[cfe-dev] Warnings not raised from a macro: -Wundefined-bool-conversion/-Wtautological-undefined-compare

[Richtarsky, Martin]

Hi,

while cleaning some code of "-Wundefined-bool-conversion"/"-Wtautological-undefined-compare" warnings I found that these are never raised when coming from a macro. I know this is intentional in clang, but I wonder whether the benefits of this really outweigh the risks.

clang 3.5 optimizes these checks away, so it is crucial to find all places that rely on them in the codebase. But when the code comes from a macro there is suddenly no way to do so at compile time. See functions f2() below.

How are others dealing with this problem e.g. at Google and Apple with large codebases likely affected by this?


$ cat undefined+macro.cpp
----
#define RET(expr) \
  return (!(expr))

class A
{

int f1()
{
  return (!this);  // warns
}

int f2()
{
  RET(this);  // does NOT warn
}

};

int f1()
{
  int a = 1;
  int &r = a;
  return (&r != 0);  // warns
}

int f2()
{
  int a = 1;
  int &r = a;
  RET(&r != 0);  // does NOT warn
}
----

$ clang++ -c  undefined+macro.cpp


undefined+macro.cpp:9:12: warning: 'this' pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true [-Wundefined-bool-conversion]
  return (!this);  // warns
          ~^~~~
undefined+macro.cpp:23:12: warning: reference cannot be bound to dereferenced null pointer in well-defined C++ code; comparison may be assumed to always evaluate to true [-Wtautological-undefined-compare]
  return (&r != 0);  // warns
           ^    ~
2 warnings generated.


Best regards,
Martin