>You should stop trying to read between the subtext, as there is none here. I have no idea where you are coming from in assuming that was my intention.

I'm sorry if I've understood you incorrectly. I want to say that mangle string should be validate in glue/backend, not frontend, because frontend know nothing about mangle limitation. 

>It is meaningless, why should the frontend care about what mangling is applied to the symbol?

If this mangling is setted explicit (in pragma(mangle)) it should be validated using glue/back-end calling. Of course we can move all check into backend and generate beck-end error if mangle is incorrect.
This way you can give user pretty error message instead of strange `as` error message.

>This postcondition is only ran in dmd/win32. So if lets say we come to agree that pragma(mangle) should be left unchecked (apart from some common things - such as zero length string) then this routine being here is useless, and better off being a static in mangle.c

After porting this code to D out block will be work on other systems.
Now validate logic is so difficult and depends on compiler (on host and on target). 
Remove check at all, maybe not the best solution, but place it into mangle.c impossible, because mangle.c is part of frontend and know nothing about validation rules.