Modified C++

Inspired by the paper "Some Were Meant for C" by Stephen Kell, I decided to show that it's possible to iterate C++ to be safer, more explicit, and less error-prone.

Here's a possible starting point: I didn't invent a new language or compiler, but took the world's best compiler, clang, and modified it to begin iterating towards a new furture of C++. Naming things is hard, so I call this 'Modified C++'. Some of the following could be implemented as tooling in a linter or checker, but the idea is to update the compiler directly. I also wanted to learn more about clang. This compiler needs a flag to enable/disable this functionality so that existing library code can be used with a 'diagnostic ignored' pragma.

You can build clang using the normal non-bootstrap process and you'll be left with a clang that compiles C++ but with the following modifications:

     - All basic types (excluding pointers and references) are const by
     default and may be marked 'mutable' to allow them to be changed after
     declaration
     - Lambda capture lists must be explicit (no [&] or [=], by themselves)
     - Braces are required for conditional statements, case and default
     statements within switches, and loops
     - Implicit conversions to bool are prohibited (e.g., pointers must be
     compared against nullptr/NULL)
     - No goto support
     - Explicit 'rule of six' for classes must be programmer-implemented
     (default, copy, and move c'tors, copy and move assignment, d'tor)
     - No C style casts

Here's an example program that's valid in Modified C++:

    mutable int main(int, char**)
    {
      mutable int x = 0;
      return x;
    }


    Here's another that will fail to compile:


    mutable int main(int, char**)
    {
      int x = 1;
      x = 0;  // x is constant
      return x;
    }

I'd like your feedback. Future changes I'm thinking about are:

     - feature flag for modified c++ to enable/disable with 'diagnostic ignored'
     pragma, to support existing headers and libraries
     - support enum classes only
     - constructor declarations are explicit by default
     - namespaces within classes
     - normalize lambda and free function syntax
     - your ideas here