Improve deduction failure diagnostics.

	* pt.c (unify): Don't diagnose no common base if we already have
	the same template.
	(do_auto_deduction): Explain deduction failure.

From-SVN: r229627

gcc/cp/ChangeLog

 2015-10-31  Jason Merrill  <jason@redhat.com>
 
+	* pt.c (unify): Don't diagnose no common base if we already have
+	the same template.
+	(do_auto_deduction): Explain deduction failure.
+
 	* pt.c (hash_tmpl_and_args): Use iterative_hash_object on template
 	uid.
 

gcc/cp/pt.c

@@ -19633,7 +19633,16 @@ unify (tree tparms, tree targs, tree parm, tree arg, int strict,
 					 explain_p, &t);
 
 		  if (!t)
-		    return unify_no_common_base (explain_p, r, parm, arg);
+		    {
+		      /* Don't give the derived diagnostic if we're
+			 already dealing with the same template.  */
+		      bool same_template
+			= (CLASSTYPE_TEMPLATE_INFO (arg)
+			   && (CLASSTYPE_TI_TEMPLATE (parm)
+			       == CLASSTYPE_TI_TEMPLATE (arg)));
+		      return unify_no_common_base (explain_p && !same_template,
+						   r, parm, arg);
+		    }
 		}
 	    }
 	  else if (CLASSTYPE_TEMPLATE_INFO (arg)
@@ -23500,6 +23509,9 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 		error ("unable to deduce lambda return type from %qE", init);
 	      else
 		error ("unable to deduce %qT from %qE", type, init);
+	      type_unification_real (tparms, targs, parms, &init, 1, 0,
+				     DEDUCE_CALL, LOOKUP_NORMAL,
+				     NULL, /*explain_p=*/true);
 	    }
 	  return error_mark_node;
 	}

gcc/testsuite/g++.old-deja/g++.pt/explicit77.C

@@ -15,5 +15,5 @@ void g() {
   
   f<0>(s0, s2);
   f(s0, s2); // { dg-error "" } no matching function
-  // { dg-message "(candidate|deduced conflicting types|ambiguous base class)" "candidate note" { target *-*-* } 17 }
+  // { dg-message "(candidate|deduced conflicting|ambiguous base class)" "candidate note" { target *-*-* } 17 }
 }