[Bug c++/95724] New: bogue error : "expected '{' before ')' token"

["haoxintu at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95724

            Bug ID: 95724
           Summary: bogue error : "expected '{' before ')' token"
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

This code test.cc, GCC-trunk get a bogus error message and then get confused
with following code parsing.

$cat test.cc
template <(union : decltype(a = 0)>)


$g++-trunk -w -c test.cc
test.cc:1:11: error: expected identifier before ‘(’ token
    1 | template <(union : decltype(a = 0)>)
      |           ^
test.cc:1:29: error: ‘a’ was not declared in this scope
    1 | template <(union : decltype(a = 0)>)
      |                             ^
test.cc:1:29: error: ‘a’ was not declared in this scope
test.cc:1:34: error: expected ‘{’ before ‘)’ token
    1 | template <(union : decltype(a = 0)>)
      |                                  ^
test.cc:1:36: error: expected unqualified-id before ‘)’ token
    1 | template <(union : decltype(a = 0)>)
      |                                    ^

$g++-10 -w -c test.cc
test.cc:1:11: error: expected identifier before ‘(’ token
    1 | template <(union : decltype(a = 0)>)
      |           ^
test.cc:1:29: error: ‘a’ was not declared in this scope
    1 | template <(union : decltype(a = 0)>)
      |                             ^
test.cc:1:29: error: ‘a’ was not declared in this scope
test.cc:1:35: error: expected ‘{’ before ‘>’ token
    1 | template <(union : decltype(a = 0)>)
      |                                   ^
test.cc:1:35: error: expected ‘,’ or ‘...’ before ‘>’ token
test.cc:1:36: error: expected ‘>’ at end of input
    1 | template <(union : decltype(a = 0)>)
      |                                    ^
test.cc:1:36: error: expected unqualified-id at end of input

I guess GCC-trunk emits the bogus error message "expected ‘{’ before ‘)’
token", and the following error messages are inconsistent with GCC-10. So,
GCC-trunk might have something wrong with while parsing this code.