A C++23 version of Eric Schmidt's Metamath database verifier which can run at compile-time using the constexpr-std-headers branch of GCC found here; and referred to as C'est 2.

Differences to the Original

1. Added the constexpr qualifier to all subroutines;
2. Changed all functions, to member functions (methods) of a trivial struct (called checkmm). Global variables (including constexpr ones) cannot be modified at compile-time;
3. Changed the static variable names within readtokens to a class member of checkmm. While constexpr static variables are now permitted in constexpr functions, they are constant, and names needs to be modified;
4. constexpr file IO is not possible, and readtokens now accepts a second string parameter, which is used if it isn't empty. An std::istringstream (from C'est 2) is then used to feed nexttoken, rather than an std::ifstream. P2448 from C++23 also allows non-constexpr file IO, when not on the control path taken by the constexpr evaluator;
5. File-includes within mm database files are not supported when processing at compile-time. An exception is thrown if this occurs; rem 6. Headers included are from the cest library, so #include "cest/vector.hpp" rather than #include <vector> etc. rem 7. Rather than replace names qualified std:: with cest::, a namespace alias ns is used throughout; declared at global scope, and set to cest by default.
6. A macro system is leveraged, where a pair of C++11-style raw string literal delimiters are placed before and after the original contents of a file. The delimit.sh bash script is provided to help with this. To use this approach, set the preprocessor macro MMFILEPATH to the script's output, during compilation of ctcheckmm-std.cpp (e.g. -DMMFILEPATH=miu.mm.raw).

In Practise

The delimit.sh script adds R"#( before, and )#" after, the contents of its input file. The output file name appends .raw to the name of the input. Within ctcheckmm-std.cpp a #include directive will bring that (raw string) file in; as a std::string. This happens if MMFILEPATH is set to a valid file path. For example, with a recent version of clang++ (e.g. Clang 18):

wget https://raw.githubusercontent.com/metamath/set.mm/develop/peano.mm
bash delimit.sh peano.mm
clang++ -std=c++23 -Winvalid-constexpr -Wl,-rpath,"$CEST2_ROOT/lib64:$LD_LIBRARY_PATH" -I $CEST2_ROOT/constexpr-std-headers/include/c++/14.0.1 -I $CEST2_ROOT/constexpr-std-headers/include/c++/14.0.1/x86_64-pc-linux-gnu -L $CEST2_ROOT/lib64 -D_GLIBCXX_CEST_CONSTEXPR=constexpr -D_GLIBCXX_CEST_VERSION=1 -fsanitize=address -fconstexpr-steps=$((2**32-1)) -DMMFILEPATH=peano.mm.raw ctcheckmm-std.cpp

With G++, a recent version is required (e.g. GCC 14), and different switches enables a non-default constexpr operation count limit:

wget https://raw.githubusercontent.com/metamath/set.mm/develop/peano.mm
bash delimit.sh peano.mm
g++ -std=c++23 -Winvalid-constexpr -Wl,-rpath,"$CEST2_ROOT/lib64:$LD_LIBRARY_PATH" -I $CEST2_ROOT/constexpr-std-headers/include/c++/14.0.1 -I $CEST2_ROOT/constexpr-std-headers/include/c++/14.0.1/x86_64-pc-linux-gnu -L $CEST2_ROOT/lib64 -D_GLIBCXX_CEST_CONSTEXPR=constexpr -D_GLIBCXX_CEST_VERSION=1 -fsanitize=address -static-libasan -fconstexpr-ops-limit=$((2**31-1)) -fconstexpr-loop-limit=$((2**31-1)) -DMMFILEPATH=peano.mm.raw ctcheckmm-std.cpp

Successful compilation (with either compiler) indicates that the Metamath database was verified. The wget commands above relate to the repository.