Problems compiling with clang (3.8.21)

[Dan Williams]

On Tue, 2019-04-02 at 15:34 +0200, Torsten Hilbrich wrote:
> Hello,
> 
> I tried to compile libmbim (1.16.0), libqmi (1.20.0) and ModemManager
> (1.8.0) with clang and got strange results.
> 
> I set the environment variables CC=clang and CXX=clang++ and ran the
> autogen.sh, configure, and make steps.
> 
> Then the configure noticed that clang was used and checked for the
> supported warning options:
> 
> CC='clang'
> configure:3408: checking for gcc
> configure:3435: result: clang
> ...
> configure:17644: checking whether gcc understands -Wno-unused-but-
> set-variable
> configure:17657: clang -c -Wall -std=gnu89 -g -O2 -Wmissing-
> declarations -Wmissing-prototypes -Wdeclaration-after-statement
> -Wstrict-prototypes -Wno-unused-parameter -Wno-sign-compare -Wno-
> deprecated-declarations -Wno-unused-but-set-variable  conftest.c >&5
> warning: unknown warning option '-Wno-unused-but-set-variable'; did
> you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]
> 1 warning generated.
> configure:17657: $? = 0
> configure:17666: result: yes

I think -Wno-unused-but-set-variable is actually a mistake and it
should be "-Wunused-but-set-variable". But that causes build errors
that we should fix.

Does clang support -Wunused-but-set-variable?

Also, does making this change in m4/compiler_warnings.m4 of
ModemManager sources help?

-               CFLAGS="$CFLAGS $option"
+               CFLAGS="$CFLAGS $option -Werror"

Dan

> 
> However, when compiling the code I got the following error messages:
> 
> make[4]: Entering directory
> '/build/client/clang/freedesktop/modemmanager/work/modemmanager/libqc
> dm/src'
>   CC       libqcdm_la-com.lo
> error: unknown warning option '-Wno-unused-but-set-variable'; did you
> mean '-Wno-unused-const-variable'?
>       [-Werror,-Wunknown-warning-option]
> Makefile:516: recipe for target 'libqcdm_la-com.lo' failed
> make[4]: *** [libqcdm_la-com.lo] Error 1
> 
> I assume that checking in configure is done without -Werror, but
> compilation at least in libqdcdm is done with -Werror.
> 
> 
> It this a known problem? Are fixes welcome to support compilation
> with clang?
> 
> The same problem happened in libmbim and libqmi.
> 
> With regards,
> 
> 	Torsten Hilbrich
> 
> 
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