[C++2x][Modules] Amend module purview constant linkage [P2788R0].

[iains]

Summary

This paper has been applied to the working draft and is believed to be
a DR against C++20, so that the patch here makes the change unconditionally.

for:

export module A;

const int mod_cst = 10;

Before the change, mod_cst would have internal linkage; after the change it
has module linkage.

[iains]

rebase, remove use of fmodule-file=X=path to attempt to resolve Windows fail.

[iains]

rebased, try to fix Windows CI, take 2.

[iains]

I should definitely check to make sure that this was adopted as a DR against C++20 (otherwise we would need to use the previous test conditionally on < 2b).

[ChuanqiXu]

LGTM.

[iains]

rebased.

clang/lib/AST/Decl.cpp

case Decl::ModuleOwnershipKind::ModulePrivate:
     return false;
   case Decl::ModuleOwnershipKind::Visible:
   case Decl::ModuleOwnershipKind::VisibleWhenImported:
     return isInModulePurview(D);
   }
   llvm_unreachable("unexpected module ownership kind");
 }
 
+static bool isDeclaredInModuleInterfaceOrPartition(const NamedDecl *D) {
+  if (auto *M = D->getOwningModule())
+    return M->isInterfaceOrPartition();
+  return false;
+}
+
 static LinkageInfo getInternalLinkageFor(const NamedDecl *D) {
   return LinkageInfo::internal();
 }
 
 static LinkageInfo getExternalLinkageFor(const NamedDecl *D) {
   return LinkageInfo::external();
 }
 
if (getStorageClass(D->getCanonicalDecl()) == SC_Static) {
     //   that is explicitly declared static; or
     // (This bullet corresponds to C99 6.2.2p3.)
     return getInternalLinkageFor(D);
   }
 
   if (const auto *Var = dyn_cast<VarDecl>(D)) {
     // - a non-template variable of non-volatile const-qualified type, unless
     //   - it is explicitly declared extern, or
-    //   - it is inline or exported, or
+    //   - it is declared in the purview of a module interface unit
+    //     (outside the private-module-fragment, if any) or module partition, or
+    //   - it is inline, or
     //   - it was previously declared and the prior declaration did not have
     //     internal linkage
     // (There is no equivalent in C99.)
-    if (Context.getLangOpts().CPlusPlus &&
-        Var->getType().isConstQualified() &&
-        !Var->getType().isVolatileQualified() &&
-        !Var->isInline() &&
-        !isExportedFromModuleInterfaceUnit(Var) &&
+    if (Context.getLangOpts().CPlusPlus && Var->getType().isConstQualified() &&
+        !Var->getType().isVolatileQualified() && !Var->isInline() &&
+        !isDeclaredInModuleInterfaceOrPartition(Var) &&
         !isa<VarTemplateSpecializationDecl>(Var) &&
         !Var->getDescribedVarTemplate()) {
       const VarDecl *PrevVar = Var->getPreviousDecl();
       if (PrevVar)
         return getLVForDecl(PrevVar, computation);
 
       if (Var->getStorageClass() != SC_Extern &&
           Var->getStorageClass() != SC_PrivateExtern &&

clang/test/CXX/module/basic/basic.def.odr/p4.cppm

 // RUN: rm -rf %t
 // RUN: mkdir %t
 // RUN: split-file %s %t
 //
 // RUN: %clang_cc1 -std=c++20 %t/Module.cppm -triple %itanium_abi_triple -emit-llvm -o - | FileCheck %t/Module.cppm --implicit-check-not unused
 //
 // RUN: %clang_cc1 -std=c++20 %t/Module.cppm -triple %itanium_abi_triple -emit-module-interface -o %t/Module.pcm
-// RUN: %clang_cc1 -std=c++20 %t/module.cpp -triple %itanium_abi_triple -fmodule-file=%t/Module.pcm -emit-llvm -o - | FileCheck %t/module.cpp --implicit-check-not=unused --implicit-check-not=global_module
+// RUN: %clang_cc1 -std=c++20 %t/module.cpp -triple %itanium_abi_triple -fmodule-file=Module=%t/Module.pcm -emit-llvm -o - | FileCheck %t/module.cpp --implicit-check-not=unused --implicit-check-not=global_module
 //
-// RUN: %clang_cc1 -std=c++20 %t/user.cpp -triple %itanium_abi_triple -fmodule-file=%t/Module.pcm -emit-llvm -o - | FileCheck %t/user.cpp --implicit-check-not=unused --implicit-check-not=global_module
+// RUN: %clang_cc1 -std=c++20 %t/user.cpp -triple %itanium_abi_triple -fmodule-file=Module=%t/Module.pcm -emit-llvm -o - | FileCheck %t/user.cpp --implicit-check-not=unused --implicit-check-not=global_module
 
