When subst:ed, this template returns parameter then, else parameter else.

Code Return Unparsed return
{{ifsubst|then=foo|else=bar}}
bar {{ifsubst|then=foo|else=bar}}
{{subst:ifsubst|then=foo|else=bar}}
foo {{{{{1|}}}#if:|bar|foo}}
{{subst:ifsubst|then=foo|else=bar|subst:}}
foo foo