Help needed: immutable struct is a type modifier, and that's wrong and broken

[FeepingCreature]

See this bug: https://issues.dlang.org/show_bug.cgi?id=20670

immutable struct S { }

static if(is(S == immutable T, T)) {
     static assert(is(S == T));
}

So what happens here is that the spec states that immutable 
struct S is "the same as if every member had been marked 
immutable." But that's not at all what the compiler actually 
does: apparently, it just stores S with an immutable modifier.

As the code shows, immutable modifiers can be stripped away. This 
is of course bad, because it means that there's a type "S", but 
there's also a type "mutable S" that can only be created by 
template specialization or Unqual, and it can't be assigned to S 
despite nominally being the exact same type.

Help?

(For now, I'll probably homebrew an Unqual that uses mixin to 
figure out when a type has a "fake immutable modifier". But I 
won't like it.)