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

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

[foom at fuhm dot net]

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.

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

[richard-gccbugzilla at metafoo dot co.uk]

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

--- Comment #1 from Richard Smith <richard-gccbugzilla at metafoo dot co.uk> ---
Somewhat more reduced:

struct X { constexpr X(int&) {} };
template<X> struct Y {};

int a;
auto h(int b) -> Y<a>; // #1
auto h(int b) -> Y<b>; // #2

GCC accepts #1, but for #2 it produces seven (!) error messages:

<source>:6:21: error: template argument 1 is invalid
    6 | auto h(int b) -> Y<b>;
      |                     ^
<source>:6:21: error: template argument 1 is invalid
<source>:6:21: error: template argument 1 is invalid
<source>:6:21: error: template argument 1 is invalid
<source>:6:18: error: invalid template-id
    6 | auto h(int b) -> Y<b>;
      |                  ^
<source>:6:21: error: use of parameter outside function body before '>' token
    6 | auto h(int b) -> Y<b>;
      |                     ^
<source>:6:6: error: deduced class type 'Y' in function return type
    6 | auto h(int b) -> Y<b>;
      |      ^
<source>:2:20: note: 'template<X <anonymous> > struct Y' declared here
    2 | template<X> struct Y {};
      |                    ^

As far as I'm aware, both #1 and #2 are valid. (I think the "use of parameter
outside function body" restriction is referring to [dcl.fct.default]/9, but
that refers only to uses of parameters within default arguments, not uses in
other contexts in the function declaration.)

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

[mpolacek at gcc dot gnu.org]

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

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-02-12
                 CC|                            |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.