Fix "Spurious shadowing warning with -Wshadow:type-parameter-shadow"

[lihaoyi]

Attempts to fix #24582. One-shot vibe coded

Root Cause: In CheckShadowing.scala, the isAnImportedType method had a bug at line 280:

val simpleSelections = qual.tpe.member(sym.name).alternatives

This looked up the type parameter's name (S) directly on the import qualifier (scala.compiletime.ops.int) regardless of what was actually imported. Even when only + was imported (import int.+), it would still find S in the int object and report it as shadowed.

Fix: Only perform this lookup when there's a wildcard import:

val hasWildcard = sels.exists(_.isWildcard)
val simpleSelections = if hasWildcard then qual.tpe.member(sym.name).alternatives else Nil

Test cases verify:

1. Specific import (import int.+) does NOT warn for unrelated type params - fixed
2. Wildcard import (import MyModule.*) DOES warn - still works
3. Explicit type import (import MyModule.A) DOES warn - still works
4. Renamed import (import MyModule.{A => RenamedA}) does NOT warn for original name - still works
5. Multiple selectors work correctly - still works

[som-snytt]

This is on my list because it is an occasion to refactor.

[Gedochao]

This is on my list because it is an occasion to refactor.

Or just refactor for refactor's sake, separately 😅
Since it's been on your mind, thoughts on this solution?

[som-snytt]

Thanks, another way to say it is that this is an excuse to look at that code!