dlib 0.19.1 seems to be failing linking with ldc2

[Dennis]

On Friday, 31 July 2020 at 14:17:14 UTC, jeff thompson wrote:
> dlib.lib(dlib.audio.io.wav.obj) : error LNK2019: unresolved 
> external symbol 
> _D4core8internal7switch___T14__switch_errorZQrFNaNbNiNfAyamZv 
> referenced in function 
> _D3std6format__T10printFloatTfTaZQrFNaNfNkAafSQBsQBr__T10FormatSpecTaZQpEQCtQCs12RoundingModeZQCa
> dlib.lib(dlib.filesystem.local.obj) : error LNK2001: unresolved 
> external symbol 
> _D4core8internal7switch___T14__switch_errorZQrFNaNbNiNfAyamZv
> .dub\build\application-debug-windows-x86_64-ldc_2092-316AB5B187D20C1F6AFBA496E604908D\test.exe : fatal error LNK1120: 1 unresolved externals


The first thing you want to do is demangle the symbols so you 
know what they really are.
You can use the tool `ddemangle` for that:
```
echo 
'_D3std6format__T10printFloatTfTaZQrFNaNfNkAafSQBsQBr__T10FormatSpecTaZQpEQCtQCs12RoundingModeZQCa' | ddemangle
```

You'll find that this symbol is missing:

pure nothrow @nogc @safe void 
core.internal.switch_.__switch_error!().__switch_error(immutable(char)[], ulong)

And it's used in this function:

pure @safe char[] std.format.printFloat!(float, 
char).printFloat(return char[], float, 
std.format.FormatSpec!(char).FormatSpec, std.format.RoundingMode)

The switch_error function is called when none of the cases of a 
`final switch` apply. There is a final switch in 
std.format.printFloat:
https://github.com/ldc-developers/phobos/blob/c43cafe53746a07dee8fa9e00d3a2256c7f05506/std/format.d#L7096

So why is the compiler not emitting the final switch error 
template function? I am not sure, it could be a bug, or maybe you 
have some funky dub settings. You might be able to work around 
this by defining a final switch in you own code somewhere, or 
explicitly defining the function.

```
pragma(mangle, 
"_D4core8internal7switch___T14__switch_errorZQrFNaNbNiNfAyamZv")
void switchError(string file, ulong line) {
     assert(0, file);
}
```

I hope someone else with more familiarity of how template symbols 
are emitted can find the root of this problem and give a proper 
fix.