The warning was:

/roux/ru/openairinterface5g/openair1/PHY/LTE_TRANSPORT/pucch.c: In function 'generate_pucch2x':
/roux/ru/openairinterface5g/openair1/PHY/LTE_TRANSPORT/pucch.c:729:73: warning: array subscript is above array bounds [-Warray-bounds]
       AssertFatal(1==0,"Illegal modulation symbol %d for PUCCH %s\n",B2,pucch_format_string[fmt]);
                                                                         ^
/roux/ru/openairinterface5g/common/utils/./itti/assertions.h:52:53: note: in definition of macro '_Assert_'
                 __FUNCTION__, __FILE__, __LINE__, ##aRGS);  \
                                                     ^~~~
/roux/ru/openairinterface5g/openair1/PHY/LTE_TRANSPORT/pucch.c:729:7: note: in expansion of macro 'AssertFatal'
       AssertFatal(1==0,"Illegal modulation symbol %d for PUCCH %s\n",B2,pucch_format_string[fmt]);
       ^
The root cause was that the array pucch_format_string was not in sync with
the structure. (This plus why force its size and put \0 in the string...)