Unconditional use of -fdiagnostics-color=always

[olafhering]

The SUSE dune rpm package was upgraded from 8118ddb to fedec66. This broke build in SLES12, which uses gcc 4.8 as default compiler.

[   28s] $ (cd _build/default/otherlibs/xdg && /usr/bin/gcc -std=gnu99 -O2 -fno-strict-aliasing -fwrapv -fno-builtin-memcmp -pthread -Werror=implicit-function-declaration -Werror=return-type -Wno-deprecated-declarations -fPIC -pipe -D_FILE_OFFSET_BITS=64 -fdiagnostics-color=always -g -I /usr/lib64/ocaml -o xdg_stubs.o -c xdg_stubs.c)
[   28s] gcc: error: unrecognized command line option '-fdiagnostics-color=always'

Was this changed intentionally? According to the changelog, this command line option was already added in 3.7 via issue #4083. I did not spot a related changelog entry between 3.15 and 3.17. Is GCC 4.8.5 still a supported system compiler with OCaml 4.14? I think I have seen no specific depreciation notice, maybe I just missed it and things work just by luck.

[nojb]

Was this changed intentionally?

I think you are being bitten by #10883, which makes it so that -fdiagnostics-color=always is passed unconditionally to gcc (even if standard output is not a terminal). This was done to avoid gratuitious recompilations under some conditions.

[olafhering]

Is there a way to avoid it, so that this old system gcc can be used? Maybe a configure knob. Not sure how much time you are willing to sink into such old code base. As a workaround I could switch to a newer, separate system compiler in SLE12 for the entire OCaml toolchain.

[nojb]

I can't think of any way you can work around this without some modification to Dune or by using a more recent compiler on your side. The support for this flag was added in GCC 4.9. I guess we could check for the GCC version in Dune or add a configure flag as you suggest, but am not sure if it is worth the trouble (GCC 4.8 was released in 2013). What do you think?

[olafhering]

For CC it is possible to hardcode /usr/bin/gcc-N during build of ocaml, which is then used for every ocaml.opt usage.
How is this supposed to be done for CXX? ocaml.opt does not seem to have a concept of CXX.

[nojb]

How is this supposed to be done for CXX? ocaml.opt does not seem to have a concept of CXX.

As far as I know the OCaml compiler never calls out to the C++ compiler.

[olafhering]

dune does invoke c++. When g++-N is available for gcc-N, mccs and re2 succeed to build.