[Bug c++/77434] warn about suspicious precedence of ternary operator (?:)

["bernd.edlinger at hotmail dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77434

--- Comment #5 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
This is an idea for a warning that does not focus on parentheses:

Here we had: a ? c1 : c2;
but in a context where a boolean is requested.
It is always suspicious, when c1, and c2 are integer constants
which a neither 0 or 1:

Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c     (revision 239944)
+++ gcc/c-family/c-common.c     (working copy)
@@ -4618,6 +4618,14 @@
                                               TREE_OPERAND (expr, 0));

     case COND_EXPR:
+      if (TREE_CODE (TREE_OPERAND (expr, 1)) == INTEGER_CST
+         && TREE_CODE (TREE_OPERAND (expr, 2)) == INTEGER_CST
+         && !integer_zerop (TREE_OPERAND (expr, 1))
+         && !integer_onep (TREE_OPERAND (expr, 1))
+         && !integer_zerop (TREE_OPERAND (expr, 2))
+         && !integer_onep (TREE_OPERAND (expr, 2)))
+       warning_at (location, 0,
+                   "?: expression using integer constants in boolean
context");
       /* Distribute the conversion into the arms of a COND_EXPR.  */
       if (c_dialect_cxx ())
        {



Bootstrap obviously fails here:
In file included from ../../gcc-trunk/gcc/dwarf2out.c:59:0:
../../gcc-trunk/gcc/dwarf2out.c: In function 'void
output_loc_operands(dw_loc_descr_ref, int)':
../../gcc-trunk/gcc/system.h:725:18: error: ?: expression using integer
constants in boolean context [-Werror]
    ((void)(!(EXPR) ? fancy_abort (__FILE__, __LINE__, __FUNCTION__), 0 : 0))
                  ^
../../gcc-trunk/gcc/dwarf2out.c:2053:2: note: in expansion of macro
'gcc_assert'
  gcc_assert (die_offset > 0
  ^~~~~~~~~~
../../gcc-trunk/gcc/system.h:725:18: error: ?: expression using integer
constants in boolean context [-Werror]
    ((void)(!(EXPR) ? fancy_abort (__FILE__, __LINE__, __FUNCTION__), 0 : 0))
                  ^
../../gcc-trunk/gcc/dwarf2out.c:2053:2: note: in expansion of macro
'gcc_assert'
  gcc_assert (die_offset > 0
  ^~~~~~~~~~
cc1plus: all warnings being treated as errors