[Cpp] Mapping dataflow nodes to their basic blocks. #21074

Hi @jackfromeast 👋🏻 Thanks for reaching out.

There are indeed lots of dataflow nodes that do not map to expressions (and hence to BasicBlocks). An obvious example specific to C/C++ are the ones that correspond to indirect expressions (e.g. implicit casts), but all languages have nodes that do not map to an expression (post-update nodes, global variable nodes, etc.)

The C++ DataFlow library builds on top of "IR", a lower-level SSA-like intermediate representation that the CodeQL C++ library computes from the C++ AST; this gives us a more explicit representation of C++ code than the AST does. IRBlocks are part of the IR layer, while BasicBlocks are part of the AST layer. So the recommendation is to ignore BasicBlocks and stick with IRBlocks as much as possible; they serve the same purpose and are more correct.

Can you give me an idea of what information you need from BasicBlocks that IRBlocks don't give you?