Can std.variant be used with std.container.rbtree?

[Paul Backus]

On Saturday, 2 April 2022 at 14:49:15 UTC, Vijay Nayar wrote:
> On Saturday, 2 April 2022 at 14:35:10 UTC, Vijay Nayar wrote:
>> The `tryMatch` method fails to compile, and instead I get the 
>> following error:
>> ```d
>> /dlang/dmd/linux/bin64/../../src/phobos/std/sumtype.d(2004): 
>> Error: static assert:  "`handlers[0]` of type `int 
>> function(ref ubyte[] _1, ref ubyte[] _2) pure nothrow @nogc 
>> @safe` never matches"
>> ```
>
> Through sheer trial and error, I discovered that changing the 
> handler arguments from `ref ubyte[]` to `const ref ubyte[]` the 
> error goes away. However, I did not find any information in the 
> documentation or code that made this requirement clear. It was 
> basically a guess based on the fact that `string` has no 
> trouble but `ubyte[]` did, and string is basically just 
> `immutable(char[])`.

The reason `const` is required is that the arguments to your 
function, `v1` and `v2`, are `const`. In D, the `const` qualifier 
is transitive, so if you have a `const VarType`, whatever value 
it contains will also be qualified with `const`.

> So far, I'm finding that learning to use `SumType` is 
> significantly more cryptic than `Variant`, by a large margin. 
> I'll still give it a shot of course, because I want to get past 
> this problem.

It's possible to get more informative error messages using the 
`-verrors=spec` compiler flag, although it's a bit of a hassle. 
The basic process is

1. Recompile the module that the error occurs in with 
`-verrors=spec` and redirect output to a file.

     ```
     dmd -verrors=spec -c source/mymodule.d >verrors.txt 2>&1
     ```

2. Use `grep` or your text editor's search function to find 
errors that occurred in that source file or in `sumtype.d`.

     ```
     grep -e 'source/mymodule.d' -e 'std/sumtype.d' verrors.txt
     ```

Following these steps on your example code yields around 20 lines 
of output, which includes the "real" error message:

```
(spec:1) /usr/include/dmd/phobos/std/sumtype.d(1768): Error: 
function literal `__lambda3(ref ubyte[] _1, ref ubyte[] _2)` is 
not callable using argument types `(const(ubyte[]), 
const(ubyte[]))`
(spec:1) /usr/include/dmd/phobos/std/sumtype.d(1768):        
cannot pass argument `_param_0` of type `const(ubyte[])` to 
parameter `ref ubyte[] _1`
```