[C++20][Modules] Disable preferred_name when writing a C++20 header unit

clang/include/clang/Serialization/ASTWriter.h

@@ -899,6 +899,10 @@ class ASTWriter : public ASTDeserializationListener,
     return WritingModule && WritingModule->isNamedModule();
   }
 
+  bool isWritingStdCXXHeaderUnit() const {
+    return WritingModule && WritingModule->isHeaderUnit();
+  }
+
   bool isGeneratingReducedBMI() const { return GeneratingReducedBMI; }
 
   bool getDoneWritingDeclsAndTypes() const { return DoneWritingDeclsAndTypes; }

clang/lib/Serialization/ASTWriter.cpp

@@ -5167,8 +5167,9 @@ void ASTRecordWriter::AddAttr(const Attr *A) {
   // FIXME: Clang can't handle the serialization/deserialization of
   // preferred_name properly now. See
   // https://github.com/llvm/llvm-project/issues/56490 for example.
-  if (!A || (isa<PreferredNameAttr>(A) &&
-             Writer->isWritingStdCXXNamedModules()))
+  if (!A ||
+      (isa<PreferredNameAttr>(A) && (Writer->isWritingStdCXXNamedModules() ||
+                                     Writer->isWritingStdCXXHeaderUnit())))
     return Record.push_back(0);
 
   Record.push_back(A->getKind() + 1); // FIXME: stable encoding, target attrs

clang/test/Modules/preferred_name_header_unit.cpp

@@ -0,0 +1,64 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -verify -w -std=c++20 -fmodule-name=h1.h -emit-header-unit -xc++-user-header h1.h -o h1.pcm
+// RUN: %clang_cc1 -verify -w -std=c++20 -fmodule-map-file=module.modulemap -fmodule-file=h1.h=h1.pcm main.cpp -o main.o
+
+//--- module.modulemap
+module "h1.h" {
+  header "h1.h"
+  export *
+}
+
+//--- h0.h
+// expected-no-diagnostics
+#pragma once
+namespace std {
+
+template <class _CharT, class = _CharT, class = _CharT> class basic_string;
+
+namespace pmr {
+using string = basic_string<char>;
+}
+
+template <class, class, class>
+class __attribute__((__preferred_name__(pmr::string))) basic_string;
+
+template <class> class basic_string_view {};
+
+template <class _CharT, class _Traits, class _Allocator> class basic_string {
+  typedef _CharT value_type;
+  typedef _Allocator allocator_type;
+  struct __rep;
+public:
+  template <class _Tp>
+  basic_string(_Tp) {}
+  basic_string operator+=(value_type);
+};
+
+namespace filesystem {
+class path {
+  typedef char value_type;
+  value_type preferred_separator;
+  typedef basic_string<value_type> string_type;
+  typedef basic_string_view<value_type> __string_view;
+  template <class _Source> void append(_Source) {
+    __pn_ += preferred_separator;
+  }
+  void __root_directory() { append(string_type(__string_view{})); }
+  string_type __pn_;
+};
+} // namespace filesystem
+} // namespace std
+
+//--- h1.h
+// expected-no-diagnostics
+#pragma once
+
+#include "h0.h"
+
+//--- main.cpp
+// expected-no-diagnostics
+#include "h0.h"
+
+import "h1.h";