[Bug c/108128] New: missing -Wshift-overflow warning

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108128

            Bug ID: 108128
           Summary: missing -Wshift-overflow warning
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider the following C program:

#include <stdio.h>
enum { A = 1 << 31 };
int main (void)
{
  printf ("%d\n", A);
  printf ("%d\n", 1 << 31);
  printf ("%d\n", 2 << 31);
  return 0;
}

In C, the 3 shifts have undefined behavior.

The GCC 12 man page says

    -Wshift-overflow=n
        These options control warnings about left shift overflows.

        -Wshift-overflow=1
            This is the warning level of -Wshift-overflow and is enabled by
            default in C99 and C++11 modes (and newer).  This warning level
            does not warn about left-shifting 1 into the sign bit.
            (However, in C, such an overflow is still rejected in contexts
            where an integer constant expression is required.)  No warning
            is emitted in C++20 mode (and newer), as signed left shifts
            always wrap.

        -Wshift-overflow=2
            This warning level also warns about left-shifting 1 into the
            sign bit, unless C++14 mode (or newer) is active.

Nothing is said about the default, but I assume that this should be
-Wshift-overflow=2 in C because undefined behavior should be warned.

But with gcc-12 (Debian 12.2.0-10) 12.2.0, I get a warning only for 2 << 31.

cventin:~> /usr/bin/gcc-12 -std=c99 tst.c -o tst
tst.c: In function ‘main’:
tst.c:7:21: warning: result of ‘2 << 31’ requires 34 bits to represent, but
‘in’ only has 32 bits [-Wshift-overflow=]
    7 |   printf ("%d\n", 2 << 31);
      |                     ^~

BTW, according to the man page, gcc should warn on "enum { A = 1 << 31 };" even
with -Wshift-overflow=1, but it doesn't. This is actually required by the
standard as constraint 6.6#4 is violated (as the evaluation is not defined).

With the UB sanitizer (-fsanitize=undefined), running the program gives as
expected:

-2147483648
tst.c:6:21: runtime error: left shift of 1 by 31 places cannot be represented
in type 'int'
-2147483648
tst.c:7:21: runtime error: left shift of 2 by 31 places cannot be represented
in type 'int'
0

Note that the sanitizer does not emit an error for "enum { A = 1 << 31 };"
since the issue occurs only at compilation (thus a warning is particularly
important).