dub test errors during linking for struct with uninstantiated templated opEquals

[Per Nordlöw]

When a module containing


module nxt.rational;

Rational!(I1) rational(I1, I2)(I1 , I2)
{
     return typeof(return)();
}

struct Rational(Int)
{
     bool opEquals(Rhs)(Rhs _) {}
}

@nogc unittest
{
     auto _ = rational(1, 2);
}


is used as the sole module of a dub project then `dub test` fails 
as

.dub/build/phobos-next-test-library-unittest-linux.posix-x86_64-dmd_2092-06C9DAF60B64114484B3FE9AC9866D5F/phobos-next-test-library.o:src/nxt/rational.d:_D40TypeInfo_S3nxt8rational__T8RationalTiZQm6__initZ: error: undefined reference to '_D6object10_xopEqualsFMxPvMxQeZb'

When I remove

     bool opEquals(Rhs)(Rhs _) {}

the error goes away.

I'm suspecting this is a bug in dmd that fails to correctly check 
if a struct has its templated `opEquals` instantiated.

For a complete minimal sample project see

https://github.com/nordlow/phobos-next/tree/master/rational.reduced