[C++20] [Module] Support reachable definition initially/partially

This patch introduces a new kind of ModuleOwnershipKind as
ReachableWhenImported. This intended the status for reachable described
at: https://eel.is/c++draft/module.reach#3.

Note that this patch is not intended to support all semantics about
reachable semantics. For example, this patch didn't implement discarded
declarations in GMF. (https://eel.is/c++draft/module.global.frag#3).

This fixes: https://bugs.llvm.org/show_bug.cgi?id=52281 and
https://godbolt.org/z/81f3ocjfW.

Reviewed By: rsmith, iains

Differential Revision: https://reviews.llvm.org/D113545

1 files changed, 12 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index b0e7d30..3aa124d 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -90,7 +90,14 @@ Sema::ActOnGlobalModuleFragmentDecl(SourceLocation ModuleLoc) {
 
   // All declarations created from now on are owned by the global module.
   auto *TU = Context.getTranslationUnitDecl();
-  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::Visible);
+  // [module.global.frag]p2
+  // A global-module-fragment specifies the contents of the global module
+  // fragment for a module unit. The global module fragment can be used to
+  // provide declarations that are attached to the global module and usable
+  // within the module unit.
+  //
+  // So the declations in the global module shouldn't be visible by default.
+  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::ReachableWhenImported);
   TU->setLocalOwningModule(GlobalModule);
 
   // FIXME: Consider creating an explicit representation of this declaration.
@@ -325,10 +332,12 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   VisibleModules.setVisible(Mod, ModuleLoc);
 
   // From now on, we have an owning module for all declarations we see.
-  // However, those declarations are module-private unless explicitly
+  // In C++20 modules, those declaration would be reachable when imported
+  // unless explicitily exported.
+  // Otherwise, those declarations are module-private unless explicitly
   // exported.
   auto *TU = Context.getTranslationUnitDecl();
-  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::ModulePrivate);
+  TU->setModuleOwnershipKind(Decl::ModuleOwnershipKind::ReachableWhenImported);
   TU->setLocalOwningModule(Mod);
 
   // We are in the module purview, but before any other (non import)