[C++20] [Modules] Emit an warning for experimental header units

Currently, the header units are rarely used and it is not well tested.
To avoid further misunderstandings, let's mark it as experimental and
emit a warning when users wants to import it.

This is discussed in modules developers meeting.

1 files changed, 3 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index e1d7c29..a1ab013 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -544,6 +544,9 @@ DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
                                    SourceLocation ExportLoc,
                                    SourceLocation ImportLoc, Module *Mod,
                                    ModuleIdPath Path) {
+  if (Mod->isHeaderUnit())
+    Diag(ImportLoc, diag::warn_experimental_header_unit);
+
   VisibleModules.setVisible(Mod, ImportLoc);
 
   checkModuleImportContext(*this, Mod, ImportLoc, CurContext);