How to debug broken inference?

[Steven Schveighoffer]

On 5/1/20 4:40 PM, Steven Schveighoffer wrote:
> 
> Does anyone have any recommendations of how to approach this? Even if I 
> wanted to report a bug, I have no idea why this is happening in this one 
> spot. Why can't the compiler *try* @safe and if it doesn't work, just 
> decay to @system?

I figured out the issue. It was something very stupid, but probably a 
bug in the compiler. In my efforts to avoid bounds checking, I did this:

char[4] buf;

auto slice = buf.ptr[idx .. buf.length];

This compiles if it's tagged @safe, but will not infer safety.

But I don't need to do this trick anyway, it's a static array! So I just 
fixed that part of the code.

But I'd say it's a bug in the compiler, it shouldn't accept that as @safe.

Indeed if I just put that in a function, it fails to compile as @safe. 
I'll need to dustmite it.

For reference, I used a manual dustmite-like process to narrow down this 
function to the part that fails.

-Steve