Huge improvement in error messages in recent dmd

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On Thursday, 12 December 2019 at 00:39:15 UTC, mipri wrote:
> Consider the following code with an error:
>
>   #! /usr/bin/env rdmd
>   import std;
>   void main() {
>       auto pts = Array!(int[])([0, 0], [5, 5], [0, 1], [0, 2]);
>       multiSort!("a[0] < b[0]", "a[1] < b[1]", 
> SwapStrategy.unstable)(pts);
>   }
>
> The latest version of dmd that's packaged with Fedora is
> 2.086.1. Of this code it says:
>
> ./x266.d(5): Error: template 
> std.algorithm.sorting.multiSort!("a[0] < b[0]", "a[1] < b[1]", 
> cast(SwapStrategy)0).multiSort cannot deduce function from 
> argument types !()(Array!(int[])), candidates are:
> /usr/include/dmd/phobos/std/algorithm/sorting.d(1463):        
> std.algorithm.sorting.multiSort!("a[0] < b[0]", "a[1] < b[1]", 
> cast(SwapStrategy)0).multiSort(Range)(Range r) if 
> (validPredicates!(ElementType!Range, less) && 
> hasSwappableElements!Range)
>
> The nightly I have is v2.089.1-beta.1, and it says:
>
> x266.d(5): Error: template 
> std.algorithm.sorting.multiSort!("a[0] < b[0]", "a[1] < b[1]", 
> cast(SwapStrategy)0).multiSort cannot deduce function from 
> argument types !()(Array!(int[])), candidates are:
> /edit/nightly/dmd2/linux/bin64/../../src/phobos/std/algorithm/sorting.d(1466):        multiSort(Range)(Range r)
>   with Range = Array!(int[])
>   must satisfy the following constraint:
>        hasSwappableElements!Range
>
> Which is a huge, huge improvement! The problem is that
> Array!(Point) doesn't satisfy hasSwappableElements.
>
> I saw that this was also in the v2.089.0 compiler, and so
> checked the blog for announcements back to v2.087, and none of
> them showed how much better the error messages got. This should
> be highlighted. I don't normally care that much about staying
> exactly up-to-date with compilers, but I'd sure have upgraded
> earlier for this.
>
> A slightly strange thing is that rdmd's error messages are
> even friendlier (v2.089.0):
>
>   ./x266.d(5): Error: template 
> std.algorithm.sorting.multiSort!("a[0] < b[0]", "a[1] < b[1]", 
> cast(SwapStrategy)0).multiSort cannot deduce function from 
> argument types !()(Array!(int[])), candidates are:
>   
> /edit/latest/dmd2/linux/bin64/../../src/phobos/std/algorithm/sorting.d(1466):        multiSort(Range)(Range r)
>     with Range = Array!(int[])
>     whose parameters have the following constraints:
>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       validPredicates!(ElementType!Range, less)
>     > hasSwappableElements!Range
>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     Tip: not satisfied constraints are marked with >
>
> wow!

Huh, I thought I was keeping up to date with the stuff in every 
new release, but apparently not, as I had no idea that template 
constraint function error messages had been improved.