/test/CXX/module/module.unit/p4/user.cpp b/clang/test/CXX/module/module.unit/p4/user.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/p4/user.cpp
@@ -0,0 +1,19 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/A-Internals.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/A-Internals.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/A-Foo.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/A-Foo.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/A.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/A.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p4/A-Impl.cpp -fmodule-file=%t/A.pcm -fprebuilt-module-path=%t -verify
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %s
+
+import A;
+
+int user() {
+    return foo()+baz();
+}
+
+int user2() {
+    return foo() + baz() + bar(); // expected-error {{use of undeclared identifier 'bar'}}
+                                  // expected-note@*{{declared here}}
+                                  // NOTE: The note above notes the possible compiling error, which is irrevelant with module
+}
diff --git a/clang/test/CXX/module/module.unit/p8/B-Impl.cppm b/clang/test/CXX/module/module.unit/p8/B-Impl.cppm
new file mode 100644
--- /dev/null
+++ b/clang/test/CXX/module/module.unit/p8/B-Impl.cppm
@@ -0,0 +1,12 @@
+// RUN: rm -fr %t
+// RUN: mkdir %t
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p8/B-Y.cppm -fprebuilt-module-path=%t -DTESTING -verify
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p8/B-Y.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/B-Y.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p8/B.cppm -fprebuilt-module-path=%t -emit-module-interface -o %t/B.pcm
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p8/B-X1.cppm -fprebuilt-module-path=%t -verify
+// RUN: %clang_cc1 -std=c++20 %S/../Inputs/p8/B-X2.cppm -fprebuilt-module-path=%t -verify
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -fmodule-file=%t/B.pcm -verify %s
+// expected-no-diagnostics
+
+module B;                       // implicitly imports B
+int &c = n;                     // OK
diff --git a/clang/test/CXX/module/module.unit/p8.cpp b/clang/test/CXX/module/module.unit/p8/ImplicitImport.cpp
rename from clang/test/CXX/module/module.unit/p8.cpp
rename to clang/test/CXX/module/module.unit/p8/ImplicitImport.cpp
--- a/clang/test/CXX/module/module.unit/p8.cpp
+++ b/clang/test/CXX/module/module.unit/p8/ImplicitImport.cpp
@@ -24,12 +24,10 @@
 export module bar;
 
 #elif MODE == 4
-module foo:bar; // expected-error {{not yet supported}}
-#define IMPORTED // FIXME
+module foo:bar;
 
 #elif MODE == 5
-export module foo:bar; // expected-error {{not yet supported}} expected-error {{redefinition}} expected-note@* {{loaded from}}
-#define IMPORTED // FIXME
+export module foo:bar; 
 
 #endif