[Bug c++/55581] [C++11] Too-eager instantiation with decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55581

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jason at gcc dot gnu.org

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> 2013-02-13 18:56:17 UTC ---
13.5.6 says, "An expression x->m is interpreted as (x.operator->())->m for a
class object x of type T if T::operator->() exists and if the operator is
selected as the best match function by the overload resolution mechanism
(13.3)."

So by repeated application of this rule a->x becomes
a.operator->().operator->().operator->().operator->()...->x, which does in fact
require 10000 instantiations of mooch to evaluate.  Given how long clang takes
to compile this testcase, it seems that they are also doing all the
instantiations, they just aren't counting them as nested and so if the
terminating specialization isn't present it will keep going until resources are
exhausted.

It's not clear to me that this behavior is better than flagging the extreme
chain as G++ does; we used to do the same as clang, but changed in response to
bug 49118.