If I have

public protocol Foo {
  @_spi(A) func foo()
}
extension Foo {
  @_spi(A) public func foo() {}
}

and a second module with

import Lib
struct A: Foo {}

I get the following error: Type 'A' does not conform to protocol 'Foo'

But the SPI protocol extension in Lib should be sufficient to satisfy the SPI protocol requirement: Either the client imports Lib with @_spi(A) in which case it would get both the requirement and the default implementation or it imports without SPI access, in which case it doesn’t see either the default implementation or the default requirement

Test.zip

rdar://147943518