Today ParenType is used:
1. As the type of ParenExpr
2. As the payload type of an unlabeled single
associated value enum case (and the type of
ParenPattern).
3. As the type for an `(X)` TypeRepr
For 1, this leads to some odd behavior, e.g the
type of `(5.0 * 5).squareRoot()` is `(Double)`. For
2, we should be checking the arity of the enum case
constructor parameters and the presence of
ParenPattern respectively. Eventually we ought to
consider replacing Paren/TuplePattern with a
PatternList node, similar to ArgumentList.
3 is one case where it could be argued that there's
some utility in preserving the sugar of the type
that the user wrote. However it's really not clear
to me that this is particularly desirable since a
bunch of diagnostic logic is already stripping
ParenTypes. In cases where we care about how the
type was written in source, we really ought to be
consulting the TypeRepr.
This removes the distinction between argument completions and postfix expr paren completions, which was meaningless since solver-based completion.
It then determines whether to suggest the entire function call pattern (with all argument labels) or only a single argument based on whether there are any existing arguments in the call.
For this to work properly, we need to improve parser recovery a little bit so that it parsers arguments after the code completion token properly.
This should make call pattern heuristics obsolete.
rdar://84809503
