commit 16219f7fd77e362d9aa4e717529cd42a9f126153
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Oct 22 11:09:42 2009 -0700

    	Core issue 899
    	* call.c (add_function_candidate): Only permit explicit conversion
    	ops if copy ctor was called with a single argument.

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index fe74d15..4542804 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -1631,7 +1631,8 @@ add_function_candidate (struct z_candidate **candidates,
 	      parmtype = build_pointer_type (parmtype);
 	    }
 
-	  if (ctype && i == 0 && DECL_COPY_CONSTRUCTOR_P (fn))
+	  if (ctype && i == 0 && DECL_COPY_CONSTRUCTOR_P (fn)
+	      && (len-skip == 1))
 	    {
 	      /* Hack: Direct-initialize copy parm (i.e. suppress
 		 LOOKUP_ONLYCONVERTING) to make explicit conversion ops
diff --git a/gcc/testsuite/g++.dg/cpp0x/explicit4.C b/gcc/testsuite/g++.dg/cpp0x/explicit4.C
new file mode 100644
index 0000000..74726a9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/explicit4.C
@@ -0,0 +1,17 @@
+// Negative explicit conv test.
+// { dg-options "-std=c++0x" }
+
+struct A {
+  A(const A&, int = 0);		// { dg-message "candidates" }
+};
+struct B
+{
+  explicit operator A();
+};
+
+int main()
+{
+  B b;
+  (A(b));			// OK
+  (A(b,1));			// { dg-error "no match" }
+}