C++ is the last language I'd add to any list of languages used for correct-by-design - it's underspecified in terms of semantics with huge areas of UB and IB. Given its vast complexity - at every level from the pre-processor to template meta-programming and concepts, I simply can't imagine any formal denotational definition of the language ever being developed. And without a formal semantics for the language, you cannot even start to think about proof of correctness.

As with Spark, proving properties over a subset of the language is sufficient. Code is written to be verified; we won’t be verifying interesting properties of large chunks of legacy code in my career span. The C (near-) subset of C++ is (modulo standard libraries) a starting point for this; just adding on templates for type system power (and not for other exotic uses) goes a long way.