Both your convenience function and the quotes in the type name can be removed via the use of static opDispatch:

struct FlagImpl(string name) { bool value; alias value this; }

struct Flag { alias opDispatch(string name) = FlagImpl!name; }

struct Yes { static auto opDispatch(string name)() { return FlagImpl!name(true); } }

struct No { static auto opDispatch(string name)() { return FlagImpl!name(false); } }

void fun(Flag.foo a) {} // Look ma, no quotes!

unittest { fun(Yes.foo); fun(Flag.foo(true)); }