Phobos should provide unit testing faciilities

[Steven Schveighoffer]

On 9/12/22 7:28 AM, Quirin Schroll wrote:
> In [*Taming Immutable Data Types in D with 
> librebindable*](https://www.youtube.com/watch?v=eGX_fxlig8I) by Mathis 
> Beer, in the Q&A, it was pointed out that it’s quite common to forget 
> testing template code against types that do exotic (yet valid) things, 
> e.g. `@disable this()`, define/`@disable` the copy constructor, contain 
> qualified members, be immutable, etc.
> 
> One way to mitigate this would be a Phobos module (name it `std.testing` 
> or something) that – possibly among other things – defines aptly named 
> types that are irregular for the sole purpose of being used as examples, 
> together with various AliasSeqs that group types that belong together 
> somehow. Then, one could do:
> 
> ```D
> unittest
> {
>      import std.testing;
>      static foreach (T; IrregularStructs)
>      {
>          T obj; // can default construct? T = NoDefaultCtor fails.
>          auto obj2 = obj; // can copy? T = NoCopyCtor fails.
>          // etc.?
>      }
> }
> ```
> 
> Anyone can find *some* irregular types and test against them, but 
> looking into *every* corner requires extensive knowledge about the 
> language’s niches.

Great idea! Many things come to mind here.

Like a forward-range-accepting functions should be tested to make sure 
it's properly using `.save` when copying, one could probably construct a 
range that doesn't work without using that method.

-Steve