[C++20] [Modules] Don't add decls from other units to undefinedButUsed set

It is still better to elide the declaration if possible. To overcome the
false positive undefinedButUsed diagnostic, it seems better to not add
declaration from other units to the set actually.

Author: ChuanqiXu9

clang/lib/Sema/SemaDecl.cpp

@@ -3829,11 +3829,9 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
 
   // If this redeclaration makes the function inline, we may need to add it to
   // UndefinedButUsed.
-  if (!Old->isInlined() && New->isInlined() &&
-      !New->hasAttr<GNUInlineAttr>() &&
-      !getLangOpts().GNUInline &&
-      Old->isUsed(false) &&
-      !Old->isDefined() && !New->isThisDeclarationADefinition())
+  if (!Old->isInlined() && New->isInlined() && !New->hasAttr<GNUInlineAttr>() &&
+      !getLangOpts().GNUInline && Old->isUsed(false) && !Old->isDefined() &&
+      !New->isThisDeclarationADefinition() && !Old->isInAnotherModuleUnit())
     UndefinedButUsed.insert(std::make_pair(Old->getCanonicalDecl(),
                                            SourceLocation()));
 
@@ -4700,7 +4698,8 @@ void Sema::MergeVarDecl(VarDecl *New, LookupResult &Previous) {
   // If this redeclaration makes the variable inline, we may need to add it to
   // UndefinedButUsed.
   if (!Old->isInline() && New->isInline() && Old->isUsed(false) &&
-      !Old->getDefinition() && !New->isThisDeclarationADefinition())
+      !Old->getDefinition() && !New->isThisDeclarationADefinition() &&
+      !Old->isInAnotherModuleUnit())
     UndefinedButUsed.insert(std::make_pair(Old->getCanonicalDecl(),
                                            SourceLocation()));
 

clang/lib/Sema/SemaExpr.cpp

@@ -18355,7 +18355,7 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
   // If this is the first "real" use, act on that.
   if (OdrUse == OdrUseContext::Used && !Func->isUsed(/*CheckUsedAttr=*/false)) {
     // Keep track of used but undefined functions.
-    if (!Func->isDefined()) {
+    if (!Func->isDefined() && !Func->isInAnotherModuleUnit()) {
       if (mightHaveNonExternalLinkage(Func))
         UndefinedButUsed.insert(std::make_pair(Func->getCanonicalDecl(), Loc));
       else if (Func->getMostRecentDecl()->isInlined() &&

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -331,12 +331,6 @@ namespace clang {
 }
 
 bool clang::CanElideDeclDef(const Decl *D) {
-  bool isExternalWithNoLinkageType = false;
-  if (auto *VD = dyn_cast<ValueDecl>(D))
-    if (VD->hasExternalFormalLinkage() &&
-        !isExternalFormalLinkage(VD->getType()->getLinkage()))
-      isExternalWithNoLinkageType = true;
-
   if (auto *FD = dyn_cast<FunctionDecl>(D)) {
     if (FD->isInlined() || FD->isConstexpr())
       return false;
@@ -346,9 +340,6 @@ bool clang::CanElideDeclDef(const Decl *D) {
 
     if (FD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
       return false;
-
-    if (isExternalWithNoLinkageType && !FD->isExternC())
-      return false;
   }
 
   if (auto *VD = dyn_cast<VarDecl>(D)) {
@@ -362,9 +353,6 @@ bool clang::CanElideDeclDef(const Decl *D) {
 
     if (VD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
       return false;
-
-    if (isExternalWithNoLinkageType && !VD->isExternC())
-      return false;
   }
 
   return true;

clang/test/Modules/external-but-not-type-external.cppm

@@ -2,8 +2,10 @@
 // RUN: split-file %s %t
 // RUN: cd %t
 //
-// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-reduced-module-interface -o %t/a.pcm
-// RUN: %clang_cc1 -std=c++20 %t/use.cc -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/a.cppm -emit-reduced-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/use.cc -fmodule-file=a=%t/a.pcm -fsyntax-only -verify
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/use.cc -fmodule-file=a=%t/a.pcm -emit-llvm -o - | \
+// RUN:     FileCheck %t/use.cc
 
 //--- a.cppm
 export module a;
@@ -25,3 +27,5 @@ void *use() {
     A a;
     return a.external_but_not_type_external(nullptr);
 }
+
+// CHECK-NOT: define {{.*}}@_ZNW1a1A30external_but_not_type_externalEPN12_GLOBAL__N_15LocalE