↓これなら複数 API の戻り値型間で代入できました。

protocol Foo {} extension Int: Foo {} protocol P { associatedtype F: Foo func makeFoo(from: Int) -> F func foo() -> F } struct S: P { func makeFoo(from: Int) -> __opaque Foo { return from } func foo() -> F { return makeFoo(from: 42) } } let s = S() var a = s.foo() let b = s.makeFoo(from: -1) print(a) a = b print(a)

42 -1