cannot call impure function ~this

[monarch_dodra]

On Tuesday, 15 October 2013 at 21:37:40 UTC, Namespace wrote:
> I get this error:
> ----
> /d701/f223.d(11): Error: pure function 'f223.getA' cannot call 
> impure function 'f223.A.~this'
> ----
>
> with this code:
> ----
> import std.stdio;
>
> struct A {
> public:
> 	~this() {
> 		writeln("DTor");
> 	}
> }
>
> A getA() pure nothrow {
> 	return A();
> }
>
> void main()
> {
> 	A a = getA();
> 	
> 	writeln("end of main");
> }
> ----
>
> But without pure and nothrow I get this output:
> ----
> end of main
> DTor
> ----
>
> Why the compiler thinks that the function should/could call 
> A::~this?

It could have something to do with the fact that RVO is an 
optimization *opportunity* that the compiler is allowed to go 
for, even if it changes the program output.

Hoewever, being an *opportunity*, the compiler still has to make 
sure the code is valid without said optimization, which in this 
case, isn't: getA would destroy it's temporary after blitting it 
on the stac, leading to an impure call.