> How much of this is because nobody puts effort into these optimizations?
restrict is rare in C and C++ but common in Fortran; array parameters in Fortran aren't allowed to alias. Intel and IBM both have great Fortran compilers so I would expect their C and C++ compilers to have good support for restrict.
The Rust compiler has repeatedly found critical bugs in LLVM's restrict / noalias support, bugs that would impact C / C++ as well if any real-world C / C++ programs actually used it.
If compilers produce straight-up broken code in these situations, I can only imagine they're not putting a lot of effort into these optimization strategies.