c++: value-dep but not type-dep decltype expr [PR105756]

Here during ahead of time instantiation of the value-dependent but not
type-dependent decltype expression (5 % N) == 0, cp_build_binary_op folds
the operands of the == via cp_fully_fold, which performs speculative
constexpr evaluation, and from which we crash for (5 % N) due to the
value-dependence.

Since the operand folding performed by cp_build_binary_op appears to
be solely for sake of diagnosing overflow, and since these diagnostics
are suppressed when in an unevaluated context, this patch avoids this
crash by suppressing cp_build_binary_op's operand folding accordingly.

	PR c++/105756

gcc/cp/ChangeLog:

	* typeck.cc (cp_build_binary_op): Don't fold operands
	when c_inhibit_evaluation_warnings.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/decltype82.C: New test.

(cherry picked from commit 0ecb6b906f215ec56df1a555139abe9ad95414fb)

2 files changed, 31 insertions, 17 deletions

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 0da6f2485d0..a6c393647b2 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -4929,7 +4929,7 @@ cp_build_binary_op (const op_location_t &location,
      convert it to this type.  */
   tree final_type = 0;
 
-  tree result, result_ovl;
+  tree result;
 
   /* Nonzero if this is an operation like MIN or MAX which can
      safely be computed in short if both args are promoted shorts.
@@ -6253,25 +6253,29 @@ cp_build_binary_op (const op_location_t &location,
     result = build2 (COMPOUND_EXPR, TREE_TYPE (result),
                      instrument_expr, result);
 
-  if (!processing_template_decl)
+  if (resultcode == SPACESHIP_EXPR && !processing_template_decl)
+    result = get_target_expr_sfinae (result, complain);
+
+  if (!c_inhibit_evaluation_warnings)
     {
-      if (resultcode == SPACESHIP_EXPR)
-        result = get_target_expr_sfinae (result, complain);
-      op0 = cp_fully_fold (op0);
-      /* Only consider the second argument if the first isn't overflowed.  */
-      if (!CONSTANT_CLASS_P (op0) || TREE_OVERFLOW_P (op0))
-        return result;
-      op1 = cp_fully_fold (op1);
-      if (!CONSTANT_CLASS_P (op1) || TREE_OVERFLOW_P (op1))
+      if (!processing_template_decl)
+        {
+          op0 = cp_fully_fold (op0);
+          /* Only consider the second argument if the first isn't overflowed.  */
+          if (!CONSTANT_CLASS_P (op0) || TREE_OVERFLOW_P (op0))
+            return result;
+          op1 = cp_fully_fold (op1);
+          if (!CONSTANT_CLASS_P (op1) || TREE_OVERFLOW_P (op1))
+            return result;
+        }
+      else if (!CONSTANT_CLASS_P (op0) || !CONSTANT_CLASS_P (op1)
+               || TREE_OVERFLOW_P (op0) || TREE_OVERFLOW_P (op1))
         return result;
-    }
-  else if (!CONSTANT_CLASS_P (op0) || !CONSTANT_CLASS_P (op1)
-           || TREE_OVERFLOW_P (op0) || TREE_OVERFLOW_P (op1))
-    return result;
 
-  result_ovl = fold_build2 (resultcode, build_type, op0, op1);
-  if (TREE_OVERFLOW_P (result_ovl))
-    overflow_warning (location, result_ovl);
+      tree result_ovl = fold_build2 (resultcode, build_type, op0, op1);
+      if (TREE_OVERFLOW_P (result_ovl))
+        overflow_warning (location, result_ovl);
+    }
 
   return result;
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype82.C b/gcc/testsuite/g++.dg/cpp0x/decltype82.C
new file mode 100644
index 00000000000..915e5e37675
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype82.C
@@ -0,0 +1,10 @@
+// PR c++/105756
+// { dg-do compile { target c++11 } }
+
+template<int N>
+void f() {
+  using ty1 = decltype((5 % N) == 0);
+  using ty2 = decltype((5 / N) == 0);
+}
+
+template void f<0>();