Restrict implicit args to using

[som-snytt]

Typer#adaptNoArgsImplicitMethod populates implicit args when an arg list is missing. To remedy missing implicits, it tries a named application using args it did find. Then Applications#tryDefault supplies a default arg if available. A previous fix to allow tryDefault to supply implicit args for implicit params is now restricted to explicit using; typer now adds using for implicit when it needs to try defaults.

This commit restores propagatedFailure and the previous condition that default params are tried if there is an error that is not an ambiguity. An additional restriction is that default params must be useful: there must be a param which has a default arg to be added (because it's not a named arg).

Fixes #22439

[som-snytt]

Aspirationally,

Since issueErrors specifies endPos, the position used for
implicitArgTree by matchArgs is also tweaked to be the endPos
(of the appPos). That places a single caret in the familiar
spot following the existing application, instead of underlining.

That needs a mechanism for distinguishing user's using from typing for default args, where user expr requires underlining, but the latter case wants trailing caret.

[EugeneFlesselle]

I'm in my exam period until 05/02, so I won't be able to review before then. I'll be happy to take a look once I can though.

[som-snytt]

@mbovel you were in this area last spring, and the LOC I restored will be familiar to you. (Maybe over-familiar.) If you wouldn't mind revisiting it for a review? This commit tweaks Eugene's previous fix by tweaking your previous fix.