Here are the grammars extracted from https://github.com/cplusplus/draft/ and converted to an EBNF understood by https://www.bottlecaps.de/rr/ui to generate navigable railroad diagrams.
The g++-3.3.6 grammar (the last using yacc/bison).
The Elsa C++ grammar.
The open-watcom-v2 grammar.
The cfront grammars.
The rose-compiler experimental-c++-parser grammar.
The cppparser satya-das-cppparser grammar.
The cppfront grammar.
The carbon-lang grammar.
There is also a had written C++ parser here.