foo1.cs

      var genericType = myIsAsync
        ? useEnumeratorType
          ? predefinedType.IAsyncEnumerator
          : predefinedType.IAsyncEnumerable
        : useEnumeratorType
          ? predefinedType.GenericIEnumerator
          : predefinedType.GenericIEnumerable;

foo2.cs

      var genericType = (myIsAsync, useEnumeratorType) switch {
          (true, true) =>  predefinedType.IAsyncEnumerator,
          (true, false) => predefinedType.IAsyncEnumerable,
          (false, true) => predefinedType.GenericIEnumerator,
          (false, false) => predefinedType.GenericIEnumerable
        };