[C++20] [Moduels] Correct the linkage of const variable in language linkage from module interfaces (#102574)

Close #99825

The root cause of the issue is that I didn't realize the things in
implicit global module (the language linkage in module interfaces)
should be considered in module purview.

Author: ChuanqiXu9

clang/lib/AST/Decl.cpp

@@ -583,12 +583,6 @@ static bool isSingleLineLanguageLinkage(const Decl &D) {
   return false;
 }
 
-static bool isDeclaredInModuleInterfaceOrPartition(const NamedDecl *D) {
-  if (auto *M = D->getOwningModule())
-    return M->isInterfaceOrPartition();
-  return false;
-}
-
 static LinkageInfo getExternalLinkageFor(const NamedDecl *D) {
   return LinkageInfo::external();
 }
@@ -642,7 +636,13 @@ LinkageComputer::getLVForNamespaceScopeDecl(const NamedDecl *D,
     // (There is no equivalent in C99.)
     if (Context.getLangOpts().CPlusPlus && Var->getType().isConstQualified() &&
         !Var->getType().isVolatileQualified() && !Var->isInline() &&
-        !isDeclaredInModuleInterfaceOrPartition(Var) &&
+        ![Var]() {
+          // Check if it is module purview except private module fragment
+          // and implementation unit.
+          if (auto *M = Var->getOwningModule())
+            return M->isInterfaceOrPartition() || M->isImplicitGlobalModule();
+          return false;
+        }() &&
         !isa<VarTemplateSpecializationDecl>(Var) &&
         !Var->getDescribedVarTemplate()) {
       const VarDecl *PrevVar = Var->getPreviousDecl();

clang/test/Modules/pr99825.cppm

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -std=c++20 %s -fsyntax-only -verify
+// expected-no-diagnostics
+export module mod;
+
+extern "C++"
+{
+    export constexpr auto x = 10;
+}