Qt and wxWidgets are also worth considering assuming you would just be "writing" the code and not taking advantage of the visual designers. These do have visual designers, but Lazarus if far easier to use, IMHO, following the footsteps of Delphi and VB classic.
https://cppcast.com/cpp-builder/ - this is the podcast where embarcadero devs tell about how they maintain number of patches to llvm's compiler to make it compatible with necessary non-standard features like closures.
this also tells us that c++ became so big and complicated that one company (embarcadero size company) is not able anymore to support own compiler, they need to maintain own patches.
If you look at C++ standard support in major compilers, you'll find that no compiler on earth supports the whole C++20 standard. Gcc is the one that gets closest, but still has several "partially supported" features.
You can have your C/C++ code exposed to Free Pascal via a C API loadable as a shared library / DLL, but beyond that there isn't much you can do (Free Pascal comes with a `h2pas` tool to help convert C headers to Free Pascal units for such use).
This is of course far from having something like C++ Builder.
To have something like C++ Builder it'd need a lot of things to work together:
1. A C++ compiler will need to be extended so it can use Free Pascal classes (in terms of memory layout) and add any necessary language functionality for "published" sections (RTTI data that allows FPC objects to describe themselves and be streamed/serialized), metaclasses (classes in FPC are objects and can have their own virtual methods, including virtual constructors), delegates (basically "fat" pointers) as well as data types like sets, strings, etc that FPC has. LCL (Lazarus' framework) relies heavily on pretty much all of FPC features.
2. FPC will need to somehow be able to import classes defined on the C++ side too so that components written in C++ will be usable in Lazarus. Also almost goes without saying but it should be possible to statically link programs using both FPC and C++ in the same executable.
3. The C and C++ runtime libraries will need to use FPC runtime library functionality for things like memory management (FPC's memory manager is pluggable) so that "crossing" memory management will work fine - even across dynamic library boundaries (e.g. calling `malloc` on the C++ side and then "FreeMem" on the Free Pascal side, even when the latter is in a dynamic library that shares a memory manager with the host program, should Just Work).
4. Lazarus' "codetools" will need to be expanded to support parsing C++ in addition to Free Pascal so that all of the IDE's automatic code writing and refactoring functionality will work for C++ too - things as simple as double clicking on a button and having the methods be written automatically in the editor with the cursor placed in the method body rely on this working.
These are from the top of my head, there might be other issues to handle, but even from the above it should be clear that it requires not only a lot of work but also a lot of different projects to work together (unless FPC and/or Lazarus devs decide to implement their own C++ compiler and runtime).
In the short term it might be easier to use Qt Creator which has some RAD-ish features. It is certainly clunkier, the API feels to be made with a code-first (as opposed to LCL's WYSIWYG-designer-first) approach and the IDE isn't as featureful (also at least for me it never works out of the box on Linux) but it is the closest open source project you can find for C++.
