[cfe-dev] [PATCH] -Wconversion-null

[Lubos Lunak]

 Hello,

 the attached patch adds option -Wconversion-null . It is pretty much the same 
like http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35669 .

 The purpose of the option can be seen in this testcase:

#include <stdlib.h>
void foo( int a )
    {
    }
int main()
    {
    foo( NULL );
    int a = NULL;
    int b = 1;
    short c = b + 1;
    (void)a;
    (void)c;
    return 0;
    }

$ clang++ -Wconversion null.cpp -c -fno-caret-diagnostics
null.cpp:9:10: warning: implicit conversion of NULL constant to integer 
[-Wconversion]
null.cpp:10:13: warning: implicit conversion of NULL constant to integer 
[-Wconversion]
null.cpp:12:15: warning: implicit conversion loses integer precision: 'int' 
to 'short' [-Wconversion]

 There are two obviously incorrect uses of NULL in this testcase. This is 
currently warned about only with -Wconversion, which however also triggers 
other conversion warnings that are not obviously incorrect. There are 
probably no realistic usage scenarios where a conversion from NULL to integer 
would be intended, but e.g. short<->int conversions may be wanted for example 
for space saving reasons and casts that would silence those warnings may not 
be deemed worth the decreased readability. In short, the benefits 
of -Wconversion may be questionable depending on the codebase, but incorrect 
usage of NULL should not (and it may not be as obvious as in this testcase).

 This can be solved by introduction of -Wconversion-null, which only warns 
about most probably incorrect usage of NULL, and is enabled by -Wconversion 
or -Wall.

 The attached patch should implement the change. This is my first clang 
contribution, so I don't know if there is something more necessary. I also 
couldn't find if/where options are documented.

-- 
 Lubos Lunak
 l.lunak at suse.cz
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