PR c++/37004 Wconversion warns for short y = 0x7fff; short z = (short) x & y;

[Manuel López-Ibáñez]

The code in c-common.c calls a function common_type that is expected
to not perform promotion of their operands. However, C++ does perform
promotion, so optimizations like shorten_binary_op do not work.
-Wconversion relies on this function to detect that cases like:

void foo (short x)
{
  short y = 0x7fff;
  short z = (short) x & y;
}

are safe.

The current patch modifies common_type to work more like its C
version. To do so, it adds a new static function cp_common_type that
behaves like type_after_usual_arithmetic_conversions but does not
perform default promotions. The function
type_after_usual_arithmetic_conversions is modified to use
cp_common_type instead of duplicating code.

The old common_type was only called twice within cp/, once for
arithmetic types that were already promoted, this is replaced by
cp_common_type, and for pointer types, this is replaced by a new
function common_pointer_type.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu with
--enable-languages=all,ada

OK for trunk?

2008-08-26  Manuel Lopez-Ibanez  <manu@gcc.gnu.org>

	PR c++/37004
cp/
	* typeck.c (cp_common_type): New. The same as
	type_after_usual_arithmetic_conversions but without promotions.
	(type_after_usual_arithmetic_conversions): Do the promotions and
	call cp_common_type.
	(common_type): Make it behave like the C version of this
	function. Do not handle pointer types.
	(common_pointer_type): Move handling of pointer types from
	common_type to here.
	(cp_build_binary_op): Use common_pointer_type instead of
	common_type in call to pointer_diff.
	Use cp_common_type instead of common_type.
	* cp-tree.h (common_pointer_type): Declare.
testsuite/
	* g++.dg/warn/Wconversion-pr34389.C: Remove XFAIL.
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