Regression when reading a type spelled via a dependent alias

[jmcarcell]

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Description

In tests in Gaudi using ROOT's master we have found out that there are tests reading a number of tracks and they do not read what was written, making tests fail.

Bisection seems to lead to 081af22.

Reproducer (🤖)

#include <TClass.h>
#include <TInterpreter.h>
#include <TStreamerElement.h>
#include <TStreamerInfo.h>

#include <iostream>

void legacy_streamer_typename() {
  gInterpreter->Declare( R"cpp(
    #include <vector>

    namespace repro {
      struct Track {
        int value{};
      };

      template <typename T>
      struct Container {
        typedef typename std::vector<T*> seq_type;
        seq_type m_sequential;
      };
    }
  )cpp" );

  auto* klass = TClass::GetClass( "repro::Container<repro::Track>" );
  auto* info = klass ? dynamic_cast<TStreamerInfo*>( klass->GetStreamerInfo() ) : nullptr;
  Int_t offset = 0;
  auto* element = info ? info->GetStreamerElement( "m_sequential", offset ) : nullptr;
  if ( !element ) {
    std::cerr << "setup failed" << std::endl;
    return;
  }

  std::cout << "stored=" << element->GetTypeName() << std::endl;

  // Simulate ROOT rebuilding metadata read from an old file against the
  // current class layout. This is the path taken by the Gaudi file read.
  info->ResetBit( TStreamerInfo::kBuildOldUsed );
  info->BuildOld();

  std::cout << "new-type=" << element->GetNewType() << std::endl;
}

Where the key line is typedef typename std::vector<T*> seq_type. Removing typename fixes it for this macro (I haven't tested yet if it does in Gaudi).

Output in ROOT 6.40.02:

Processing legacy_streamer_typename.C...
stored=vector<repro::Track*>
new-type=300

Output in ROOT master:

Processing legacy_streamer_typename.C...
stored=typename std::vector<repro::Track*>
new-type=-2
Warning in <TStreamerInfo::BuildOld>: Cannot convert repro::Container<repro::Track>::m_sequential from type: typename std::vector<repro::Track*> to type: vector<repro::Track*>, skip element

ROOT version

6.40.02 works, master doesn't

Installation method

LCG stacks

Operating system

Linux

Additional context

No response

[ferdymercury]

Maybe related: https://its.cern.ch/jira/browse/ROOT-9113

[guitargeek]

Thanks a lot for getting the AI to build this nice reproducer! Here is the take of the AI on the solution:

• [cling][AST] Strip elaborated keyword exposed by partial desugaring #23060