[Bug c/79204] New: improve -Woverflow by mentioning types and values

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79204

            Bug ID: 79204
           Summary: improve -Woverflow by mentioning types and values
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

In a non-trivial program where types are "hidden" behind typedefs and constant
expressions by layers of macros and/or enums, diagnostics such as overflow
could be improved by mentioning the types of the operands and their (constant)
values.  For example, in the program below the typedefs and macros can be
defined in headers and far removed from initialization of x, and neither the
warning nor the note may give a clue as to what they expand to.

$ cat t.c && gcc -S -Wall t.c
typedef signed char T;
#define X 123
#define Y 234
#define N (X * Y)

T x = N;
t.c:4:11: warning: overflow in implicit constant conversion [-Woverflow]
 #define N (X * Y)
           ^
t.c:6:7: note: in expansion of macro ‘N’
 T x = N;
       ^

As an example of a possible improvement consider the Clang output for the same
test case:

t.c:6:7: warning: implicit conversion from 'int' to 'T' (aka 'signed char')
      changes value from 28782 to 110 [-Wconstant-conversion]
T x = N;
  ~   ^
t.c:4:14: note: expanded from macro 'N'
#define N (X * Y)
           ~~^~~