[Bug c++/37004] [C++ only] Wconversion warns for short y = 0x7fff; short z = (short) x & y;

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

------- Comment #3 from manu at gcc dot gnu dot org  2008-08-23 18:06 -------
In C:

/* Wrapper around c_common_type that is used by c-common.c and other
   front end optimizations that remove promotions.  ENUMERAL_TYPEs
   are allowed here and are converted to their compatible integer types.
   BOOLEAN_TYPEs are allowed here and return either boolean_type_node or
   preferably a non-Boolean type as the common type.  */
tree
common_type (tree t1, tree t2)

In C++:

/* Return the common type of two types.
   We assume that comptypes has already been done and returned 1;
   if that isn't so, this may crash.

   This is the type for the result of most arithmetic operations
   if the operands have the given two types.  */

tree
common_type (tree t1, tree t2)


They are not the same. C++ performs integral promotion, C doesn't. Which is a
waste of time in C++ because everywhere common_type is called, integral
promotions have been performed already.

Duplicationg type_after_usual_arithmetic_conversions with a variant that does
not perform promotions fixes the problem. But there should be a more elegant
way to handle this than just duplicating the whole function...


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37004