Inconsistent warning flags specified in CMakeLists.txt for different compilers (possibly by mistake)

[oddlama]

While skimming through the CMakeLists.txt, I found that several warning flags have inconsistent (opposite) values for gcc and clang, which seems to be a mistake (please elaborate if I'm wrong).

First inconsistency -Wzero-as-null-pointer-constant

For gcc, -Wzero-as-null-pointer-constant is explicitly enabled if the host uses gcc >= 5.0,
while for clang -Wno-zero-as-null-pointer-constant (Note the -Wno-) is set and therefore the warning is disabled.

Second inconsistency -Wshadow

Also in gcc's PEDANTIC_COMPILE_FLAGS, -Wshadow is followed by -Wno-shadow:

set(PEDANTIC_COMPILE_FLAGS -pedantic-errors -Wall -Wextra -pedantic
    -Wold-style-cast -Wundef
    -Wredundant-decls -Wshadow [...] -Wno-shadow)

Other issues

Possibly there are other inconsistencies or mistakes, but as the authors intent does not appear clearly to me, I am unable to further judge the other flags.
I just wanted to point out, that almost every flag passed to clang is prefixed with -Wno-, and as -Wno-zero-as-null-pointer-constant appears to be a mistake, it might be worth having a look at the other flags, too.

[eliaskosunen]

I was the one that wrote those flags, see #736.

See git blame for CMakeLists.txt for the commit that introduced these changes. You can see, that gcc follows the scheme of enabling every flag possible, while clang uses -Weverything and then disables some of the noisier warnings.

Point 1:
I honestly don't remember what was my reasoning behind this, but it looks like a honest mistake. That warning should be enabled on both gcc and clang, and I can't think of any reason to disable it on clang, given that it's also enabled on gcc.

Point 2:
This was changed later here, I don't know why the change was done nor why it was done this way instead of simply removing the flag.

[oddlama]

Fair enough. Although I cannot find that -Weverything is being used here, only -Wall for both compilers.

[eliaskosunen]

See the blame from my previous comment. Line 96 has -Weverything. This was rather sloppily changed in 50b18a, without removing the -Wnos.

[vitaut]

Looks like I accidentally messed up the warnings. Should be cleaned in 34030de.