[Bug c++/85266] New: Inconsistent _Pragma behavior for macro with and without arguments

["sdaniel.cole at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85266

            Bug ID: 85266
           Summary: Inconsistent _Pragma behavior for macro with and
                    without arguments
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sdaniel.cole at gmail dot com
  Target Milestone: ---

I'm trying to use the _Pragma operator to disable warnings for code inside a
macro. It seems to work when the macro has no arguments (without parenthesis),
but doesn't work when it has arguments (even having empty parenthesis causes
the change in behavior).

The following works as expected (shadow warning disabled for macro only):

    struct RAII{
        RAII();
        ~RAII();
    };

    #define RAII_BLOCK                                      \
        _Pragma("GCC diagnostic push \"-Wshadow\"")         \
        _Pragma("GCC diagnostic ignored \"-Wshadow\"")      \
        auto a = RAII();                                    \
        _Pragma("GCC diagnostic pop")

    void foo() {
        RAII_BLOCK;
        {
            RAII_BLOCK;
        }
    }


The following doesn't work as expected (a warning is still generated):

    struct RAII{
        RAII();
        ~RAII();
    };

    #define RAII_BLOCK()                                    \
        _Pragma("GCC diagnostic push \"-Wshadow\"")         \
        _Pragma("GCC diagnostic ignored \"-Wshadow\"")      \
        auto a = RAII();                                    \
        _Pragma("GCC diagnostic pop")

    void foo() {
        RAII_BLOCK();
        {
            RAII_BLOCK();
        }
    }

I compile the above using:
g++ -c test.cpp -Wshadow

https://godbolt.org/g/D22MYu