c++: variable partial spec redeclaration [PR113612]

If register_specialization finds a previous declaration and throws the new
one away, we shouldn't still add the new one to
DECL_TEMPLATE_SPECIALIZATIONS.

	PR c++/113612

gcc/cp/ChangeLog:

	* pt.cc (process_partial_specialization): Return early
	on redeclaration.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ85.C: New test.

gcc/cp/pt.cc

@@ -5417,9 +5417,14 @@ process_partial_specialization (tree decl)
     }
   if (VAR_P (decl))
-    /* We didn't register this in check_explicit_specialization so we could
-       wait until the constraints were set.  */
-    decl = register_specialization (decl, maintmpl, specargs, false, 0);
+    {
+      /* We didn't register this in check_explicit_specialization so we could
+	 wait until the constraints were set.  */
+      tree reg = register_specialization (decl, maintmpl, specargs, false, 0);
+      if (reg != decl)
+	/* Redeclaration.  */
+	return reg;
+    }
   else
     associate_classtype_constraints (type);

gcc/testsuite/g++.dg/cpp1y/var-templ85.C

+// PR c++/113612
+// { dg-do compile { target c++14 } }
+
+template <typename T> T t;
+template <typename T> extern T *t<T *>;
+template <typename T> T *t<T *> = t<int>;