Pat var must be isVarPattern

[som-snytt]

Fixes #25700

How much have you relied on LLM-based tools in this contribution?

But can one rely on LLM-based tools?

How was the solution tested?

Apparently, there was not a test for val def with stable identifier pattern. The ticket minimization supplies it. Edit: a previous test was erroring incorrectly but flew under the radar. Its correct error is restored.

Additional notes

Follow-up to #25566 that tweaked #24972.

Obviously, a pattern variable must be isVarPattern, but the degrees of freedom are lowercase, underscore, and backquoted.

The code comment, that "royal we" want wildcards, suggests our bases are not covered. IdPattern is for parsing and would not be useful here. But how do I ask whether I'm looking at a patvar? There is an attachment. Are we too early or late for it?

"One does not simply ask if an identifier is a pattern variable."

[som-snytt]

I was mistaken, the test was the one that used to error for each subpattern.

-- [E045] Cyclic Error: /home/amarki/projects/scala3/tests/neg/i4935.scala:2:7 -
2 |  val (A, B) = () // error
  |       ^
  |       Recursive value $2$ needs type

The previous message was

-- Error: tests/neg/i4935.scala:2:6 --------------------------------------------
2 |  val (A, B) = () // error
  |      ^^^^^^
  |      cannot test if value of type Unit is a reference of class Tuple2

The text was not tested, so the change went unnoticed.

This commit restores erroring twice with "not found".

[SolalPirelli]

Thanks!