Missing completions when give a backticked prefix in various positions

[ckipp01]

Compiler version

3.2.0-RC1-bin-20220307-6dc591a-NIGHTLY-git-6dc591a

Explanation

As a follow-up to the work done in #14594 there are still a couple situations that still don't offer you a completions if your preface already includes an opened backtick.

Minimized code

The first case be illustrated by the following:

val `foo-bar` = 3

val x = `foo<TAB>

In this situation no completion is offered because the tree that is contained for the current position is Literal(Constant(null)). This seems to come from:

https://github.com/lampepfl/dotty/blob/808c66925daa518294e15419dffec9b7c1490848/compiler/src/dotty/tools/dotc/parsing/Parsers.scala#L386

The second situation is when this is a type instead of a term like so:

type Foo = `Lis<TAB>

The tree that is returned here is TypeDef(Foo,Ident(<error>))

One final example is:

val `foo-bar` = 3

`foo<TAB>

In this situation Interactive.patTo just returns the entire document.

Expectation

I'd expect the correct backticked completions to be offered in all 3 of these examples.

Potential solutions

The first way I tried to solve this was by adding specific cases for the first two in various places to catch those trees. For example in:

https://github.com/lampepfl/dotty/blob/808c66925daa518294e15419dffec9b7c1490848/compiler/src/dotty/tools/dotc/interactive/Completion.scala#L61-L79
and
https://github.com/lampepfl/dotty/blob/808c66925daa518294e15419dffec9b7c1490848/compiler/src/dotty/tools/dotc/interactive/Completion.scala#L85-L110
and
https://github.com/lampepfl/dotty/blob/808c66925daa518294e15419dffec9b7c1490848/compiler/src/dotty/tools/dotc/interactive/Completion.scala#L128-L137

While this works for those two situations it's quite a bit of code for very specific scenarios, and we're probably missing some as well that would also need to be added in. An alternative proposed by @bishabosha here is as follows:

We had a small discussion and it could be worth trying to make a new Token for an unfinished backticked identifier, which we can then make a valid tree for