[RFC][C++20][Modules] Relax ODR check in unnamed modules

clang/include/clang/AST/DeclBase.h

@@ -687,6 +687,9 @@ class alignas(8) Decl {
   /// Whether this declaration comes from a named module.
   bool isInNamedModule() const;
 
+  /// Whether this declaration comes from a header unit.
+  bool isFromHeaderUnit() const;
+
   /// Return true if this declaration has an attribute which acts as
   /// definition of the entity, such as 'alias' or 'ifunc'.
   bool hasDefiningAttr() const;

clang/include/clang/Serialization/ASTReader.h

@@ -2527,7 +2527,7 @@ class BitsUnpacker {
 
 inline bool shouldSkipCheckingODR(const Decl *D) {
   return D->getASTContext().getLangOpts().SkipODRCheckInGMF &&
-         D->isFromGlobalModule();
+         (D->isFromGlobalModule() || D->isFromHeaderUnit());
 }
 
 } // namespace clang

clang/lib/AST/DeclBase.cpp

@@ -1168,6 +1168,10 @@ bool Decl::isInNamedModule() const {
   return getOwningModule() && getOwningModule()->isNamedModule();
 }
 
+bool Decl::isFromHeaderUnit() const {
+  return getOwningModule() && getOwningModule()->isHeaderUnit();
+}
+
 static Decl::Kind getKind(const Decl *D) { return D->getKind(); }
 static Decl::Kind getKind(const DeclContext *DC) { return DC->getDeclKind(); }
 

clang/test/Headers/header-unit-common-cmp-cat.cpp

@@ -0,0 +1,40 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -verify -std=c++20 -fskip-odr-check-in-gmf -I. -emit-header-unit -xc++-user-header bz1.h
+// RUN: %clang_cc1 -verify -std=c++20 -fskip-odr-check-in-gmf -I. -emit-header-unit -xc++-user-header bz2.h
+// RUN: %clang_cc1 -verify -std=c++20 -fskip-odr-check-in-gmf -I. -emit-header-unit -xc++-user-header -fmodule-file=bz1.pcm -fmodule-file=bz2.pcm bz.cpp
+
+//--- compare
+namespace std {
+namespace __detail {
+
+template<typename _Tp>
+inline constexpr unsigned __cmp_cat_id = 1;
+
+template<typename... _Ts>
+constexpr auto __common_cmp_cat() {
+  (__cmp_cat_id<_Ts> | ...);
+}
+
+} // namespace __detail
+} // namespace std
+
+//--- bz0.h
+template <class T>
+int operator|(T, T);
+
+//--- bz1.h
+#include "bz0.h"
+#include <compare>
+// expected-no-diagnostics
+
+//--- bz2.h
+#include <compare>
+// expected-no-diagnostics
+
+//--- bz.cpp
+#include <compare>
+
+import "bz1.h"; // expected-warning {{the implementation of header units is in an experimental phase}}
+import "bz2.h"; // expected-warning {{the implementation of header units is in an experimental phase}}