When is a dynamic array really a static array?

[MoonlightSentinel]

On Monday, 30 December 2019 at 15:39:00 UTC, Steven Schveighoffer 
wrote:
> What do you think this should print?
>
> import std.stdio;
>
> void foo(size_t N)(ubyte[N])
> {
>     writeln("static");
> }
>
> void foo()(ubyte[])
> {
>     writeln("dynamic");
> }
>
> void main()
> {
>     ubyte[16] x;
>     foo(x);
>     foo(x[]);
> }
>
> Up until 2.068.2 (possibly 2.068.0), this printed:
>
> static
> dynamic
>
> Since 2.068.2 it now prints
>
> static
> static
>
> Why? Is there a good reason for this, or should it be a 
> regression? I always thought if you sliced a static array, that 
> became a dynamic array.
>
> Note that doing this:
>
> auto y = x[];
> foo(y);
>
> does print dynamic as I expected.
>
> The context for this is 
> https://issues.dlang.org/show_bug.cgi?id=16519. I wanted to say 
> in there "at least there's a workaround, just slice the input". 
> But I guess that doesn't work!
>
> -Steve

Digger blames https://github.com/dlang/dmd/pull/4779

This is probably a bug but DMD sometimes detects when the sliced 
array is static and uses this information to enable certain 
rewrites IIRC, e.g. when initializing another static array