Re: Identifying function-lookup failures due to argument name mismatches

I wrote:
> Another thing not to like is that it seems like this is doing violence
> to several APIs in exchange for not very much improvement in the error
> messages. I feel like maybe we ought to be trying for more
> specificity about additional cases, but I'm not very sure what else
> to improve or what the API could look like.

I couldn't quite let go of this, and after some thought I hit on the
idea of making FuncnameGetCandidates pass back a bitmask of flags
showing how far the match succeeded. This seems to work pretty
nicely, allowing quite-detailed reports with only minimal added
overhead or code restructuring. There's probably room for further
improvement, but it has less of a whiff of "quick single-purpose
hack". See draft commit message for more details.

regards, tom lane