I like this updated syntax. @ftynse WDYT?
The custom<Space> is ugly but I see its motivation. Maybe file a bug to add support for explicit whitespace to the assembly format?
I'd prefer to expose the parsing of just the argument list instead of parsing a result type and then failing.
Thanks. Just some nits.
Can you update the example, too?
You could add an option here whether it may have attributes, like allowAttributes or something and then parsing would just fail with a parse error.
Nit: This could be in the assemblyFormat