[Bug c++/107953] New: Greater-than operator misparsed inside a lambda expression used as a template argument

[Bug c++/107953] New: Greater-than operator misparsed inside a lambda expression used as a template argument

[majerech.o at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107953

            Bug ID: 107953
           Summary: Greater-than operator misparsed inside a lambda
                    expression used as a template argument
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: majerech.o at gmail dot com
  Target Milestone: ---

template <auto>
void
f() { }

constexpr auto g = f<[] (int x, int y) { return x > y; }>;

This produces the following diagnostic:
<source>: In lambda function:
<source>:5:50: error: expected ';' before '>' token
    5 | constexpr auto g = f<[] (int x, int y) { return x > y; }>;
      |                                                  ^~
      |                                                  ;
<source>:5:51: error: expected primary-expression before '>' token
    5 | constexpr auto g = f<[] (int x, int y) { return x > y; }>;
      |                                                   ^
ASM generation compiler returned: 1
<source>: In lambda function:
<source>:5:50: error: expected ';' before '>' token
    5 | constexpr auto g = f<[] (int x, int y) { return x > y; }>;
      |                                                  ^~
      |                                                  ;
<source>:5:51: error: expected primary-expression before '>' token
    5 | constexpr auto g = f<[] (int x, int y) { return x > y; }>;
      |                                                   ^

Godbolt link to the above: https://godbolt.org/z/KG6d5E6ev

Changing the body to return (x > y); makes the error go away as a workaround.
So I speculate that the greater-than operator is misparsed as the end of the
template argument list.

[Bug c++/107953] Greater-than operator misparsed inside a lambda expression used as a template argument

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107953

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=57

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect this is the same issue as PR 57 .

[Bug c++/107953] Greater-than operator misparsed inside a lambda expression used as a template argument

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107953

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Even template defaults has issues:
template <auto = [] (int x, int y) { return x > y; }>
int t = 0;

Which is why I Pointed to PR 57.

[Bug c++/107953] Greater-than operator misparsed inside a lambda expression used as a template argument

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107953

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I wonder if this is still an ambiguous part of the C++ grammar and all.

[Bug c++/107953] Greater-than operator misparsed inside a lambda expression used as a template argument

[majerech.o at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107953

--- Comment #4 from Ondřej Majerech <majerech.o at gmail dot com> ---
It seems that the core of PR 57 is that `A<T` could syntactically be a valid
expression, and it would require a look-up to determine that A and T are in
fact a template and a type, and not values.

Here, however, the fragment `[] (int x, int y) { return x` doesn't form a
syntactically valid expression. So, to me at least, it seems there should be no
ambiguity.