[Bug c++/84727] New: [8 regression] Constant class initialization not detected as such

[romain.geissler at amadeus dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84727

            Bug ID: 84727
           Summary: [8 regression] Constant class initialization not
                    detected as such
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: romain.geissler at amadeus dot com
  Target Milestone: ---

Hi,

There appear to be a recent regression, triggered when compiling tcmalloc. This
snippet:

#include <stdint.h>

// A safe way of doing "(1 << n) - 1" -- without worrying about overflow
// Note this will all be resolved to a constant expression at compile-time
#define N_ONES_(IntType, N)                                     \
  ( (N) == 0 ? 0 : ((static_cast<IntType>(1) << ((N)-1))-1 +    \
                    (static_cast<IntType>(1) << ((N)-1))) )

template <int> class A
{
    public:
        static const int kValuebits = 7;

    private:
        // With the template version, this fails.
        static const uint32_t kValueMask = N_ONES_(uint32_t, kValuebits);
};

class B 
{
    public:
        static const int kValuebits = 7;

    private:
        // With the non-template version, this works.
        static const uint32_t kValueMask = N_ONES_(uint32_t, kValuebits);
};

used to build fine (with no special options) with g++ (GCC) 8.0.1 20180224
(experimental) but fails with g++ (GCC) 8.0.1 20180301 (experimental):

file.cpp:7:59: error: non-constant in-class initialization invalid for static
member 'A<<anonymous> >::kValueMask'
                     (static_cast<IntType>(1) << ((N)-1))) )
                                                           ^
file.cpp:16:44: note: in expansion of macro 'N_ONES_'
         static const uint32_t kValueMask = N_ONES_(uint32_t, kValuebits);
                                            ^~~~~~~
file.cpp:7:59: note: (an out of class initialization is required)
                     (static_cast<IntType>(1) << ((N)-1))) )
                                                           ^
file.cpp:16:44: note: in expansion of macro 'N_ONES_'
         static const uint32_t kValueMask = N_ONES_(uint32_t, kValuebits);
                                            ^~~~~~~
Cheers,
Romain