[Bug c/66618] New: Failure to diagnose non-constant initializer for static object with -O1

[Keith.S.Thompson at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618

            Bug ID: 66618
           Summary: Failure to diagnose non-constant initializer for
                    static object with -O1
           Product: gcc
           Version: 4.8.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Keith.S.Thompson at gmail dot com
  Target Milestone: ---

$ uname -a
Linux m5 3.13.0-37-generic #64-Ubuntu SMP Mon Sep 22 21:28:38 UTC 2014 x86_64
x86_64 x86_64 GNU/Linux
$ gcc --version | head -n 1
gcc (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4
$ cat bug.c
int main(void) {
    const int not_a_constant = 0;
    static int n = not_a_constant; /* constraint violation */
    return n;                      /* avoid unused variable warning */
}
$ gcc -O0 -c -std=c11 -pedantic-errors -Wall -Wextra bug.c
bug.c: In function ‘main’:
bug.c:3:5: error: initializer element is not constant
     static int n = not_a_constant; /* constraint violation */
     ^
$ gcc -O1 -c -std=c11 -pedantic-errors -Wall -Wextra bug.c
$ 

In the test program "bug.c", the object "not_a_constant" is defined as "const",
but its name is not a constant expression. The initializer for a static
expression must be a constant expression; using a non-constant expression is a
constraint violation, requiring a diagnostic (N1570 6.7.9p4).

With "-O0", gcc correctly diagnoses this. I see the same problem with "-O1",
"-O2", and "-O3", and with "-std=c90", "-std=c99", and "-std=c11".

This is based on a question posted to Stack Overflow:
http://stackoverflow.com/q/30962512/827263
by user "meet". Grzegorz Szpetkowski gets the credit for realizing that the
"-O" option triggers the bug.