Try to fix #24573: Don't treat SAM class with imcompatible Unit result types as a platform SAM

[noti0na1]

Try to fix #24573, not sure if this is a correct fix.

Try a more strict check for platform SAM

[noti0na1]

Still WIP

[Copilot]

Pull request overview

This PR fixes issue #24573 by adding stricter checks for platform SAM (Single Abstract Method) compatibility. The core change prevents SAM classes with incompatible Unit result types (or other types that LambdaMetaFactory cannot auto-adapt) from being treated as platform SAMs, requiring them to be expanded to anonymous classes instead.

Key changes:

• Added samDoesNotNeedExpansion method in TypeErasure to check if LambdaMetaFactory can handle required signature adaptations
• Integrated the new check into JavaPlatform.isSam to filter out incompatible SAM types
• Added comprehensive test coverage for various SAM type scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
compiler/src/dotty/tools/dotc/core/TypeErasure.scala	Adds samDoesNotNeedExpansion method that checks whether overridden methods have compatible erased signatures for LambdaMetaFactory adaptation; converts object declaration to Scala 3 indentation style
compiler/src/dotty/tools/dotc/config/JavaPlatform.scala	Integrates the new SAM expansion check into isSam method; adds import statement
tests/run/i24573.scala	Comprehensive test file with various SAM trait configurations testing parameter and result type adaptations
tests/run/i24573.check	Expected output for the test cases

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The documentation comment is misleading. Line 593-594 states "This method returns true if any overridden method has an incompatible erased signature that LMF cannot auto-adapt", but the actual behavior is the opposite: the method returns true when LambdaMetaFactory CAN handle all overridden methods (i.e., when the SAM does NOT need expansion).

The contradiction is between "any overridden method has an incompatible erased signature" (which would indicate a problem) and "the SAM class does not need to be expanded" (line 600), which would be the opposite.

The correct description should be: "This method returns true if all overridden methods have compatible erased signatures that LMF can auto-adapt (or don't need adaptation)."

Suggested change

	* for LambdaMetaFactory to work. This method returns true if any overridden method
	* has an incompatible erased signature that LMF cannot auto-adapt.
	* for LambdaMetaFactory to work. This method returns true if all overridden methods
	* have compatible erased signatures that LMF can auto-adapt (or don't need adaptation).

[Copilot]

The import Phases.* appears to be unused in this file. Consider removing it if it's not needed for the new functionality.

Suggested change

import Phases.*