clang: used type 'union u' where arithmetic or pointer type is required

[llvmbot]

Bugzilla Link	3000
Resolution	DUPLICATE
Resolved on	Mar 12, 2010 00:57
Version	unspecified
OS	All
Blocks	#3150
Reporter	LLVM Bugzilla Contributor
CC	@efriedma-quic,@nunoplopes

Extended Description

union u { int a, b; };

void f2(union u);

void f1() {
f2((union u) 0);
}

:6:8: error: used type 'union u' where arithmetic or pointer type is required
f2((union u) 0);
^ ~
1 diagnostic generated.

llvm/clang r58543 (trunk)

[llvmbot]

What is the bug here? Does gcc accept this? If so, is this common in some code you are testing?

I don't think this code is standards conforming (but would need to check), and I don't yet see a convincing reason to support it (if not).

[llvmbot]

What is the bug here? Does gcc accept this? If so, is this common in some code
you are testing?

Yes, gcc accepts this. It is the most common error produced by clang on DragonFly BSD's kernel source code. (See bug 2778.)

I don't think this code is standards conforming (but would need to check), and
I don't yet see a convincing reason to support it (if not).

gcc agrees that it is not standards conforming:

% gcc-4.2 -pedantic -fsyntax-only llvm/llvm-bugzilla-archive#3000 .c
llvm/llvm-bugzilla-archive#3000 .c: In function ‘f1’:
llvm/llvm-bugzilla-archive#3000 .c:6: warning: ISO C forbids casts to union type

I will look at this a bit more closely. Even though my testcase elicits the same error from clang that DragonFly BSD's kernel source does, I may have reduced it so far that it doesn't accurately portray what's going on there.

[efriedma-quic]

gcc docs: http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Cast-to-Union.html

[nunoplopes]

*** This bug has been marked as a duplicate of bug #3118 ***