Would it break the language or existing code if we'd add safe signed/unsigned compares to C/C++?

After reading this question on signed/unsigned compares (they come up every couple of days I'd say):

• Signed / unsigned comparison and -Wall

I wondered why we don't have proper signed unsigned compares and instead this horrible mess? Take the output from this small program:

#include <stdio.h>
#define C(T1,T2)\
 {signed   T1 a=-1;\
 unsigned T2 b=1;\
  printf("(signed %5s)%d < (unsigned %5s)%d = %d\n",#T1,(int)a,#T2,(int)b,(a<b));}\

 #define C1(T) printf("%s:%d\n",#T,(int)sizeof(T)); C(T,char);C(T,short);C(T,int);C(T,long);
int main()
{
 C1(char); C1(short); C1(int); C1(long); 
}

Compiled with my standard compiler (gcc, 64bit), I get this:

char:1
(signed  char)-1 < (unsigned  char)1 = 1
(signed  char)-1 < (unsigned short)1 = 1
(signed  char)-1 < (unsigned   int)1 = 0
(signed  char)-1 < (unsigned  long)1 = 0
short:2
(signed short)-1 < (unsigned  char)1 = 1
(signed short)-1 < (unsigned short)1 = 1
(signed short)-1 < (unsigned   int)1 = 0
(signed short)-1 < (unsigned  long)1 = 0
int:4
(signed   int)-1 < (unsigned  char)1 = 1
(signed   int)-1 < (unsigned short)1 = 1
(signed   int)-1 < (unsigned   int)1 = 0
(signed   int)-1 < (unsigned  long)1 = 0
long:8
(signed  long)-1 < (unsigned  char)1 = 1
(signed  long)-1 < (unsigned short)1 = 1
(signed  long)-1 < (unsigned   int)1 = 1
(signed  long)-1 < (unsigned  long)1 = 0

If I compile for 32 bit, the result is the same except that:

long:4
(signed  long)-1 < (unsigned   int)1 = 0

The "How?" of all this is easy to find: Just goto section 6.3 of the C99 standard or chapter 4 of C++ and dig up the clauses which describe how the operands are converted to a common type and this can break if the common type reinterprets negative values.

Bu