That would be easy, the problem is that Rice's Theorem tells us the compiler definitionally cannot be sure whether every possible source code represents a valid

That would be easy, the problem is that Rice's Theorem tells us the compiler definitionally cannot be sure whether every possible source code represents a valid program if it has any non-trivial semantic requirements (C++, like Rust, has a whole lot of such requirements). So while "What should we do when we're sure it's not a valid program?" is easy and so it's very silly that some C++ compilers will compile that anyway, it's not enough, you also need to decide "What should we do when we're not sure?" and there the ISO document provides a firm answer, which all the compiler vendors follow - press on anyway, too bad. "Ill-formed - No Diagnostic Required" is how the ISO document describes this situation. The diagnostic isn't required because it's technically impossible to guarantee. You might insist that "Not sure" can't happen. Unfortunately the reason it's called Rice's Theorem is that a guy named Henry Rice got himself a Maths PhD proving this in 1951.