Tried using tagged release fish 3.0.2 as well as current master, built from source on Fedora 29, running in Gnome Terminal.

Problem

The command true && and true sets the exit code to 1 but is otherwise not indicated to be misbehaving.

Changing the command to true && and echo word reveals that the final statement is never executed (nothing is printed). Running true && and exits with code 2 instead and prints the help information for and. Running true && or exits with code 2 and prints the help info for or.

Similar results are observed with ||. false || and and false || or exit with code 2 and print the help info for and or or.

In all these cases, a command placed after the and or or is never executed.

Expected behaviour

I think that these two constructs shouldn't be allowed to be mixed in this way, as the results are unintuitive. The parser should expressly reject and or or when they follow && or ||, to prevent users accidentally creating structures that don't behave as they may anticipate, with commands that never execute.