[Bug c++/56239] New: parse error calling operator() on parenthesized value-initialized temporary

["ed at catmur dot co.uk" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56239

             Bug #: 56239
           Summary: parse error calling operator() on parenthesized
                    value-initialized temporary
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ed@catmur.co.uk


From
http://stackoverflow.com/questions/14732132/global-initialization-with-temporary-function-object

Under gcc 4.8.0 (20130127):

struct S { int operator()(); };
int main() { (S())(); }

source.cpp:2:20: error: expected primary-expression before ')' token
 int main() { (S())(); }
                    ^

Motivation for parenthesizing the temporary is to use it in a constructor call:

struct S { int operator()(); };
struct T { T(int); };
int main() { T t((S())()); }

source.cpp:3:24: error: expected primary-expression before ')' token
 int main() { T t((S())()); }
                        ^

It appears that gcc is parsing the expression (S())() using production 5.4p2:

cast-expression:
    unary-expression
    ( type-id ) cast-expression

Since an empty pair of parentheses is not a cast-expression, this should
instead be parsed using 5.2p1:

postfix-expression:
    [...]
    postfix-expression ( expression-list[opt] )
    [...]
where the postfix-expression is (S()) and the expression-list is omitted.

Same error occurs in both C++03 and C++11 mode.

icc 13.0.1 has the same issue; clang 3.2 compiles it successfully.

Two workarounds are available: enclose (S()) in an extra pair of parentheses,
or use C++11 universal initialization syntax.