>>> I don't understand -- this will still return a symbol.  (And error out
>>> if you enter stuff like "foo bar".)
>>
>> Yes, entering a symbol returns a symbol, entering a string
>> like "foo bar" returns a string "foo bar" literally.
>
> Oh, right.  Hm.  Well, entering things with "..." is very unusual in
> Emacs prompts, so if we want that, the prompt should at least say that
> that's what's expected...

Fixed in the following patch.

>> Currently text-property-search-forward has more problems:
>> today I needed to search the property ‘face’ with the value
>> ‘hi-yellow’ in the buffer with regexps highlighted by hi-lock.el.
>> Executing interactively:
>>
>>   M-x text-property-search-forward RET face RET hi-yellow RET
>>
>> failed to find the property because all hi-lock occurrences were
>> combined with font-lock text properties, i.e. all they had the
>> property ‘face’ with the value ‘(hi-yellow font-lock-keyword-face)’
>> and text-property-search-forward fails to find a value in the list
>> of values.
>
> Yes, you can't really use text-property-search-forward to do that in any
> meaningful manner, which is why I didn't add that to the interactive
> bit.  It's a function useful almost only when programming, and the only
> useful interactive thing is to vaguely poke around in the buffer.

Implemented in this patch: