[Clang] Correctly initialize placeholder fields from their initializers

clang/docs/ReleaseNotes.rst

@@ -591,6 +591,7 @@ Bug Fixes to C++ Support
 - Clang now correctly ignores previous partial specializations of member templates explicitly specialized for
   an implicitly instantiated class template specialization. (#GH51051)
 - Fixed an assertion failure caused by invalid enum forward declarations. (#GH112208)
+- Name independent data members were not correctly initialized from default member initializers. (#GH114069)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/ASTContext.h

@@ -1041,7 +1041,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   void setInstantiatedFromUsingShadowDecl(UsingShadowDecl *Inst,
                                           UsingShadowDecl *Pattern);
 
-  FieldDecl *getInstantiatedFromUnnamedFieldDecl(FieldDecl *Field);
+  FieldDecl *getInstantiatedFromUnnamedFieldDecl(FieldDecl *Field) const;
 
   void setInstantiatedFromUnnamedFieldDecl(FieldDecl *Inst, FieldDecl *Tmpl);
 

clang/lib/AST/ASTContext.cpp

@@ -1592,14 +1592,17 @@ ASTContext::setInstantiatedFromUsingShadowDecl(UsingShadowDecl *Inst,
   InstantiatedFromUsingShadowDecl[Inst] = Pattern;
 }
 
-FieldDecl *ASTContext::getInstantiatedFromUnnamedFieldDecl(FieldDecl *Field) {
+FieldDecl *
+ASTContext::getInstantiatedFromUnnamedFieldDecl(FieldDecl *Field) const {
   return InstantiatedFromUnnamedFieldDecl.lookup(Field);
 }
 
 void ASTContext::setInstantiatedFromUnnamedFieldDecl(FieldDecl *Inst,
                                                      FieldDecl *Tmpl) {
-  assert(!Inst->getDeclName() && "Instantiated field decl is not unnamed");
-  assert(!Tmpl->getDeclName() && "Template field decl is not unnamed");
+  assert((!Inst->getDeclName() || Inst->isPlaceholderVar(getLangOpts())) &&
+         "Instantiated field decl is not unnamed");
+  assert((!Inst->getDeclName() || Inst->isPlaceholderVar(getLangOpts())) &&
+         "Template field decl is not unnamed");
   assert(!InstantiatedFromUnnamedFieldDecl[Inst] &&
          "Already noted what unnamed field was instantiated from");
 

clang/lib/Sema/SemaExpr.cpp

@@ -5560,6 +5560,24 @@ ExprResult Sema::BuildCXXDefaultArgExpr(SourceLocation CallLoc,
                                    Init, InitializationContext->Context);
 }
 
+static FieldDecl *FindFieldDeclInstantiationPattern(const ASTContext &Ctx,
+                                                    FieldDecl *Field) {
+  if (FieldDecl *Pattern = Ctx.getInstantiatedFromUnnamedFieldDecl(Field))
+    return Pattern;
+  auto *ParentRD = cast<CXXRecordDecl>(Field->getParent());
+  CXXRecordDecl *ClassPattern = ParentRD->getTemplateInstantiationPattern();
+  DeclContext::lookup_result Lookup =
+      ClassPattern->lookup(Field->getDeclName());
+  auto Rng = llvm::make_filter_range(
+      Lookup, [](auto &&L) { return isa<FieldDecl>(*L); });
+  if (Rng.empty())
+    return nullptr;
+  // FIXME: this breaks clang/test/Modules/pr28812.cpp
+  // assert(std::distance(Rng.begin(), Rng.end()) <= 1
+  //       && "Duplicated instantiation pattern for field decl");
+  return cast<FieldDecl>(*Rng.begin());
+}
+
 ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
   assert(Field->hasInClassInitializer());
 
@@ -5588,15 +5606,8 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
     // Maybe we haven't instantiated the in-class initializer. Go check the
     // pattern FieldDecl to see if it has one.
     if (isTemplateInstantiation(ParentRD->getTemplateSpecializationKind())) {
-      CXXRecordDecl *ClassPattern = ParentRD->getTemplateInstantiationPattern();
-      DeclContext::lookup_result Lookup =
-          ClassPattern->lookup(Field->getDeclName());
-
-      FieldDecl *Pattern = nullptr;
-      for (auto *L : Lookup) {
-        if ((Pattern = dyn_cast<FieldDecl>(L)))
-          break;
-      }
+      FieldDecl *Pattern =
+          FindFieldDeclInstantiationPattern(getASTContext(), Field);
       assert(Pattern && "We must have set the Pattern!");
       if (!Pattern->hasInClassInitializer() ||
           InstantiateInClassInitializer(Loc, Field, Pattern,

