I have this a.cu file:

extern "C" __device__ int test42();
__global__ void get42(int* toStore){
    *toStore = test42();
}

int main() {
    int host{0};
    int* device;
    cudaMalloc(&device, sizeof(int));
    get42<<<1, 1>>>(device);
    cudaDeviceSynchronize();
    cudaMemcpy(&host, device, sizeof(int), cudaMemcpyDeviceToHost);
}

and this b.cu file:

extern "C" __device__ int test42(){
    return 42;
}

I compiled b.cu into a fatbinary stored at somepath/b.fatbin.
I have also linked both into an object file:

nvcc -arch=sm_60 -dlink -o c.o somepath/b.fatbin a.cu  

The object dump clearly shows both functions defined:

$: cuobjdump -sass c.o

Fatbin elf code:
================
arch = sm_60
code version = [1,7]
host = linux
compile_size = 64bit

        code for sm_60
                Function : test42
        .headerflags    @"EF_CUDA_TEXMODE_UNIFIED EF_CUDA_64BIT_ADDRESS EF_CUDA_SM60 EF_CUDA_VIRTUAL_SM(EF_CUDA_SM60)"
                                                            /* 0x001ffc00ffe007f0 */
        /*0008*/                   MOV32I R4, 0x2a ;        /* 0x0100000002a7f004 */
        /*0010*/                   RET ;                    /* 0xe32000000007000f */
        /*0018*/                   BRA 0x18 ;               /* 0xe2400fffff87000f */
                                                            /* 0x001f8000fc0007e0 */
        /*0028*/                   NOP;                     /* 0x50b0000000070f00 */
        /*0030*/                   NOP;                     /* 0x50b0000000070f00 */
        /*0038*/                   NOP;                     /* 0x50b0000000070f00 */
                ..........


                Function : _Z5get42Pi
        .headerflags    @"EF_CUDA_TEXMODE_UNIFIED EF_CUDA_64BIT_ADDRESS EF_CUDA_SM60 EF_CUDA_VIRTUAL_SM(EF_CUDA_SM60)"
                                                            /* 0x001fc400ffa007f6 */
        /*0008*/                   MOV R1, c[0x0][0x20] ;   /* 0x4c98078000870001 */
        /*0010*/                   JCAL 0x0 ;               /* 0xe220000000000040 */
        /*0018*/                   MOV R2, c[0x0][0x140] ;  /* 0x4c98078005070002 */
                                                            /* 0x001fbc00fe2007f2 */
        /*0028*/                   MOV R3, c[0x0][0x144] ;  /* 0x4c98078005170003 */
        /*0030*/                   STG.E [R2], R4 ;         /* 0xeedc200000070204 */
        /*0038*/                   NOP ;                    /* 0x50b0000000070f00 */
                                                            /* 0x001ffc00fc6007ef */
        /*0048*/                   NOP ;                    /* 0x50b0000000070f00 */
        /*0050*/                   NOP ;                    /* 0x50b0000000070f00 */
        /*0058*/                   EXIT ;                   /* 0xe30000000007000f */
                                                            /* 0x001f8000fc0007ff */
        /*0068*/                   BRA 0x60 ;               /* 0xe2400fffff07000f */
        /*0070*/                   NOP;                     /* 0x50b0000000070f00 */
        /*0078*/                   NOP;                     /* 0x50b0000000070f00 */

when I try to build an executable:
nvcc -arch=sm_60 -o executable a.cu c.o -lcudadevrt

I get:

ptxas fatal   : Unresolved extern function 'test42'

What am I missing?