Attachment: Download

Issue Description:

When calling an internal method which is marked with @usableFromInline inside a public @inlinable method but the Type on which the method is defined is internal and not marked with @usableFromInline, the compiler complains rightfully but gives an unhelpful error message.

Example code:

internal protocol InternalProtocol {}

extension InternalProtocol {
    @usableFromInline
    func internalMethod() {}
}

extension Int: InternalProtocol {}

@inlinable
public func publicMethod() {
    Int().internalMethod()
}

Error message: Instance method 'internalMethod()' is not '@usableFromInline' or public

``

The compiler should instead state that the problem is the protocol and not the method e.g.:
Protocol 'InternalProtocol' is internal and cannot be referenced from an '@inlinable' function
Protocol 'InternalProtocol' is not '@usableFromInline' or public

Example Project:
usableFromInline.zip