Make shadowing mixin template names an error (5 years old problem)

[mw]

On Wednesday, 6 May 2026 at 02:51:33 UTC, Kapendev wrote:
> On Tuesday, 5 May 2026 at 21:51:41 UTC, mw wrote:
>> This problem is at least 5 years old.
>>
>> Seriously, can we fix it?
>
> It's weird and I don't depend on it, but I like that the basic 
> method wins over the mixin method here.

All needed is an error message, let the programmer fix it.