Safer Exceptions for Scala 3

[hamzaremmal]

This PR improve the implementation of safer exceptions as introduced in Scala 3.1 (see PR #11721 by @odersky).

The suggested implementation completes the safer exceptions project by adding missing checks. The following changes have been added:

• Changes to the syntax to allow throws clauses in function definition. When enabling the safer exceptions feature, the following function definition is allowed : def foo throws Exception = ???

• Fix overriding problems when functions throws exceptions

• Drop contextual parameter lists to bring a CanThrow capabilities into a certain scope

• Favor contextual functions and union types when desugaring throws clauses

• Change the semantics of implicit search of CanThrow capabilities

• Add to the bytecode the exceptions a function might throw

• Require a CanThrow capability for all function annotated with the @throws annotation

• Add override checks for methods with @throws annotations

• Implement special handling for CanThrow capabilities for main-methods

• Change the semantics of the $throws infix type to reflect the changes

• Add "ghost" capabilities to the corresponding capture set

• Syntax changes - throws clauses : When enabled, the following code is accepted

import scala.language.experimental.saferExceptions

class A extends Exception
class B extends Exception

// New syntax to declare checked exceptions
def foo throws A, B = ???

// Declare checked exceptions using $throws infix type
def foo : Unit throws A | B = ???

// Desugared representation of both syntaxes
def foo : CanThrow[A | B] ?=> Unit = ???

• @throws annotation : The following function declarations are equivalent

@throws[Exception]
def foo : Unit = ???

def foo throws Exception : Unit = ???

Please note that this PR is part of my semester project in @lampepfl. The project was done under the supervison of @abgruszecki and @odersky.

[hamzaremmal]

This PR should close issue #15510

[tgodzik]

@hamzaremmal do you plan to work on this again? Could we close it for now otherwise?