[LLVMbugs] [Bug 19899] New: Warn user about comparision of reference to nullptr

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19899

            Bug ID: 19899
           Summary: Warn user about comparision of reference to nullptr
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ofv at wanadoo.es
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

In http://llvm.org/viewvc/llvm-project?rev=209723&view=rev an optimization was
introduced for reducing the following case of undefined behavior:

bool foo(int &x) { return &x == 0; }

to:

bool foo(int &x) { return false; }

I noticed this new optimization because some code started crashing. AFAIK
neither of gcc, MSVC, Intel C++ apply this optimization. All of them behave on
the same way when the function is invoked as

foo((int*)nullptr);

It is reasonable to expect more instances of this problem. A behavior that
previously was undefined (by the book) but fairly predictable (by how the
available implementations work) now effectively becomes undefined.

I propose to implement a warning (active by default) for communicating the user
about the occuerrences of that type of code.

A toggle for disabling the optimization would also be a good thing as a quick
fix for those on a hurry.

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