[clang-tidy] Ignore `modernize-use-integer-sign-comparison` between signed wide type and unsigned narrow type

[zufuliu]

For following code (online at https://godbolt.org/z/evGzzTdb7), I'd like the checker emits no warnings for foo1 and foo2 (the narrow y implicitly promoted to int), also bar1 missed the warning even with -fsigned-char.

bool foo1(int x, unsigned char y) {
    return x == y;
}
bool foo2(int x, unsigned short y) {
    return x == y;
}
bool bar1(unsigned int x, char y) {
    return x == y;
}
bool bar2(unsigned int x, short y) {
    return x == y;
}

current output:

[<source>:2:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]](javascript:;)
    1 | bool foo1(int x, unsigned char y) {
    2 |     return x == y;
      |            ^ ~~  
      |            std::cmp_equal( , )
[<source>:5:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]](javascript:;)
    5 |     return x == y;
      |            ^ ~~  
      |            std::cmp_equal( , )
[<source>:11:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]](javascript:;)
   11 |     return x == y;
      |            ^ ~~  
      |            std::cmp_equal( , )
3 warnings generated.

CC @qt-tatiana as author of the checker (PR #113144).

[zufuliu]

Update clang compiler flags to -std=c++20 -fsigned-char -Wall -Wextra.
https://godbolt.org/z/eYdcxG4Th

[<source>:2:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]](javascript:;)
    1 | bool foo1(int x, unsigned char y) {
    2 |     return x == y;
      |            ^ ~~  
      |            std::cmp_equal( , )
[<source>:5:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]](javascript:;)
    5 |     return x == y;
      |            ^ ~~  
      |            std::cmp_equal( , )
[<source>:8:14: warning: comparison of integers of different signs: 'unsigned int' and 'char' [clang-diagnostic-sign-compare]](javascript:;)
    8 |     return x == y;
      |            ~ ^  ~
[<source>:11:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]](javascript:;)
   11 |     return x == y;
      |            ^ ~~  
      |            std::cmp_equal( , )
[<source>:11:14: warning: comparison of integers of different signs: 'unsigned int' and 'short' [clang-diagnostic-sign-compare]](javascript:;)
   11 |     return x == y;
      |            ~ ^  ~
5 warnings generated.

[qt-tatiana]

For following code (online at https://godbolt.org/z/evGzzTdb7), I'd like the checker emits no warnings for foo1 and foo2 (the narrow y implicitly promoted to int), also bar1 missed the warning even with -fsigned-char.

bool foo1(int x, unsigned char y) {
return x == y;
}
bool foo2(int x, unsigned short y) {
return x == y;
}
bool bar1(unsigned int x, char y) {
return x == y;
}
bool bar2(unsigned int x, short y) {
return x == y;
}
current output:

:2:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]
1 | bool foo1(int x, unsigned char y) {
2 | return x == y;
| ^ ~~
| std::cmp_equal( , )
:5:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]
5 | return x == y;
| ^ ~~
| std::cmp_equal( , )
:11:12: warning: comparison between 'signed' and 'unsigned' integers [modernize-use-integer-sign-comparison]
11 | return x == y;
| ^ ~~
| std::cmp_equal( , )
3 warnings generated.
CC @qt-tatiana as author of the checker (PR #113144).

Hi,

A note regarding the 'bar1' case: it is not about missing the warning, see https://en.cppreference.com/w/cpp/utility/intcmp
'cmpint' functions don't allow the 'char' type. But 'signed char' and 'unsigned char' types are allowed.
So we cannot use the fix-it in case of 'bar1', even if it is a real sign-compare case.

Docs about -funsigned-char, -fsigned-char options say :
The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. (see https://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/C-Dialect-Options.htm)

I agree that foo1() and foo2() should be skiped by the check. Will create an update.