C++ PATCH to checking of explicit instantiation namespace

[Jason Merrill <jason@redhat.com>]

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In the testcase, the using-declaration was confusing namespace 
comparison into thinking that we were instantiating a template from an 
enclosing namespace.  Given using-declarations, we need to wait until 
we've chosen a template before donig this comparison.

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

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commit 43aa655785ca8da40d07e1320f532950fc9a83b5
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Nov 12 11:22:04 2015 -0500

    	* pt.c (check_explicit_specialization): Check the namespace after
    	we choose a template.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 62659ec..2e3d48b 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -2800,14 +2800,6 @@ check_explicit_specialization (tree declarator,
 		  error ("%qD is not a template function", dname);
 		  fns = error_mark_node;
 		}
-	      else
-		{
-		  tree fn = OVL_CURRENT (fns);
-		  if (!is_associated_namespace (CP_DECL_CONTEXT (decl),
-						CP_DECL_CONTEXT (fn)))
-		    error ("%qD is not declared in %qD",
-			   decl, current_namespace);
-		}
 	    }
 
 	  declarator = lookup_template_function (fns, NULL_TREE);
@@ -2941,6 +2933,14 @@ check_explicit_specialization (tree declarator,
 	return error_mark_node;
       else
 	{
+	  if (!ctype && !was_template_id
+	      && (specialization || member_specialization
+		  || explicit_instantiation)
+	      && !is_associated_namespace (CP_DECL_CONTEXT (decl),
+					   CP_DECL_CONTEXT (tmpl)))
+	    error ("%qD is not declared in %qD",
+		   tmpl, current_namespace);
+
 	  tree gen_tmpl = most_general_template (tmpl);
 
 	  if (explicit_instantiation)
diff --git a/gcc/testsuite/g++.dg/template/explicit-instantiation4.C b/gcc/testsuite/g++.dg/template/explicit-instantiation4.C
new file mode 100644
index 0000000..72417b4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/explicit-instantiation4.C
@@ -0,0 +1,7 @@
+void f();
+
+namespace A {
+  template <class T> void f(T) { }
+  using ::f;
+  template void f(int);
+}