new DIP40: Template parameter deduction for constructors

[Peter Alexander]

On Monday, 13 May 2013 at 02:39:22 UTC, timotheecour wrote:
> The link:
> http://wiki.dlang.org/DIP40
> The paper links mentioned in the abstract are given in this DIP.

This DIP is lacking in detail.

struct A(T1)
if(!is(T1==float))
{
	this(T2)(T2 a, T1 b){}
	this()(T1 b){}
	this()(){}
}
struct A(T1)
if(is(T1==float))
{
	this()(){}
}

auto a=A(1,1.0); //deduced to A!(double)(1,1.0)
auto a=A(1.0); //deduced to A!(double)(1.0)
auto a=A(); //error: T1 cannot be deduced.


How does the compiler decide which templates to attempt to 
instantiate? Does it just ignore conditional compilation 
conditions? If so, what would it do with this?

struct A(T)
if (is(T==float) && is(T!=float))
{
     this()(T a) {}
}


If the conditions are ignored then it will match this 
uninstantiable template. If it doesn't ignore the conditions then 
how does it determine T ahead of time to evaluate the conditions?

One possible solution could be to first ignore the conditions, 
match the constructor, then check that the condition is okay. 
This is an extension on how normal function type deduction works 
though, so whatever mechanisms you have in mind need to be part 
of the proposal.