Clang Analyzer: false positive or am I missing something?

I’m trying to understand an issue reported by Clang’s static analysis tool. The code below demonstrates the issue:

$ cat problem.c
#include <stdint.h>

int main() {
#if VARIANT==1
uint32_t data = 0xdeadbeef;
uint8_t* byte = (uint8_t*)&data;
uint8_t value = byte[0];
#elif VARIANT==2
uint32_t data = 0xdeadbeef;
uint8_t* byte = (uint8_t*)&data;
uint8_t value = byte[1];
#elif VARIANT==3
uint32_t data[1] = {0xdeadbeef};
uint8_t* byte = (uint8_t*)&data[0];
uint8_t value = byte[0];
#elif VARIANT==4
uint32_t data[1] = {0xdeadbeef};
uint8_t* byte = (uint8_t*)&data[0];
uint8_t value = byte[1];
#else
#error “Define VARIANT={1,2,3,4}”
#endif
return value;
}

Now, when I throw Clang’s static analysis at it with VARIANT 1,2, or 3 it says everything’s a-OK. But with VARIANT=4 it complains:

$ scan-build-3.8 --use-cc=clang-3.8 /usr/share/clang/scan-build-3.8/libexec/ccc-analyzer -D VARIANT=4 problem.c

scan-build: Using ‘/usr/lib/llvm-3.8/bin/clang’ for static analysis
problem.c:19:5: warning: Assigned value is garbage or undefined
uint8_t value = byte[1];
^~~~~~~~~~~~~ ~~~~~~~
1 warning generated.
scan-build: 1 bug found.
scan-build: Run ‘scan-view /tmp/scan-build-2016-06-25-104600-17811-1’ to examine bug reports.

My question is why is byte[1] undefined in VARIANT 4 but not anywhere else? I would think if it’s complaining that the value is dependent on endianness, then they should all be reported. Is there some detail of the C spec that I’m missing, or have I stumbled on a false positive (would be a first for me – every other issue reported has been legit thus far).

Thanks,
-Andrew

I recall seeing this before, but I don't recall if its valid C:

    uint8_t* byte = (uint8_t*)&data;
    uint8_t value = byte[0];

Are you getting other compiler warnings with it? If not, try -Wall -Wextra.

Jeff

Thanks for the reply, Jeff. Clang 3.8 is warning-free with -Wall and
-Weverything going through all 4 code paths (VARIANT set to 1-4). Ditto
for gcc 5.2.1 with -Wall -Wextra. Only scan-build grumbles.

-Andrew