Checking the Return Type of a Template Alias Parameter in assertCTFEable

[John Colvin]

On Friday, 28 February 2014 at 14:02:35 UTC, Nordlöw wrote:
> I have a solution to Issue 388 at
>
> https://github.com/nordlow/dmd/commits/master
>
> that works nicely for my projects.
>
> This patch however generates a handful of warnings in Phobos 
> unittests.
>
> All of these are IMO fixable except for one which I don't know 
> how to fix namely the definition of assertCTFEable in 
> exception.d
>
> I need to change this to explicitly capture the return value of 
> the template alias argument dg when this is non-void. My try so 
> far was to change
>
> version(unittest) package
> @property void assertCTFEable(alias dg)()
> {
>     static assert({ dg(); return true; }());
>     dg();
> }
>
> to
>
> version(unittest) package
> @property void assertCTFEable(alias dg)()
> {
>     static assert
>         ({
>             static if (is(typeof(db()) == void)) {
>                 dg();
>             } else {
>                 auto x = dg();
>             }
>             return true;
>         }());
>     static if (is(typeof(db()) == void)) {
>         dg();
>     } else {
>         auto x = dg();
>     }
> }
>
> gives the following error when in Phobos make unittest:
>
> std/exception.d(1392): Error: variable 
> std.exception.assertCTFEable!(function ()
> {
> S[] r = array(repeat((S __ctmp1582 = 0;
>  , __ctmp1582).this(1), 2LU));
> assert(equal(r, [(S __ctmp1591 = 0;
>  , __ctmp1591).this(1), (S __ctmp1592 = 0;
>  , __ctmp1592).this(1)]));
> }
> ).assertCTFEable.__lambda1.x type void is inferred from 
> initializer (*function ()
> {
> S[] r = array(repeat((S __ctmp1582 = 0;
>  , __ctmp1582).this(1), 2LU));
> assert(equal(r, [(S __ctmp1591 = 0;
>  , __ctmp1591).this(1), (S __ctmp1592 = 0;
>  , __ctmp1592).this(1)]));
> }
> )(), and variables cannot be of type void
> std/exception.d(1392): Error: expression (*function ()
> {
> S[] r = array(repeat((S __ctmp1582 = 0;
>  , __ctmp1582).this(1), 2LU));
> assert(equal(r, [(S __ctmp1591 = 0;
>  , __ctmp1591).this(1), (S __ctmp1592 = 0;
>  , __ctmp1592).this(1)]));
> }
> )() is void and has no value
>
> How can I in a general way check if dg evaluates to void or not?

Either use std.traits.ReturnType, or see from it's implementation.