Allow attributes on template explicit instantiations

[Matthew Woehlke]

Consider the following code which exports an explicit instantiation of a
template (e.g. imagine this is in a .cpp file of a library):

template <typename T> T foo(T*)
{
// ...
}
template FOO_EXPORT int foo<int>(int*);

Modern C++ allows FOO_EXPORT on ELF platforms using GCC and Clang to be
defined as `[[gnu::visibility("default")]]`... right?

Alas, no. Attributes on explicit instantiations are explicitly forbidden
([dcl.attr.grammar]/5). (Despite that, GCC is happy to DTRT here. Clang,
however, enforces the standard, even though this particular prohibition
- especially for the attribute in this example - seems to be of
questionable value.)

This is obviously unfortunate. Is there any reason for this restriction
to be in place? If not, I propose to remove it.

--
Matthew

[Matthew Woehlke]

That exciting, huh?

Well, here's a paper. Comments appreciated!

--
Matthew