Re: [PATCH v2 08/10] Fix up a bunch of "gcc -Werror=sign-compare" complaints

[Bruno Haible]

> especially given it's in the gcc -Wextra set, not some random flag

Warnings in the '-Wall' category are typically worth eliminating: they
regularly point to real bugs.

Eliminating '-Wsign-compare' warnings, OTOH, is usually a waste of time, in
my experience: I have hardly ever found a bug while chasing them.

Robbie Harwood wrote:
> > As you point out with coreutils, gnulib taking a position like this on
> > flags has the knock-on effect that, for our grub, we'll need to do one
> > of the following:
> > 
> > - Carry a gnulib patch to make the flag work (what we've been doing).
> > - Change flags in the outer work (i.e., change build options for grub)
> > - Maintain logic to keep flags gnulib-disliked flags out when building
> >    the gnulib modules

Paul Eggert replied:
> Every other Gnulib-using project I know takes the third approach.

So, how about adding the third approach to gnulib-tool?

Rationale:
Gnulib does not want to dictate their preferred GCC flags to coreutils,
grub, etc.
And similarly, we don't want coreutils, grub, etc. to dictate the coding
style in which gnulib is written.

Implementation idea:
Since 2021-06-10, gnulib-tool already ensures that the Gnulib modules are
compiled with '-Wno-error'. This code could be extended to add
'-Wno-sign-compare' and other warning-disabling options.

Question:
Which warning options should we disable this way?
1) We have some comments in build-aux/gcc-warning.spec.
2) When I collect all warnings in the gllib/ directory of a full testdir,
I get (with gcc-9.3):

$ grep warning: gllib.log | sed -e 's/^.*\[\(-W.*\)\]$/\1/' | LC_ALL=C sort | 
LC_ALL=C uniq -c
     72 -Wattribute-warning
    169 -Wcast-qual
    457 -Wconversion
      1 -Wempty-body
     10 -Wfloat-conversion
     24 -Wfloat-equal
      7 -Wimplicit-fallthrough=
      9 -Wmaybe-uninitialized
      5 -Wmissing-field-initializers
      1 -Wpedantic
      7 -Wredundant-decls
      8 -Wrestrict
    156 -Wsign-compare
   4324 -Wsign-conversion
      2 -Wtype-limits
    888 -Wundef
    139 -Wunsuffixed-float-constants
      2 -Wunused-function
    129 -Wunused-parameter

And with gcc-11.2.0:
$ grep warning: gllib.log | sed -e 's/^.*\[\(-W.*\)\]$/\1/' | LC_ALL=C sort | 
LC_ALL=C uniq -c
     72 -Wattribute-warning
    172 -Wcast-qual
    509 -Wconversion
      4 -Wcpp
      1 -Wempty-body
      9 -Wfloat-conversion
     26 -Wfloat-equal
      7 -Wimplicit-fallthrough=
      9 -Wmaybe-uninitialized
      5 -Wmissing-field-initializers
    432 -Wpedantic
      7 -Wredundant-decls
      8 -Wrestrict
      2 -Wreturn-local-addr
    168 -Wsign-compare
   4362 -Wsign-conversion
      2 -Wtype-limits
    878 -Wundef
    155 -Wunsuffixed-float-constants
      2 -Wunused-function
    131 -Wunused-parameter

I therefore propose to disable these warnings:
  -Wattribute-warning
  -Wcast-qual
  -Wconversion
  -Wfloat-conversion
  -Wfloat-equal
  -Wimplicit-fallthrough
  -Wmaybe-uninitialized
  -Wpedantic
  -Wrestrict
  -Wsign-compare
  -Wsign-conversion
  -Wtype-limits
  -Wundef
  -Wunsuffixed-float-constants
  -Wunused-function
  -Wunused-parameter

Bruno