[C++20] [Modules] Warn for importing implementation partition unit in interface units (#108493)

Recently, there are multiple false positive issue reports about the
reachability of implementation partition units:
- https://github.com/llvm/llvm-project/issues/105882
- https://github.com/llvm/llvm-project/issues/101348
- https://lists.isocpp.org/core/2024/08/16232.php

And according to our use experience for modules, we find it is a pretty
good practice to not import implementation partition units in the
interface units. It can help developers to have a pretty good mental
model for when to use an implementation partition unit: that any unit in
the module but not in the module interfaces can be in the implementation
partition unit.

So I think it is good to add the diagnostics.

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index 3b84e7b..d6ebc38 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -650,6 +650,14 @@ DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
   else
     VisibleModules.setVisible(Mod, ImportLoc);
 
+  assert((!Mod->isModulePartitionImplementation() || getCurrentModule()) &&
+         "We can only import a partition unit in a named module.");
+  if (Mod->isModulePartitionImplementation() &&
+      getCurrentModule()->isModuleInterfaceUnit())
+    Diag(ImportLoc,
+         diag::warn_import_implementation_partition_unit_in_interface_unit)
+        << Mod->Name;
+
   checkModuleImportContext(*this, Mod, ImportLoc, CurContext);
 
   // FIXME: we should support importing a submodule within a different submodule