`std::basic_string`'s `operator==` set invisible at template-instantiation time from PCMs

[guitargeek]

The std::basic_string's operator== set seems invisible at template-instantiation time from PCMs because bits/basic_string.h appears to be unmapped in cling's std.modulemap

Reproducer:

• Revert acc9402 from [DF] Avoid operator==(string const &, const char*) in RInterface.hxx #23481, so we're using std::basic_string's operator== in the RInterface.hxx header that ends up in the ROOTDataFrame module

• Run this code with root -q repro:

   #include "ROOT/RDataFrame.hxx"
  
   int repro() {
      // Coes doesn't even need to be executed
      if (false) {
         ROOT::RDataFrame df("t", "in.root");
         df.Snapshot("t", "out.root", {"b"});
      }
      return 0;
   }

Expected error (truncated):

Processing ../root_src/repro.C...
In module 'ROOTDataFrame':
/home/rembserj/code/root/root_build/include/ROOT/RDF/RInterface.hxx:3264:53: error:
      invalid operands to binary expression ('std::string' (aka
      'basic_string<char>') and 'const char[10]')
         if (RDFInternal::GetDataSourceLabel(*this) == "RNTupleDS" &&
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~
/home/rembserj/code/root/root_build/include/ROOT/RDF/RInterface.hxx:3379:14: note:
      in instantiation of member function
      'ROOT::RDF::RInterface<ROOT::Detail::RDF::RLoopManager>::Snapshot'
      requested here
      return Snapshot(treename, filename, selectedColumns, options);
             ^
/home/rembserj/code/root/root_src/repro.C:8:10: note: in instantiation of
      member function
      'ROOT::RDF::RInterface<ROOT::Detail::RDF::RLoopManager>::Snapshot'
      requested here
      df.Snapshot("t", "out.root", {"b"});
         ^
/home/rembserj/code/root/root_build/include/ROOT/RResultPtr.hxx:422:6: note:
      candidate template ignored: could not match 'RResultPtr' against
      'basic_string'
bool operator==(const RResultPtr<T1> &lhs, const RResultPtr<T2> &rhs)
     ^
/home/rembserj/code/root/root_build/include/ROOT/RResultPtr.hxx:434:6: note:
      candidate template ignored: could not match 'RResultPtr' against
      'basic_string'
bool operator==(const RResultPtr<T1> &lhs, std::nullptr_t rhs)
     ^

[hageboeck]

@guitargeek this seems to be somewhat of a duplicate of #23470, wouldn't you say? Maybe we can close that one?

[guitargeek]

Yes, my intention was to create a more general version of #23470 that doesn't relate to TMVA and can be handed off to the C++ modules experts, @vgvassilev and @hahnjo.

[hahnjo]

As far as I understand, the problem is not always reproducible and depends on the platform, compiler, and configuration. That information is critically missing (compared to #23470)

[guitargeek]

Good point! I'll continue the investigation by doing a CI sweep to check on which platforms my reproducer fails.

[guitargeek]

Fixed in "non applicable" because this was a temporary problem after the LLVM 22 upgrade in this development cycle.