In bash, if I write

if foo="$(some command)"; then

then the if statement will actually conditionalize based on the exit status of $(some command). I don't see any way to get the equivalent functionality in fish. The fundamental problem is variable assignment in fish must be done via the set command, which overwrites $status, and so there is no reasonable way to capture the output of a command and its return value separately. The only unreasonable way I can think of looks like

set -l oldstatus
set -l foo (some command; set oldstatus $status)