Null-checked reference types

[Tim]

On Tuesday, 6 August 2024 at 14:55:46 UTC, Quirin Schroll wrote:
> Add the following type suffixes to the language: `?` and `!`.

Using `!` seems to be ambiguous with templates. For example:
```
void f(T!x)
{}
```
Is `T!x` a template instance with template argument `x` or the 
new type `T!` and parameter name `x`?