Activity

1. llvmorg-github-actions

   added

   clangClang issues not falling into any other categoryClang issues not falling into any other category

   on Nov 12, 2026
2. frederick-vs-ja

   added

   clang:modulesC++20 modules and Clang Header ModulesC++20 modules and Clang Header Modules

   crash-on-valid

   and removed

   clangClang issues not falling into any other categoryClang issues not falling into any other category

   on Nov 12, 2026
3. frederick-vs-ja

   added the

   Bug

   issue type on Nov 12, 2026
4. llvmorg-github-actions commented on Nov 12, 2026

   

   llvmorg-github-actionsBot

   on Nov 12, 2026

   More actions

   @llvm/issue-subscribers-clang-modules

   Author: Denis Yaroshevskiy (DenisYaroshevskiy)

   Details

   Reproduction on godbolt: trunk https://godbolt.org/z/Kr1YdG5dd 23.1.0: https://godbolt.org/z/z9vr9PW4v

   Below is the explanation from AI

   Clang crash in CodeGen with C++20 modules: auto member function of a class template

   Clang segfaults during IR generation on valid code. CodeGen emits a call to
   Inner<unsigned long>::size(), but on the declaration it uses, the auto return type
   was never deduced. Clang then reads a null type while classifying the return type.

   Godbolt: <link>

   Reproducer

   CMakeLists.txt

   cmake_minimum_required(VERSION 3.28)
   project(repro CXX)
   set(CMAKE_CXX_STANDARD 20)
   add_library(repro OBJECT main.cpp)
   target_sources(repro PUBLIC FILE_SET CXX_MODULES FILES
     table.cppm use_size.cppm holder.cppm user.cppm)

   table.cppm

   export module table;
   export template <class> struct Inner {
     auto size() { return 0; }
   };
   export template <class T> struct Table {
     Inner<decltype(sizeof(T))> inner;
     constexpr Table() { size(); }
     void size() { inner.size(); }
   };

   use_size.cppm

   export module use_size;
   import table;
   export inline void useSize(Table<int>& t) { t.size(); }

   holder.cppm

   export module holder;
   export import table;
   export struct Holder { Table<int> t; };

   user.cppm

   export module user;
   import use_size;
   import holder;
   export struct A { Table<char> t{}; };

   main.cpp

   import holder;
   struct B { Table<char> t{}; };
   import user;
   void f() { B b; }

   Crash

   Per-file LLVM IR generation
   table.cppm:8:8: Generating code for declaration 'Table<char>::size'
    clang::QualType::getTypePtr() const
    clang::CodeGen::isAggregateTypeForABI(clang::QualType)
    X86_64ABIInfo::classifyReturnType(clang::QualType) const
    clang::CodeGen::CodeGenTypes::arrangeCXXMethodDeclaration(clang::CXXMethodDecl const*)
    clang::CodeGen::CodeGenFunction::EmitCXXMemberOrOperatorMemberCallExpr(...)
   
5. meta-codesync

   added a commit that references this issue on Nov 12, 2026

   Explicit return types for size/chunkShift/chunkCount to work around a…

   ...

   78ed36e