c++: constexpr folding in unevaluated context [PR105931]

Changing the type of N from int to unsigned in decltype82.C (from
r13-986-g0ecb6b906f215e) reveals another spot where we perform constexpr
evaluation in an unevaluated context for sake of warnings, this time
from the call to shorten_compare in cp_build_binary_op, which calls
fold_for_warn.

We could (and probably should) suppress the shorten_compare warnings
when in an unevaluated context, but there's probably other callers of
fold_for_warn that are similarly affected.  So this patch takes the
approach of directly suppressing fold_for_warn when in an unevaluated
context.

	PR c++/105931

gcc/cp/ChangeLog:

	* expr.cc (fold_for_warn): Don't fold when in an unevaluated
	context.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/decltype82a.C: New test.

gcc/cp/expr.cc

@@ -397,6 +397,11 @@ fold_for_warn (tree x)
 {
   /* C++ implementation.  */
 
+  if (cp_unevaluated_operand)
+    /* In an unevaluated context we don't care about the reduced value
+       of an expression, so neither should any warnings.  */
+    return x;
+
   /* Prevent warning-dependent constexpr evaluation from changing
      DECL_UID (which breaks -fcompare-debug) and from instantiating
      templates.  */

gcc/testsuite/g++.dg/cpp0x/decltype82a.C

@@ -0,0 +1,11 @@
+// PR c++/105931
+// { dg-do compile { target c++11 } }
+// A version of decltype82.C where N is unsigned.
+
+template<unsigned N>
+void f() {
+  using ty1 = decltype((5 % N) == 0);
+  using ty2 = decltype((5 / N) == 0);
+}
+
+template void f<0>();