RFC: change in std::type_info's equality operator

[Matt Austern]

I sent out a message yesterday suggesting a change in 
std::type_info::operator==.  I didn't get a response, possibly because 
my suggestion was buried in a message that largely talked about other 
things.  (I began with a detailed discussion leading up to why the 
change would be useful for certain purposes.)  So let's try again, and 
this time I'll begin at the beginning.

Right now, depending on whether or not a certain macro is set when 
libstdc++ is built, t1==t2 is equivalent to one of these two 
expressions:
  (1) t1.__name == t2.__name, where __name is the address of the string 
representing the type's name.
  (2) t1.__name == t2.__name || strcmp(t1.__name, t2.__name) == 0.

The library uses version (1) iff the compiler defines __GXX_WEAK__ to 
be true.

My proposal: change the library so it unconditionally uses version 2.  
That is, always fall back to string comparison if the address 
comparison

The disadvantage: it hurts the performance of anything that uses 
typeinfo comparison where negative  comparisons are common.  (I suspect 
that nothing performance-critical uses type_info::operator==.  I'll 
drop this proposal if someone shows me otherwise.) The advantage: I 
would like to introduce a feature that has the effect, if users specify 
a particular command line option, of disabling merging of typeinfo 
names across shared libraries.  This mode will not be the default.  But 
the problem is that I can't introduce it at all if libstdc++ assumes 
that names will always be merged.

			--Matt