Relaxing the definition of isSomeString and isNarrowString

[Andrew Godfrey via Digitalmars-d]

The OP and the question of auto-decoding share the same root 
problem: Even though D does a lot better with UTF than other 
languages I've used, it still confuses characters with code 
points somewhat. "Element type is some character" is an example 
from OP. So clarify for me:
If a programmer makes an array of either 'char' or 'wchar', does 
that always, unambiguously, mean a UTF8 or UTF16 code point? E.g. 
If interoperating with C code, they will never make the mistake 
of using these types for a non-string byte/word array?

If and only if this is true, then D has done well and I'm 
unafraid of duck-typing here.