Deprecate implicit conversion between signed and unsigned integers

[Walter Bright]

On 2/17/2025 1:11 PM, Nick Treleaven wrote:
> In this case, we can use these with IFTI instead of explicit casts:
> 
> https://dlang.org/phobos/std_conv.html#signed
> https://dlang.org/phobos/std_conv.html#unsigned

Yes (those were Andrei's initiative).

Up to a point. An explicit use of a signed template doesn't work if one is 
refactoring to an unsigned type.