[Bug c/80383] New: wrong caret location and missing detail in warning: initializer element is not a constant expression on a signed overflow

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80383

            Bug ID: 80383
           Summary: wrong caret location and missing detail in warning:
                    initializer element is not a constant expression on a
                    signed overflow
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The caret in the pedantic warnings issued for the following test case points to
the wrong subexpression, making the warning confusing.  The caret should point
to the shift expression and the operands should be underlined.  It would help
if the warning also explained why the result of the shift expression isn't a
constant expression.  Perhaps -Wshift-overflow=2 should be enabled by
-Wpedantic?

$ cat b.c && gcc -O2 -S -Wall -Wextra -Wpedantic b.c
const int i = 1 << (sizeof (int) * __CHAR_BIT__ - 1);
const int j = 1 << (sizeof (int) * __CHAR_BIT__);
b.c:1:15: warning: initializer element is not a constant expression
[-Wpedantic]
 const int i = 1 << (sizeof (int) * __CHAR_BIT__ - 1);
               ^
b.c:2:17: warning: left shift count >= width of type [-Wshift-count-overflow]
 const int j = 1 << (sizeof (int) * __CHAR_BIT__);
                 ^~
b.c:2:15: warning: initializer element is not a constant expression
[-Wpedantic]
 const int j = 1 << (sizeof (int) * __CHAR_BIT__);
               ^

In contrast, with -Wshift-overlow=2, GCC prints the following.  With
-Wshift-overflow and -Wshift-count-overflow the caret is in the right place
(but the operands aren't underlined).

b.c:1:17: warning: result of ‘1 << 31’ requires 33 bits to represent, but ‘int’
only has 32 bits [-Wshift-overflow=]
 const int i = 1 << (sizeof (int) * __CHAR_BIT__ - 1);
                 ^~
b.c:1:15: warning: initializer element is not a constant expression
[-Wpedantic]
 const int i = 1 << (sizeof (int) * __CHAR_BIT__ - 1);
               ^
b.c:2:17: warning: left shift count >= width of type [-Wshift-count-overflow]
 const int j = 1 << (sizeof (int) * __CHAR_BIT__);
                 ^~
b.c:2:15: warning: initializer element is not a constant expression
[-Wpedantic]
 const int j = 1 << (sizeof (int) * __CHAR_BIT__);
               ^

With -Weverything Clang prints the following warnings for the problems
discussed above:

t.c:1:17: warning: signed shift result (0x80000000) sets the sign bit of the
      shift expression's type ('int') and becomes negative
      [-Wshift-sign-overflow]
const int i = 1 << (sizeof (int) * __CHAR_BIT__ - 1);
              ~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.c:2:17: warning: shift count >= width of type [-Wshift-count-overflow]
const int j = 1 << (sizeof (int) * __CHAR_BIT__);
                ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~