[C++20] [Modules] Enhance better diagnostic for implicit global module and module partitions

Due to an oversight, when users use an unexported declaration from
implicit global module, the diagnostic will show "please #include"
instead of "please import". This patch corrects the behavior.

Also previously, when users use an unexported declarations from module
partitions, the diagnostic message will always show the partition name
no matter if that partition name is visible to the users. Now the users
may only see the partition name if the users are in the same module with
the partition unit.

Author: ChuanqiXu9

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;
@@ -5764,15 +5786,15 @@ void Sema::diagnoseMissingImport(SourceLocation UseLoc, const NamedDecl *Decl,
         ModuleList += "[...]";
         break;
       }
-      ModuleList += M->getFullModuleName();
+      ModuleList += GetModuleNameForDiagnostic(M);
     }
 
     Diag(UseLoc, diag::err_module_unimported_use_multiple)
       << (int)MIK << Decl << ModuleList;
   } 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();

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 {{}}
        // [email protected]:2 {{declaration here is not visible}}
        // [email protected]:2 {{definition here is not reachable}} [email protected]: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 {{}}
        // [email protected]:2 {{declaration here is not visible}}
        // [email protected]:2 {{definition here is not reachable}} [email protected]:2 {{}}
 }

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}}
                       // [email protected]:5 {{passing argument to parameter 'b' here}}

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@* {{}}
 }

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}}
                    // [email protected]: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}}
                 // [email protected]:4 {{declaration here is not visible}}
 }