[Bug c++/98804] New: GCC misparses template in pack expansion as comparison

["foom at fuhm dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98804

            Bug ID: 98804
           Summary: GCC misparses template in pack expansion as comparison
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foom at fuhm dot net
                CC: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

GCC parses the following program (by Richard Smith) as `g` less-than `a`
greater-than `(a)` instead of a call to the function `g` with template-args
`a`, and argument-list `(a)`. And thus, it fails to compile.

(Compile with -std=c++20)
----
struct X { constexpr X(int&) {} };
template<class F> void g(F);
template<X> void g(int);
void g(...);

template<class ...A> auto h(A ...a)->decltype (g (0, g < a > (a) ...)) {
}

void test () {
  h(0);
}
-----

I originally noticed this bug with the following test from Clang's test-suite,
while working on a mangling bug in Clang. In this original test-case, GCC does
successfully compile, but the mangling is weird, exposing that the same
misparse occurred.

----
namespace pr30440 {
template<class F> void g(F);

template<class ...A> auto h(A ...a)->decltype (g (0, g < a > (a) ...)) {
}
void pr30440_test () {
  h();
}
}
----

GCC mangles as:
_ZN7pr304401hIJEEEDTcl1gLi0Espgtlt1gfp_fp_EEDpT_
                              ^^^^
with greater/less operators in the mangling.

But it should've been:
_ZN7pr304401hIJEEEDTcl1gLi0Espcl1gIXfp_EEfp_EEEDpT_
                                  ^^^^^^^
with a template-arg.