Error should be raised for public typealias with internal generic parameter

[CrazyFanFan]

Description

When using the following Swift code:

protocol Foo {
    var foo: Int { get }
}

public typealias Alias<T: Foo> = (T) -> Void

A warning is issued:

Type alias should not be declared public because its generic parameter uses an internal type.

However, I believe this should be an error instead of a warning.

Upon examining the source code, I discovered a FIXME comment indicating that there are plans to promote this warning to an error in a future -swift-version break:

// swift/lib/Sema/TypeCheckAccess.cpp:line:491
// FIXME: Promote these to an error in the next -swift-version break.
if (isa<SubscriptDecl>(ownerDecl) || isa<TypeAliasDecl>(ownerDecl))
    downgradeToWarning = DowngradeToWarning::Yes;

Furthermore, when BUILD_LIBRARY_FOR_DISTRIBUTION is set to YES, this issue leads to module verifier failures. The inconsistent behavior between the two scenarios adds to the problem.

Reproduction

protocol Foo {
    var foo: Int { get }
}

public typealias Alias<T: Foo> = (T) -> Void

Expected behavior

The warning should be upgraded to an error for the public typealias with an internal generic parameter. This change would ensure consistency and prevent module verifier failures when building libraries for distribution.

Environment

swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0a

Additional information

No response