 //--- Module.cppm
 // CHECK-DAG: @extern_var_global_module = external {{(dso_local )?}}global
 // CHECK-DAG: @inline_var_global_module = linkonce_odr {{(dso_local )?}}global
 // CHECK-DAG: @_ZL24static_var_global_module = internal global
 // CHECK-DAG: @_ZL23const_var_global_module = internal constant
 //
 // With strong-ownership, the module is mangled into exported symbols
 // that are attached to a named module.
 // CHECK-DAG: @_ZW6Module19extern_var_exported = external {{(dso_local )?}}global
 // FIXME: Should this be 'weak_odr global'? Presumably it must be, since we
 // can discard this global and its initializer (if any), and other TUs are not
 // permitted to run the initializer for this variable.
 // CHECK-DAG: @_ZW6Module19inline_var_exported = linkonce_odr {{(dso_local )?}}global
 // CHECK-DAG: @_ZW6Module18const_var_exported = {{(dso_local )?}}constant
 //
 // CHECK-DAG: @_ZW6Module25extern_var_module_linkage = external {{(dso_local )?}}global
 // FIXME: Should this be 'weak_odr global'? Presumably it must be, since we
 // can discard this global and its initializer (if any), and other TUs are not
 // permitted to run the initializer for this variable.
 // CHECK-DAG: @_ZW6Module25inline_var_module_linkage = linkonce_odr {{(dso_local )?}}global
 // CHECK-DAG: @_ZL25static_var_module_linkage = internal
-// CHECK-DAG: @_ZL24const_var_module_linkage = internal
+// CHECK-DAG: @_ZW6Module24const_var_module_linkage = {{(dso_local )?}}constant
 //
 // CHECK-DAG: @_ZW6Module25unused_var_module_linkage = {{(dso_local )?}}global i32 4
 
 module;
 
 static void unused_static_global_module() {}
 static void used_static_global_module() {}
 

 
 // CHECK-DAG: @_ZW6Module19extern_var_exported = external {{(dso_local )?}}global
 // CHECK-DAG: @_ZW6Module19inline_var_exported = linkonce_odr {{(dso_local )?}}global
 // CHECK-DAG: @_ZW6Module18const_var_exported = available_externally {{(dso_local )?}}constant i32 3,
 //
 // CHECK-DAG: @_ZW6Module25extern_var_module_linkage = external {{(dso_local )?}}global
 // CHECK-DAG: @_ZW6Module25inline_var_module_linkage = linkonce_odr {{(dso_local )?}}global
 // CHECK-DAG: @_ZL25static_var_module_linkage = internal {{(dso_local )?}}global i32 0,
-// CHECK-DAG: @_ZL24const_var_module_linkage = internal {{(dso_local )?}}constant i32 3,
+// CHECK-DAG: @_ZW6Module24const_var_module_linkage = available_externally {{(dso_local )?}}constant i32 3,
 
 module Module;
 
 void use() {
   // CHECK: define linkonce_odr {{.*}}@_ZW6Module20used_inline_exportedv
   used_inline_exported();
   // CHECK: declare {{.*}}@_ZW6Module18noninline_exportedv
   noninline_exported();

clang/test/CXX/module/basic/basic.link/p3.cppm

 // RUN: %clang_cc1 -std=c++20 -triple x86_64-linux %s -emit-module-interface -o %t
 // RUN: %clang_cc1 -std=c++20 -triple x86_64-linux -x pcm %t -emit-llvm -o - | FileCheck %s
 
 export module M;
 
-// CHECK-NOT: @_ZW1M1a ={{.*}}
+// CHECK: @_ZW1M1a ={{.*}} constant i32 1
 const int a = 1;
 // CHECK: @_ZW1M1b ={{.*}} constant i32 2
 export const int b = 2;
 
 export int f() { return a + b; }