C array index, signed or unsigned

Firstly, according to the C standard, is the C array index variable interpreted as a signed or unsigned integer? Secondly, what does Keil do?
C array index, signed or unsigned

Firstly, according to the C standard, is the C array index variable interpreted as a signed or unsigned integer? Secondly, what does Keil do?
Array indexing with unsigned int

C251 v3.12 The C251 compiler does not handle arithmetic for array indices consistently. Consider char tt[30] ; char test ; unsigned char aaa = 27 ; unsigned int bbb = 27 ; test = tt[aaa - 20] ;...
Array indexing with unsigned int

C251 v3.12 The C251 compiler does not handle arithmetic for array indices consistently. Consider char tt[30] ; char test ; unsigned char aaa = 27 ; unsigned int bbb = 27 ; test = tt[aaa - 20] ;...
Using unsigned with signed int

I have a signed integer which I have to assign to an unsigned one after addition with unsigned one. Like int16_t x = -9; uint16_t y = 11; if(x < y) { y = y+x; } How to ensure that...