C++ PATCH for c++/48557 (SFINAE failure with void operands)

[Jason Merrill <jason@redhat.com>]

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We were just failing to check complain for this one error.

Tested x86_64-pc-linux-gnu, applying to trunk.

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commit e807a327e3e667c8d79356cf644e493fbb82033f
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Apr 13 18:14:45 2011 -0400

    	PR c++/48557
    	* typeck.c (cp_build_binary_op): Don't decay void operands.

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index ecd7d41..b0e2110 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -3646,16 +3646,16 @@ cp_build_binary_op (location_t location,
       || code == TRUTH_OR_EXPR || code == TRUTH_ORIF_EXPR
       || code == TRUTH_XOR_EXPR)
     {
-      if (!really_overloaded_fn (op0))
+      if (!really_overloaded_fn (op0) && !VOID_TYPE_P (TREE_TYPE (op0)))
 	op0 = decay_conversion (op0);
-      if (!really_overloaded_fn (op1))
+      if (!really_overloaded_fn (op1) && !VOID_TYPE_P (TREE_TYPE (op1)))
 	op1 = decay_conversion (op1);
     }
   else
     {
-      if (!really_overloaded_fn (op0))
+      if (!really_overloaded_fn (op0) && !VOID_TYPE_P (TREE_TYPE (op0)))
 	op0 = default_conversion (op0);
-      if (!really_overloaded_fn (op1))
+      if (!really_overloaded_fn (op1) && !VOID_TYPE_P (TREE_TYPE (op1)))
 	op1 = default_conversion (op1);
     }
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/sfinae14.C b/gcc/testsuite/g++.dg/cpp0x/sfinae14.C
new file mode 100644
index 0000000..305f96e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/sfinae14.C
@@ -0,0 +1,27 @@
+// PR c++/48557
+// { dg-options -std=c++0x }
+
+template<class T>
+struct add_rval_ref
+{
+  typedef T&& type;
+};
+
+template<>
+struct add_rval_ref<void>
+{
+  typedef void type;
+};
+
+template<class T>
+typename add_rval_ref<T>::type create();
+
+template<class T, class U,
+  class = decltype(create<T>() + create<U>())
+>
+char f(int);
+
+template<class, class>
+char (&f(...))[2];
+
+static_assert(sizeof(f<void, int>(0)) != 1, "Error");  // (a)