[Bug middle-end/72818] New: No warning for dereferencing casted pointer

[pmatos at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72818

            Bug ID: 72818
           Summary: No warning for dereferencing casted pointer
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pmatos at gcc dot gnu.org
  Target Milestone: ---

GCC appropriately generates a warning for:
void f(char c){*(unsigned*)&c=0;}

$ gcc -Wall -fsanitize=undefined -Os -c -ocast1.s cast1.c
cast1.c: In function ‘f’:
cast1.c:1:1: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
 void f(char c){*(unsigned*)&c=0;}
 ^
$

But not for:
void foo(unsigned char x){unsigned int *y=(unsigned int *)&x; *y=0x12345678;}

$ gcc -Wall -fsanitize=undefined -Os -c -ocast.s cast.c
$

It would seem this latter case should be caught and reported as well.