[cling][c++20] Interpreter crash on concept definition

[jalopezg-git]

Describe the bug

An interactive ROOT session crashes on definition of a simple C++20 concept, e.g.

template <typename T> concept Hashable = requires(T a) {
   { std::hash<T>{}(a) } -> std::convertible_to<std::size_t>;
};

For reference, the relevant stack trace frames is provided below.

#5  0x00007f388ee59180 in cling::isDefinition(clang::Decl const*) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#6  0x00007f388ee59c60 in cling::DefinitionShadower::invalidatePreviousDefinitions(clang::NamedDecl*) const () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#7  0x00007f388ee5b37b in cling::DefinitionShadower::Transform(clang::Decl*) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#8  0x00007f388ee509c2 in cling::DeclCollector::TransformDecl(clang::Decl*) const () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#9  0x00007f388ee50b75 in cling::DeclCollector::Transform(clang::DeclGroupRef&) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#10 0x00007f388ee50d41 in cling::DeclCollector::HandleTopLevelDecl(clang::DeclGroupRef) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#11 0x00007f388ee11089 in cling::IncrementalParser::ParseInternal(llvm::StringRef) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#12 0x00007f388ee118ac in cling::IncrementalParser::Compile(llvm::StringRef, cling::CompilationOptions const&) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#13 0x00007f388ed6e87c in cling::Interpreter::DeclareInternal(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, cling::CompilationOptions const&, cling::Transaction**) const () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#14 0x00007f388ed71e7e in cling::Interpreter::process(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, cling::Value*, cling::Transaction**, bool) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#15 0x00007f388ee6443f in cling::MetaProcessor::process(llvm::StringRef, cling::Interpreter::CompilationResult&, cling::Value*, bool) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so
#16 0x00007f388ec6fd0c in HandleInterpreterException(cling::MetaProcessor*, char const*, cling::Interpreter::CompilationResult&, cling::Value*) () from /home/jalopezg/CERN/root/_build.cxx20/lib/libCling.so

Setup

1. ROOT version: master
2. Operating system: ArchLinux x86_64