Don't worry; a function blocking indefinitely (i.e., there is some point where it stops giving off side effects, and never returns) is also UB. C++ attempts to guarantee a certain amount of forward progress, for now.
In theory, the compiler could know that since `printf` is a well-known standard function. In practice, `printf` might even exit the program via SIGPIPE, so I don't think any compiler will assume that it definitely will return.
Not just sigpipe. Getting sigttou is part and parcel of how command line apps share control of the tty.