[LLVMbugs] [Bug 21724] New: Void expression address does not generate an error (C only)

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21724

            Bug ID: 21724
           Summary: Void expression address does not generate an error (C
                    only)
           Product: clang
           Version: 3.4
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hamsa.bugs at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

This compiles without warnings:

    extern void foo; // 1

    int main()
    {
      &foo; // 2
    }

(1) The validity of the `foo` declaration is questionable. There is no apparent
direct prohibition in the standard to declare (as opposed to define) void
objects, but as one cannot define a `void` object, it is not clear how such a
declaration could refer to a valid object. Apart from that, the declared
identifier cannot be used in any way.

(2) The expression appears to be outright invalid, because of 6.3.22.1/1 (in
C11):

    An lvalue is an expression (with an object type other than void) that 
    potentially designates an object;

Thus `foo` is not an lvalue, nor it is a function designator, so its address
cannot be taken.

To reiterate, all of this applies to the C front-end only.

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