778984 - Enable clang -Werror=null-conversion on OS X

Status: RESOLVED WONTFIX

(Firefox Build System :: General, defect)

Description

[Chris Peterson [:cpeterson]]

Given the following example code:

  char a = nullptr;
  int  b = nullptr;
  char c = NULL;
  int  d = NULL;

* clangs emits these nullptr errors and NULL warnings on OS X:

  error: assigning to 'char' from incompatible type 'nullptr_t'
  error: assigning to 'int' from incompatible type 'nullptr_t'
  warning: implicit conversion of NULL constant to 'char' [-Wnull-conversion]

Note that clang warns about implicit conversion of nullptr to int, but not NULL to int!

To make clang's NULL warnings fatal like its nullptr errors, we may want to add -Werror=null-conversion to clang's CFLAGS/CXXFLAGS.

For comparison, gcc 4.5 emits these warnings on Linux (which might be treated as errors by bug 778980):

  warning: converting to non-pointer type 'char' from NULL
  warning: converting to non-pointer type 'char' from NULL
  warning: converting to non-pointer type 'int' from NULL
  warning: converting to non-pointer type 'int' from NULL

Comment 1

[Chris Peterson [:cpeterson]]

gcc 4.7 added a similar nullptr warning:

-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
    Warn when a literal '0' is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11.

Comment 2

[Chris Peterson [:cpeterson]]

Resolving this bug as WONTFIX because:

1. clang's -Wnull-conversion flag no longer actually warns about implicit conversion of NULL constant to char, which kind of defeats the purpose of this bug. I filed a clang bug upstream:

https://bugs.llvm.org//show_bug.cgi?id=31842

2. Our current approach to C/C++ warnings is to not force individual warnings-as-errors with -Werror=whatever because it is unfriendly to developers with different toolchains. We enable -Werror in our automation so we will catch these warnings before the code lands in mozilla-central. Developers should --enable-warnings-as-errors in their mozconfig to catch this warnings on their local machines.