Fix "No check for abstract methods when Erased instances are around"

[lihaoyi]

Attempts to fix #23932. Vibe coded

Problem

Extension methods with erased parameters could incorrectly be counted as implementations of non-extension abstract methods, because when erased parameters are filtered out during signature matching, the signatures could appear to match.

Fix

In compiler/src/dotty/tools/dotc/typer/RefChecks.scala, I added a check to the isImplemented function:

&& !(impl.symbol.is(ExtensionMethod) && !mbr.is(ExtensionMethod))

This ensures:

• Extension methods cannot implement non-extension abstract methods (the bug)
• Extension methods can implement abstract extension methods (legitimate use case like type classes)

Test File

Created tests/neg/i23932.scala with 9 test cases:

• Cases 1-6 (errors): Extension methods incorrectly trying to implement non-extension abstract methods
• Cases 7-9 (OK): Extension methods properly implementing abstract extension methods

Verification

• The test file produces exactly 5 expected errors
• Existing extension tests (tests/run/extension-methods.scala, tests/neg/extension-methods.scala, tests/neg/override-extension_normal-methods.scala) continue to work correctly
• The fix is consistent with existing behavior where normal methods cannot override extension methods and vice versa

[odersky]

The problem is, if the signature matches then the extension method does in fact override the abstract method by the JVM semantics. So the test cannot be simply dropped; we risk getting a wrong override instead.