After a rule head, an expression that parses to a set term is discarded
and re-parsed as `{ BODY }`, so that `p if { true }` is a body and not a
one-element set. That reinterpretation is only correct when the
expression starts with a `{`. `p if not {...}`, `p if set()` and `p if
({1, 2})` all parse to a set, but none have a leading brace, so there is
no ambiguity with a rule body and they should be kept as a sets.
Note: `not { ... }` with `future.keywords.not` imported parses to an
explicit body, not a set term, and so was never affected.
---------
Signed-off-by: Johan Fylling <johan.dev@fylling.se>
Johan Fylling
f34d62bea0