What are (were) the most difficult parts of D?

[jmh530]

On Thursday, 12 May 2022 at 12:13:32 UTC, Basile B. wrote:
> [snip]
> ```
> is ( Type : TypeSpecialization , TemplateParameterList )
> is ( Type == TypeSpecialization , TemplateParameterList )
> is ( Type Identifier : TypeSpecialization , 
> TemplateParameterList )
> is ( Type Identifier == TypeSpecialization , 
> TemplateParameterList )
> ```
>
> I never remember those variants, because basically you never 
> need them...
> They were required for std.traits and that's it.

What's the difference between a Type and Type Identifier?