Unhelpful error message when trying to use named extraction, when not matching case class or named tuple

[wwbakker]

Fixes #22903

Worked on together by @jan-pieter @nmcb @RoccoMathijn @thijsnissen and me.

PR should still be finished.

[som-snytt]

The new message is emitted for

[E215] Pattern Match Error: tests/neg/named-tuples.scala:44:10

which is the unnamed tuple case in the failing test. I wonder if the text could say "this is not a named tuple".

Selector type Tuple does not define any named elements

I would be confused and say I thought they supported named tuples now.

The PR doesn't have the test from the OP.

The phrase "selector type" might be confusing. I think of selector match { case ... => } because it selects which case. I don't think of it to mean "whatever value is matched in a subpattern". (My usage may be naive.) In this case, it sounds like "mechanism".

In the OP, the selector expression was 3

+19 |    case ProductMatch(someName = x) => println (x) // error
+   |                      ^^^^^^^^^^^^
+   |                      Selector type CustomProduct does not define any named elements

I don't know if there is a term that is precise yet friendly.

[wwbakker]

Thanks @som-snytt, we (@jan-pieter , @RoccoMathijn @nmcb ) have had another look and we think we have improved the logic and error message a bit.

Next up is adding and fixing tests, and optionally looking at the underlined text in the error message. My colleagues are currently looking into that.