diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index be011b9f4368d..54b48f7ab80eb 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -5723,7 +5723,7 @@ void Sema::diagnoseMissingImport(SourceLocation UseLoc, const NamedDecl *Decl,
   llvm::SmallVector<Module*, 8> UniqueModules;
   llvm::SmallDenseSet<Module*, 8> UniqueModuleSet;
   for (auto *M : Modules) {
-    if (M->isGlobalModule() || M->isPrivateModule())
+    if (M->isExplicitGlobalModule() || M->isPrivateModule())
       continue;
     if (UniqueModuleSet.insert(M).second)
       UniqueModules.push_back(M);
@@ -5755,6 +5755,28 @@ void Sema::diagnoseMissingImport(SourceLocation UseLoc, const NamedDecl *Decl,
 
   Modules = UniqueModules;
 
+  auto GetModuleNameForDiagnostic = [this](const Module *M) -> std::string {
+    if (M->isModuleMapModule())
+      return M->getFullModuleName();
+
+    Module *CurrentModule = getCurrentModule();
+
+    if (M->isImplicitGlobalModule())
+      M = M->getTopLevelModule();
+
+    bool IsInTheSameModule =
+        CurrentModule && CurrentModule->getPrimaryModuleInterfaceName() ==
+                             M->getPrimaryModuleInterfaceName();
+
+    // If the current module unit is in the same module with M, it is OK to show
+    // the partition name. Otherwise, it'll be sufficient to show the primary
+    // module name.
+    if (IsInTheSameModule)
+      return M->getTopLevelModuleName().str();
+    else
+      return M->getPrimaryModuleInterfaceName().str();
+  };
+
   if (Modules.size() > 1) {
     std::string ModuleList;
     unsigned N = 0;
@@ -5764,7 +5786,7 @@ void Sema::diagnoseMissingImport(SourceLocation UseLoc, const NamedDecl *Decl,
         ModuleList += "[...]";
         break;
       }
-      ModuleList += M->getFullModuleName();
+      ModuleList += GetModuleNameForDiagnostic(M);
     }
 
     Diag(UseLoc, diag::err_module_unimported_use_multiple)
@@ -5772,7 +5794,7 @@ void Sema::diagnoseMissingImport(SourceLocation UseLoc, const NamedDecl *Decl,
   } else {
     // FIXME: Add a FixItHint that imports the corresponding module.
     Diag(UseLoc, diag::err_module_unimported_use)
-      << (int)MIK << Decl << Modules[0]->getFullModuleName();
+        << (int)MIK << Decl << GetModuleNameForDiagnostic(Modules[0]);
   }
 
   NotePrevious();
diff --git a/clang/test/CXX/module/module.import/p2.cpp b/clang/test/CXX/module/module.import/p2.cpp
index 0c02d253f3a89..ef6006811e776 100644
--- a/clang/test/CXX/module/module.import/p2.cpp
+++ b/clang/test/CXX/module/module.import/p2.cpp
@@ -23,8 +23,8 @@ export A f();
 //--- Use.cpp
 import M;
 void test() {
-  A a; // expected-error {{declaration of 'A' must be imported from module 'M:impl'}}
-       // expected-error@-1 {{definition of 'A' must be imported from module 'M:impl'}} expected-error@-1 {{}}
+  A a; // expected-error {{definition of 'A' must be imported from module 'M' before it is required}}
+       // expected-error@-1 {{definition of 'A' must be imported from module 'M' before it is required}} expected-error@-1 {{}}
        // expected-note@impl.cppm:2 {{declaration here is not visible}}
        // expected-note@impl.cppm:2 {{definition here is not reachable}} expected-note@impl.cppm:2 {{}}
 }
@@ -41,8 +41,8 @@ void test() {
 export module B;
 import M;
 void test() {
-  A a; // expected-error {{declaration of 'A' must be imported from module 'M:impl'}}
-       // expected-error@-1 {{definition of 'A' must be imported from module 'M:impl'}} expected-error@-1 {{}}
+  A a; // expected-error {{declaration of 'A' must be imported from module 'M'}}
+       // expected-error@-1 {{definition of 'A' must be imported from module 'M'}} expected-error@-1 {{}}
        // expected-note@impl.cppm:2 {{declaration here is not visible}}
        // expected-note@impl.cppm:2 {{definition here is not reachable}} expected-note@impl.cppm:2 {{}}
 }
diff --git a/clang/test/CXX/module/module.reach/ex1.cpp b/clang/test/CXX/module/module.reach/ex1.cpp
index a1e38c4c88a28..96d06f9a3e0f6 100644
--- a/clang/test/CXX/module/module.reach/ex1.cpp
+++ b/clang/test/CXX/module/module.reach/ex1.cpp
@@ -37,10 +37,10 @@ export void f(B b = B());
 //--- X.cppm
 export module X;
 import M;
-B b3; // expected-error {{definition of 'B' must be imported from module 'M:B' before it is required}} expected-error {{}}
+B b3; // expected-error {{definition of 'B' must be imported from module 'M' before it is required}} expected-error {{}}
       // expected-note@* {{definition here is not reachable}} expected-note@* {{}}
 // FIXME: We should emit an error for unreachable definition of B.
 void g() { f(); }
-void g1() { f(B()); } // expected-error 1+{{definition of 'B' must be imported from module 'M:B' before it is required}}
+void g1() { f(B()); } // expected-error 1+{{definition of 'B' must be imported from module 'M' before it is required}}
                       // expected-note@* 1+{{definition here is not reachable}}
                       // expected-note@M.cppm:5 {{passing argument to parameter 'b' here}}
diff --git a/clang/test/CXX/module/module.reach/p2.cpp b/clang/test/CXX/module/module.reach/p2.cpp
index 6ccf6ee6d0566..9242e6f2457e8 100644
--- a/clang/test/CXX/module/module.reach/p2.cpp
+++ b/clang/test/CXX/module/module.reach/p2.cpp
@@ -17,6 +17,6 @@ export A f();
 //--- UseStrict.cpp
 import M;
 void test() {
-  auto a = f(); // expected-error {{definition of 'A' must be imported from module 'M:impl' before it is required}} expected-error{{}}
+  auto a = f(); // expected-error {{definition of 'A' must be imported from module 'M' before it is required}} expected-error{{}}
                 // expected-note@* {{definition here is not reachable}} expected-note@* {{}}
 }
diff --git a/clang/test/Modules/export-language-linkage.cppm b/clang/test/Modules/export-language-linkage.cppm
index 420e4be0ce45b..462b28d36cb44 100644
--- a/clang/test/Modules/export-language-linkage.cppm
+++ b/clang/test/Modules/export-language-linkage.cppm
@@ -37,7 +37,7 @@ int use() {
     f1();
     f2();
     f3();
-    unexported(); // expected-error {{missing '#include'; 'unexported' must be declared before it is used}}
+    unexported(); // expected-error {{declaration of 'unexported' must be imported from module 'a' before it is required}}
                    // expected-note@a.cppm:15 {{declaration here is not visible}}
     return foo();
 }
@@ -58,6 +58,6 @@ int use() {
 }
 
 int use_of_nonexported() {
-    return h(); // expected-error {{missing '#include'; 'h' must be declared before it is used}}
+    return h(); // expected-error {{declaration of 'h' must be imported from module 'c' before it is required}}
                 // expected-note@c.cppm:4 {{declaration here is not visible}}
 }