[C++20] [Modules] Disable preferred_name when writing a C++20 Module interface

Currently, the use of preferred_name would block implementing std
modules in libcxx. See https://github.com/llvm/llvm-project/issues/56490
for example.
The problem is pretty hard and it looks like we couldn't solve it in a
short time. So we sent this patch as a workaround to avoid blocking us
to modularize STL. This is intended to be fixed properly in the future.

Reviewed By: erichkeane, aaron.ballman, tahonermann

Differential Revision: https://reviews.llvm.org/D130331

2 files changed, 10 insertions, 0 deletions

diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index aff0dbb..5c84e2f 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -5040,6 +5040,12 @@ general this requires the template to be declared at least twice. For example:
                                 clang::preferred_name(wstring)]] basic_string {
     // ...
   };
+
+
+Note that the ``preferred_name`` attribute will be ignored when the compiler
+writes a C++20 Module interface now. This is due to a compiler issue
+(https://github.com/llvm/llvm-project/issues/56490) that blocks users to modularize
+declarations with `preferred_name`. This is intended to be fixed in the future.
   }];
 }
 
diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
index 6a3532d..83bc7dc 100644
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -703,6 +703,10 @@ public:
   bool hasChain() const { return Chain; }
   ASTReader *getChain() const { return Chain; }
 
+  bool isWritingNamedModules() const {
+    return WritingModule && WritingModule->isModulePurview();
+  }
+
 private:
   // ASTDeserializationListener implementation
   void ReaderInitialized(ASTReader *Reader) override;