-emit-irを使うとメソッドにどのwitness tableが渡されてるかわかるぽい。 callFoo(B()) // hello

%112 = call { i64, i64, i64 } @main.callFoo <A where A: main.Foo> (A) -> Swift.String(%swift.opaque* noalias nocapture %111, %swift.type* %21, i8** getelementptr inbounds ([1 x i8*], [1 x i8*]* @protocol witness table for main.A : main.Foo in main, i32 0, i32 0))

class C: Foo, FooStandard, FooFatal {} callFoo(C()) // fatal

 %122 = call { i64, i64, i64 } @main.callFoo <A where A: main.Foo> (A) -> Swift.String(%swift.opaque* noalias nocapture %121, %swift.type* %118, i8** getelementptr inbounds ([1 x i8*], [1 x i8*]* @protocol witness table for main.C : main.Foo in main, i32 0, i32 0))

(edited)