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digitalmars.D.learn - Should the compiler be failing to infer template args here?

reply SealabJaster <sealabjaster gmail.com> writes:
If you take a look at this code here: https://godbolt.org/z/4T3uLh

You can see that when using a templated alias, the compiler fails 
to infer the T template parameter, but only when using the 
function that also asks for the alias, instead of the original 
type.

I was just wondering if this is correct behavior, since it's a 
slightly annoying, but easy to deal with difference.
Jun 19 2020
parent reply Paul Backus <snarwin gmail.com> writes:
On Friday, 19 June 2020 at 16:16:55 UTC, SealabJaster wrote:

 If you take a look at this code here: 
 https://godbolt.org/z/4T3uLh

 You can see that when using a templated alias, the compiler 
 fails to infer the T template parameter, but only when using 
 the function that also asks for the alias, instead of the 
 original type.

 I was just wondering if this is correct behavior, since it's a 
 slightly annoying, but easy to deal with difference.


This is a known issue:

https://issues.dlang.org/show_bug.cgi?id=1807
Jun 19 2020
parent reply SealabJaster <sealabjaster gmail.com> writes:
On Friday, 19 June 2020 at 16:31:50 UTC, Paul Backus wrote:

 This is a known issue:

 https://issues.dlang.org/show_bug.cgi?id=1807


"Reported: 2008"... yikes.

Thanks anyway, glad to know I wasn't just going mad :)
Jun 19 2020
parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 6/19/20 12:38 PM, SealabJaster wrote:

 On Friday, 19 June 2020 at 16:31:50 UTC, Paul Backus wrote:

 This is a known issue:

 https://issues.dlang.org/show_bug.cgi?id=1807


 
 "Reported: 2008"... yikes.
 
 Thanks anyway, glad to know I wasn't just going mad :)


It's somewhat difficult to solve, because reverse-templating is not 
necessarily one-to-one, or even have any tractible relationship at all.

Consider, for instance:

struct Foo(T) {}

alias Bar(T) = Foo!int;

void takefoo(X)(Bar!X) {}

takefoo(Foo!int());

How does the compiler decide what type T should be? It could be anything.

But this could be special cased for a *direct* alias translation. How to 
define that is tricky, but should match the pattern:

alias Template1(T) = Template2!(T, and, possibly, other, things)

Where Template2!(T, and, possibly, other, things) can be deduced for 
Template2 and T using IFTI directly.

As the age of that bug report shows, it's not easy to define or solve.

-Steve
Jun 19 2020