[friendlydude12]

I’m not saying that 1<<x is generally meaningless. I’m saying that whenever the shift operand is negative, it’s meaningless. Just like log(x) isn’t generally meaningless, it’s only meaningless when the argument is not positive.

Syntactic constructs in general should and do have meaning. The meaning isn’t inherent, but nothing has inherent meaning. log(x) has no inherent meaning, but when x is positive we have decided that it means that it evaluates to the value to which you would raise 10 to get x.

Bad run-time values only cause UB in the context of improper usage in syntactic constructs. They aren’t independent of syntactic constructs: both syntactic constructs and values dictate behavior.

You’re right that UB only manifests itself at run-time. And you’re right that we wouldn’t need to use UB for constructs that only manifest their behavior at compile-time. Indeed, there exists syntactically valid yet meaningless C++ template substitutions that don’t result in UB, they result in compilation errors.