[Bug c++/77564] New: Code fails to link when compiled with -O0

[csaba_22 at yahoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77564

            Bug ID: 77564
           Summary: Code fails to link when compiled with -O0
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: csaba_22 at yahoo dot co.uk
  Target Milestone: ---

The following code fails to link when compiled with -O0. It links when compiled
with -O1 or higher, or -Og:

template <typename R>
class du {
    R r;
public:
    constexpr du() = default;
    du(const du&)  = default;

    template<typename R2>
    constexpr explicit du(const R2& rep)
    : r(static_cast<R>(rep)) { }
};

typedef du<int> secs;

class DWP {
    secs to;
public:
    DWP(secs x);
};

DWP::DWP(secs x)
: to(x)
{}

class FHT {
public:
    FHT()
    : dwp{secs(READ_TIMEOUT)}
    {}
protected:
    static auto constexpr READ_TIMEOUT = 1;
    DWP dwp;
};

int main()
{
    FHT fht;
}

$ g++-620 -Wall -pedantic -Wextra -std=c++11 constexpr.cc -o constexpr
/tmp/ccoUUTbu.o: In function `FHT::FHT()':
constexpr.cc:(.text._ZN3FHTC2Ev[_ZN3FHTC5Ev]+0x11): undefined reference to
`FHT::READ_TIMEOUT'


$ nm constexpr.o | c++filt 
0000000000000017 T main
0000000000000000 W du<int>::du<int>(int const&)
0000000000000000 T DWP::DWP(du<int>)
0000000000000000 T DWP::DWP(du<int>)
                 U FHT::READ_TIMEOUT
0000000000000000 W FHT::FHT()
0000000000000000 W FHT::FHT()
0000000000000000 n FHT::FHT()

If compiling with -O1:

$ nm constexpr.o | c++filt 
0000000000000003 T main
0000000000000000 T DWP::DWP(du<int>)
0000000000000000 T DWP::DWP(du<int>)

Seen with a hand-built GCC 6.2:

$ g++-620 -v
Using built-in specs.
COLLECT_GCC=g++-620
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-pc-linux-gnu/6.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-6.2.0/configure --program-suffix=-620
--enable-version-specific-runtime-libs --enable-languages=c,c++,lto
Thread model: posix
gcc version 6.2.0 (GCC) 

This has been seen with GCC as far back as 
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.3)