This avoids constraint explosions in some cases, notably when working
with polydots and plain variable arity functions at the same time.
However, this also weakens inference a bit (breaks it for
unholy-terror and related tests), but not in a way that affects any
practical use cases that I know of.
original commit: 80a9027f1eee6ea56a29aa73c65675f6cf86cdb2