placement new inout issue

[TurkeyMan]

class C
{
    this(int) inout;
}
void main()
{
    void[__traits(classInstanceSize, C)] instance = void;
    C c = new(instance) C(10);
}

error : `inout` constructor `C.this` creates inout object, not mutable

This error seems wrong; the constructor signature doesn't determine the cost-ness of the constructed object; the argument does (in this case, it's a C)
Think this should work. It is conventional for constructors to be inout.

[TurkeyMan]

@WalterBright I think this is a bug?

[WalterBright]

An inout object cannot be modified, but you're assigning it to a mutable c. The inout applies to the this reference, which is the object being constructed.

The compiler is behaving correctly.

[TurkeyMan]

The input is mutable C, so shouldn't the output be mutable C? That's literally the point of inout... It should be const during the constructor, but it should resolve to match the const-ness of the input on return; that's the point no?

People always write inout constructors...

[TurkeyMan]

We mark copy constructors inout routinely; in the event it copies a mutable thing, the resulting object is mutable... no?

[TurkeyMan]

Okay, I just realised that this works:

class C
{
    this(inout int) inout;  // const-ness matches SECOND arg
}
void test()
{
    C c1 = new C(10); // `10` is mutable so `this` is mutable
}

Obviously inout needs a point of reference, and there was no point of reference in my sample.
Sorry for the noise.

This is interesting to think about though; we find inout particularly useful for copy constructors (where the point of reference is the object we're copying from), but it seems there's no similar way to have a normal constructor apply to any const-qual state... we need to implement each individual qualified constructor.