Implicit Conversion of Template Instantiations

[WalterBright]

DIP proposal:

https://github.com/WalterBright/documents/blob/master/templatecast.md

Doesn't implement covariance of non-static member functions yet.

Will be talking about it at DConf:

https://dconf.org/2024/online/index.html#walterb

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub run digger -- build "master + dmd#16314"

[WalterBright]

Looks like I was wrong that nobody would be relying on this behavior:

std\typecons.d(333): Error: static assert:  `!__traits(compiles, delegate () nothrow @system
{
Unique!(Object) uo = cast(Unique!(Object))uc;
uo.~this();
}
)` is false

and, in vibe.d:

../../../.dub/packages/vibe-inet/1.0.0/vibe-inet/source/vibe/inet/url.d(99,7): Error: `vibe.inet.url.URL.__ctor` called with argument types `(string, typeof(null), int, GenericPath!(InetPathFormat))` matches both:
--
  | ../../../.dub/packages/vibe-inet/1.0.0/vibe-inet/source/vibe/inet/url.d(84,2):     `vibe.inet.url.URL.this(string schema, string host, ushort port, GenericPath!(InetPathFormat) path)`
  | and:
  | ../../../.dub/packages/vibe-inet/1.0.0/vibe-inet/source/vibe/inet/url.d(120,2):     `vibe.inet.url.URL.this(string schema, string host, ushort port, GenericPath!(WindowsPathFormat) path)`

[WalterBright]

Blocked by dlang/phobos#8950