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -1352,7 +1352,7 @@ Decl *TemplateDeclInstantiator::VisitFieldDecl(FieldDecl *D) {
   if (Invalid)
     Field->setInvalidDecl();
 
-  if (!Field->getDeclName()) {
+  if (!Field->getDeclName() || Field->isPlaceholderVar(SemaRef.getLangOpts())) {
     // Keep track of where this decl came from.
     SemaRef.Context.setInstantiatedFromUnnamedFieldDecl(Field, D);
   }

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -1547,7 +1547,8 @@ void ASTDeclReader::VisitFieldDecl(FieldDecl *FD) {
   else if (Bits & 1)
     FD->setBitWidth(Record.readExpr());
 
-  if (!FD->getDeclName()) {
+  if (!FD->getDeclName() ||
+      FD->isPlaceholderVar(Reader.getContext().getLangOpts())) {
     if (auto *Tmpl = readDeclAs<FieldDecl>())
       Reader.getContext().setInstantiatedFromUnnamedFieldDecl(FD, Tmpl);
   }

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1038,7 +1038,7 @@ void ASTDeclWriter::VisitFieldDecl(FieldDecl *D) {
   else if (D->BitField)
     Record.AddStmt(D->getBitWidth());
 
-  if (!D->getDeclName())
+  if (!D->getDeclName() || D->isPlaceholderVar(Writer.getLangOpts()))
     Record.AddDeclRef(Context.getInstantiatedFromUnnamedFieldDecl(D));
 
   if (D->getDeclContext() == D->getLexicalDeclContext() &&

clang/test/SemaCXX/cxx2c-placeholder-vars.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang -cc1 -fsyntax-only -verify -std=c++2c -Wunused-parameter -Wunused -Wpre-c++26-compat %s
+// RUN: %clang_cc1 -fsyntax-only -verify -ast-dump -std=c++2c -Wunused-parameter -Wunused -Wpre-c++26-compat %s | FileCheck %s
 
 void static_var() {
     static int _; // expected-note {{previous definition is here}} \
@@ -254,3 +254,37 @@ namespace Bases {
         }
     };
 }
+
+namespace GH114069 {
+
+template <class T>
+struct A {
+    T _ = 1;
+    T _ = 2;
+    T : 1;
+    T a = 3;
+    T _ = 4;
+};
+
+void f() {
+    [[maybe_unused]] A<int> a;
+}
+
+// CHECK: NamespaceDecl {{.*}} GH114069
+// CHECK: ClassTemplateSpecializationDecl {{.*}} struct A definition
+// CHECK: CXXConstructorDecl {{.*}} implicit used constexpr A 'void () noexcept'
+// CHECK-NEXT: CXXCtorInitializer Field {{.*}} '_' 'int'
+// CHECK-NEXT: CXXDefaultInitExpr {{.*}} 'int' has rewritten init
+// CHECK-NEXT: IntegerLiteral {{.*}} 'int' 1
+// CHECK-NEXT: CXXCtorInitializer Field {{.*}} '_' 'int'
+// CHECK-NEXT: CXXDefaultInitExpr {{.*}} 'int' has rewritten init
+// CHECK-NEXT: IntegerLiteral {{.*}} 'int' 2
+// CHECK-NEXT: CXXCtorInitializer Field {{.*}} 'a' 'int'
+// CHECK-NEXT: CXXDefaultInitExpr {{.*}} 'int' has rewritten init
+// CHECK-NEXT: IntegerLiteral {{.*}} 'int' 3
+// CHECK-NEXT: CXXCtorInitializer Field {{.*}} '_' 'int'
+// CHECK-NEXT: CXXDefaultInitExpr {{.*}} 'int' has rewritten init
+// CHECK-NEXT: IntegerLiteral {{.*}} 'int' 4
+// CHECK-NEXT: CompoundStmt {{.*}}
+
+}